Ultrasonic Anemometer Part 29: Transducer Comparison

It’s been way too long since the last post in my Ultrasonic Anemometer series. But better late then never.

So far I have always used the same type of transducers. When I started this project I looked around and found the Multicomp MCUSD16A40S12RO. They were comparatively cheap and readily available so I ordered some and used nothing else for the next two or so years.

However, they are not hermetically sealed and therefore not the most suitable type to use outdoors all year around. Some of you have already experimented with other, truely watertight types and have reported that the signal level with those is dramatically lower. So I decided to see what’s available and do a more systematic comparison. That’s what this post is about.

The Contestants

  1. Multicomp MCUSD16A40S12R0 (Farnell 2362677),  16mm, not waterproof, no wires attached.
  2. Multicomp MCUSD14A40S09RS (Farnell 2362679), 14mm, waterproof, no wires attached.
  3. Multicomp MCUSD14A40S09RS-30C (Farnell 2362678), 14mm, waterproof, with attached wires.
  4. Multicomp MCUSD18A40S09RS-30C (Farnell 2362680), 18mm, waterproof, with attached wires.

You might wonder why I’m only considering Multicomp models but those 3 models are pretty much all the 40kHz watertight transducers I can get from Farnell.

So what I did is I attached all 4 models to the same anemometer so that I can switch back and forth between them. That turned out to be rather tedious and time consuming and resulted in a big mess of wires so I was happy that I only had 4 types to worry about.

I’ve measured the signal after the first (fixed gain) stage of the op-amp. So the signal reported here is already boosted up by a factor of 11.

The Benchmark

Let’s first look at the model used so far. As we already know it delivers a solid signal amplitude.

Zooming in at the burst at the center of the screen (the one with the smalles amplitude) gives us a reading of 1.9 volts peak-to-peak.

14mm waterproof transducer without wires

Let’s look at the first waterproof model. The signal amplitude us barely noticable at first.

Zooming in (both horizontally and vertically) then reveals the amplitude: 47 millivolts, a factor of 40 less than the benchmark model.

14mm waterproof with wires

Next in line is a transducer that looks virtually identical but has a pair of drilled wires at the back as opposed to just two pins. The overview looks just like before.

Zooming in shows an even smaller signal amplitude of only 37millivolts. That’s surprising because it has somewhat better specs compared to the previous model.

18mm waterproof with wires

Once again, the overview shows hardly anything for the 18mm model.

Zooming in we find an amplitude of 38 millivolts, basically the same as the last model.


The first fact is obvious: with hermetically sealed transducers you can only expect a small fraction of the signal at the input.  I knew that but was still surprised to find how much smaller that singal is.  In this comparison it’s a factor of 40in the very best case.

But an anemometer is useless if it cannot withstand the elements so I guess we have to live with that. I will use the 14mm without wires type going forward. Not only did it perfom best in my comparison but it is also the most affordable (waterproof) model. Apart from that I like its small 14mm form factor allowing for slim designs posing less resistance to the airflow.

The road ahead

We obviously need to make some hardware changes to deal with the low signal amplitude. The most obvious solution is more gain. We can get more gain of the first (fixed gain) stage by just changing a resistor. However, we need to be careful not to push the op amp beyond its limits. I’ve looked at the LMC6482 datasheet again and found a gain/bandwidth product of 1.5MHz. That means we can get a maximum gain of 37.5 at 40kHz. You probably don’t want to push the op amp right to its limit so something like 31 (changing the 10k resistor to 30k)  is probably about the best we can do here.  That gives us an extra factor of 2.82.

Making the physical design smaller and therefore reducing the distance between the transducers is another thing we can do.  The current desing is 245mm which is rather large. Someone has reported to me that he reduced that all the way down to 100mm. I don’t think I will go that far but will try 120mm which is about half of what I have now. That should double the signal amplitude. And the smaller transducers should make the task easier.

The two measures combined give an extra gain of factor 5.6. That leaves us with another factor 7 to be compensated elsewhere. Fortunately, we still have a second op amp stage (the one controlled by software) which hasn’t been doing much so far.  With the previous transducer model it only had to deliver something like a factor 1.5. If we boos this up to 10 we pretty much have all we need.

A software bug

During my tests I also discovered some weird software behaviour when the signal (after both stages of amplification) is not in the expected range. USB may hang, among other things. The pulse-sending sequence can also get upset. Someone had reported this to me earlier but I wasn’t able to reproduce the problem until now. I don’t expect this to be a major thing to fix but you never know. But I’ll definitely look into this.




ReflowR on indiegogo.com

This will be my shortest post ever. But I just spotted a project on indiegogo.com that I think is worth mentioning: A small and affordable tool to do reflow soldering. It’s basically a heating plate specifically designed for reflow soldering. So it can reproduce JEDEC temperature profiles, it does data logging and and you can even control it via a web interface if you spend an extra 9$ on the wifi upgrade.

Here’s the link: https://igg.me/at/reflowr/x/15560432 or http://reflowr.com

I’ve been looking around for some kind of reflow oven for a while but nothing affordable quite convinced me. This morning I just stumbled accross this project by chance. I’ve ordered a “Large ReflowR” with the wifi upgrade and an external thermoucuple  and will report on how it performs once I get it which should be some time in February next year.

MPPT Solar Charger Testing II

It’s time to follow up on the MPPT Solar Charger project. Progress has been slow since I’m currently working full time and doing a master’s degree at the same time. Given that this blog has previously been something close to a 50% job at times things will necessarily slow down a bit. But all the projects, including this one and the ultrasonic anemometer are alive and well and I’m working on them whenever I find some time.


User Interface

So what’s new with the MPPT solar charger? First of all, it got this nice user interface. Unfortunately there were still some issues with the first version so there is an identically looking but much-improved and bug-free Rev B.  I’ll do a separate post on that documenting all the things I’ve changed.

I re-used the white-on-blue display from the initial version for the Rev B. as opposed to the black-on-white used with the Rev A. They are pin compatible so you can use whichever you prefer. I more and more start liking that black-on-white look. It’s about 10 bucks cheaper as well. What’s your opinion on this?


Solar Panel drains Battery

My intention with this design was to just leave the solar panel connected to the input of the buck converter. There’s a problem with that, however. The input of a buck is always at least at the output voltage minus a diode drop. With a 12V lead-acid battery at the output the input (i.e. where the solar panel is connected) the voltage never drops much below 12 volts.

That’s a serious problem since even a small solar panel like the monocrystalline 30W panel I have here sinks around 10mA at that voltage. So whenever the panel is not providing any power it considerably drains the battery.

Try a Diode

Obviously we need a way to avoid that. The first solution that comes to mind is a diode. But there’s a problem with that, too. To keep the power loss in the diode acceptable, the diode needs to have a low forward voltage. However, any diode with a low forward voltage also comes with a high reverse leakage.  Physics seems to dictate that.

Leakage can easily be in the range of milliamps for a diode that can handle, say, 4A like needed here. And that reverse leakage is also highly temperature dependent. It may be ok-ish at room temperature and then detoriate by an order of magnitude at 50 degrees centigrade.

To get an acceptable reverse leakage one needs to tolerate at least half a volt of forward drop which is something like 3% in efficiency. Since our converter operates at around 97% efficiency that would mean doubling the energy dissipated.

P-Channel or N-Channel Mosfet

So the next obvious thing to try is a mosfet acting as a switch.  Yes, a mosfet also has a body diode that also has a reverse leakage. But the body diode is a bad diode in terms of forward drop and has a correspondingly low reverse leakage.

A nice way of solving our problem would be to use a p-channel high-side switch disconnecting the battery from the converter whenever the panel is not producing. That not only disconnects the panel but powers off the entire buck including the input and output caps and everything. Where there is no voltage no power can get wasted. Perfect.


Unfortunately, powerful (i.e. low Rds-on) p-fets are relatively large,  rare and expensive. As a rule of thumb a p-fet needs twice as much silicon area for the same performance compared to a n-fet. Besides that, that would be another type of component and I’m trying to not use too many distinct components.

So the other solution is to use an n-fet as low-side switch just disconnecting the panel. We already have 6 powerful n-fets in our design and they are cheap, too. So this is what I’ll do in the next version.

For now I’ve used one of the power outputs to connect the panel. I’ve cut some traces and soldered in some wires so that the fet is on whenever the buck is powered on.

Buck Software

During early testing of the buck converter I managed to kill the bottom fet several times. Since I was able to run the buck at relatively high power levels for prolonged periods of time without any issues (and without getting hot) I suspected that this was caused by carelessly written software. This seems to have been the case indeed. Whenever I killed a fet it happened at startup or shutdown, usually the latter. In the mean time I’ve written well-defined startup and shutdown routines and never had any issue since.


Synchronous converters are dangerous in this respect. It is very easy to short the battery to ground via the bottom fet. All it takes is a duty cycle that’s too low or a timer is running too slow or not at all. Or you stop the PWM module at a time when the bottom fet is on and it will stay on forever.  Or at least a few milliseconds until it’s dead.

Asynchronous topologies are much more forgiving in this respect since you can’t short anything. So an easy solution would be to start up and shut down in asynchronous mode. We can easily make this an asynchronous converter in software by just not utilizing the bottom fet. The diode in parallel with it will then automatically take over.

Unfortunately we cannot start up in asynchronous mode. Why? To enable the upper fet we need a bootstrap voltage above the input voltage. And we don’t have that unless the converter is running. It’s a chicken and egg problem, really. So we need to start up in synchronous mode at least for a few switching cycles for the bootstrap diode to charge up.

That’s exactly what the startup routine does. It completes 16 cycles in synchronous mode at a neutral duty cycle. What do I mean by neutral? Simple: Duty Cycle = Output Voltage / Input Voltage. That means no current actually flows on average. So this is a nice, soft way to start up. After those 16 duty cycles the buck enters asynchronous mode. As the screenshot above shows, the bootstrap capacitor is fully charged after just one full duty cycle so that number is more than sufficient.

This routine is critical. If it doesn’t do precisely what it should chances are high that the buck is destroyed. So I’ve checked it carefully by both looking at the code and the scope. I’ve run it many times and observed closely what it does just to be sure it starts up nicely very time.

Why run in asynchronous mode? Because it’s more efficient at low power levels. Once the current rises above a software-defined threshold the converter will change to synchronous mode. If the current later falls below a second (lower) threshold, the buck changes back into async mode.

The optimal values for the two threshold will have to be determined experimentally but will likely be in the range of a few hundred milliamps.


The shutdown sequence is simple. The buck is shut down if the current falls below a very low threshold (say, 10mA) below which it cannot run profitably. If the current falls even lower we are better off shutting down the buck and entering a low-power mode. Since the current is low when we shut it off, the converter is already in async mode. So shutting it down is now easy and uncritical. We just turn off the top fet and can then also turn off the timer and supply voltage to the buck. The screenshot above shows an example of that.

Conserving power

A main feature of this solar charger is it’s ability to (hopefully…) run at a very low (<100microamps) current when not in use. So we obviously need to turn off everything we don’t need and run the PIC at a much lower frequency as well.

The PIC’s maximum operating frequency is 48MHz. This is the frequency it runs at when the buck and/or USB (not yet implemented) is on. These two features need that clock frequency to perform their task.

If both the buck and USB are off we can clock the PIC down to 8MHz which already saves a great deal of power. At 8MHz we can still do everything else, including running the user interface. Updating the display and particularly calculating the display content consumes quite some computation time and so we need a few MHz to do this.

If the user interface is not used for a certain time (say, 10 seconds), it is turned off. We can then lower the board voltage to 2.3 volts and lower the CPU frequency even further to 32.768kHz.  This is the frequency of the real-time clock that is running anyway. At such a low frequency the PIC’s computational power is low but still sufficient to do some housekeeping tasks.

One can wake up the user interface at any time by pressing the push button. Turning the encoder won’t help because that, too, has been powered off.

While in this low power mode every 6 seconds the board voltage is raised to 3.3 volts and all temperatures as well as the input and output voltages are measured. That all happens at 32.768kHz. After the measurement is complete the PIC decides if the panel voltage is high enough to start harvesting energy or not. If this is not the case the board voltage is lowered again and a new set of measurements is captured 6 seconds later.


Despite all those efforts the board consumes something like 530 microamps which is much more than it should. About 100 microamps comes from the voltage divider used to measure the panel voltge. That’s an easy to solve design problem that I’ve described earlier already. But that still means that something is drawing 400 microamps. Not much at all but still way too much.

I’ve spent an evening trying to find the problem but I still don’t know. I’ve suspected the capacitors but when I measured some spare caps of the same type they hardly drew any current at 12 volts. I also un-soldered the buck’s mosfet driver but that made hardly any difference so that’s also not the problem. It may even just be some near-short on my board homemade board. So I leave that for now and check again once I have a revised design with a proper PCB.

Buck testing

I’ve saved the most interesting part for last 😉 I’ve established before that this converter has a similarly high efficiency than the Arduino version published some time ago. But at that time I was only able to test up to 35 or so watts.

This solar charger is capable of handling much more power than that. Testing how much was a bit more difficult than expected. Unless the battery is very empty it won’t draw as much as this charger is able to provide. And you don’t want to discharge a lead acid battery too much, they don’t appreciate it at all. So once again my constant current dummy load came to the rescue.


With the dummy load at one of the power outputs I was able to draw some serious current. I set the dummy load to 4 amps and the battery absorbed (or provided) whatever was left. Obviously, I can only do that for a limited amount of time until the dummy load gets too hot. It automatically shuts down once the heat sink reaches 70 degrees centigrade which is soon the case at 4 amps.

I let the charger draw 4.5 amps at a bit more than 17 volts for as long as I could. The coil got quite warm, slightly hot even, maybe something like 60 degrees. The mosfet only got lukewarm and I’m not sure if this was because of their own power dissipation or due to their proximity to the coil.


So from this test I’d conclude that this was about the power which the charger can handle for a prolonged period of time. So I think something like 75 watts is a realistic power rating.

I’ve also had another look at the datasheet of the coil and it pretty much confirmed my findings. 75 watts at (say) 13 volts output corresponds to 5.8 amps through the coil. The datasheet states that the coil starts saturating at 8.2 amps which means we’re still safe with 5.8 amps plus the ripple current. The datasheet also states a 40 degree temperature rise at 5.3 amps. So thermally the coil is pretty much at its limit at 75 watts. That, too, seems realistic to me having touched it after some time at that power.

The road ahead

So how to continue? As we have seen, there is a number of design issues but the general concept works. During testing and debugging I’ve cut and re-soldered many traces and also made some modifications like the one with the n-fet described above. I’ve unsoldered and changed many components as well. All of that doesn’t improve the reliability of the board. At some point it makes sense to design a new version, build it and take it from there. A clean slate kind of.

I think this point has been reached here. So I will work on a new revision from now on. The concept won’t change at all but I’ll try to apply what I’ve learned so far. I also hope to reduce the number of different components, reduce the size and total cost while not sacrificing performance. In other words move a step towards a design that may one day be built as a small series. Let’s see.

Low Power User Interface


As you may have noticed I’m quite busy working on the MPPT Solar Charger project. The latest version uses a 4 lines x 20 characters LCD that connects via I2C as well as a rotary encoder with a push button.


While the display got its own little board, the encoder connected directly with the solar charger where its signals aredebounced in hardware and then routed to the PIC.


After a few little fixes (see my last post) that worked but I had to use the signal intended for controlling the  backlight brightness for the display’s reset signal. I don’t want to run any more wires to the display and there are no spare pins on the PIC anyway so I had to come up with another solution.


And it was not very elegant that the display was powered off by just cutting the entire power supply. Having a digital enable/disable signal would be preferable


Design a universal User Interface

I then decided to design an new board that also includes the rotary encoder with all the necessary debouncing. That way I get a quite universal, easy to use user interface that I can use for other projects as well. That eliminates the need to design the same functionality (think debouncing) again and again. The size of the board is mainly given by the size of the display so there is plenty of board space.


So far so good but that doesn’t solve the actual problems yet. I still need a way to control both the backight as well as the reset signal without needing any more singnals. Since the display connects via I2C it was quite straight forward to use I2C communication for those purposes as well. My first thought was to add a I2C port expander just like I’ve done on the solar charger board. Those chips aren’t expensive but they typically provide 8 or 16 extra GPIO pins which seemed a bit wasteful. And PWM control of the backlight would require a great deal of I2C communication which puts a burdon on the microcontroller.


I also considered adding a sub-dollar PIC16 which could be programmed as an I2C slave and control both the backlight and the reset signal (as well as giving the option of getting the encoder input via I2C). But I wasn’t eager to write any software for this thing.


Requirement: Ultra Low Power State

Another design requirement was to keep this a very low-power design. To be sure: when the display and especially the backlight are on it will consume several milliamps to several dozen milliamps. Illumination doesn’t work without power, such is life. But when the user interface is not in use, we need a way to put it in a very low power state where the display as well as the rotary encoder is off but the push button still works in order to wake up the microcontroller. In that state the user interface must not use more than a few microamps. Furthermore the encoder signals must assume a high impedance state when the user interface is disabled.


My final solution was as follows:

  • Use the previous (backlight) PWM signal as an enable signal. Power can now always stay on. When the enable signal is high, the display is on, when enable is low, the user interface goes into its low-power state.
  • Add a dual I2C digipot to control both the reset signal as well as the backlight.

The reset signal is connected directly to the wiper of one of the two digipots. Of course any intermediate digipot values don’t make sense. The digipot should always be either at its minimum or its maximum.

The backlight brightness is controlled by the other digipot via an op-amp and a n-channel mosfet.

The debouncing is basically unchanged. Like in the solar charger design, a 74HC126 quad tri-state buffer is used together with some resistors and capacitors.

Since there are 4 gates on the 74HC126 and I only need to debounce 3 signals, there is still one gate left. I used that gate to power the display as well as the encoder and the encoder’s gates(without the push button which is always powered on). So the enable signal only controls the input of that gate. I thought that’s a nice and elegant solution. Well, if you think so too, think again. More on that later.



First of all nothing worked at all. I asserted the enable signal and nothing happened. Nothing was powered on. The error was soon found. I made a design error with the result that the respective gate was permanently in a high-impedance state. Somehow I had tied the gate’s enable input to ground instead of VCC. The mistake was corrected by cutting the ground connection and conecting the respective pin to the enable singal itself which was available from the pin right next to it. Not pretty (see below) but problem solved.


Then ok, power was there but when I tried to turn off the reset singal (i.e. pulling the reset signal high) nothing happened – again. Again, the reason was simple. The digipot’s upper terminal was floating instead of being connected to VCC. On the schematic the wire touched the respective pin but there was no connection. While that was really hard to notice on the schematic I should have noticed the problem when I laid out the board. There was obviously no connection to that pin.


This was a bit harder to fix than the previous error. There were no signals to cut but the digipot’s pins are much more closely spaced and the right signal was not to be found nearby. I ended up soldering a thin wire accross the digipot to a resistor that is connected to VCC on the upper end.


Even less pretty but problem solved anyway. Turning on the backlight basically worked as intended but the load was a bit much for the 74HC126. While it can supply up to 35mA according to the data sheet, its output voltage is nowhere near its positive supply rail when it supplies that much current. Even with a more modest 5mA load the gate’s output voltage was about half a volt below the input voltage. As a result, the display as well as all the other components only get to see about 2.8 volts instead of 3.3. This didn’t pose any problems so far but it’s not nice.

Another issue I noticed was that the LM321 op amp I had chosen is not a rail-to-rail op amp. The LM321 is inexpensive and I had quite a few left from another project so I used it without giving it much thought. I measured that the output voltage doesn’t get closer than about 1.4 volts of the positive supply rail. Since the n-mosfet used to control the backlight has a very low threshold voltage that didn’t pose a problem but the LM321 is not really a good choice here. While it does work from as little as 3 volts it’s made for split supply and higher voltages, say plus/minus 12 volts. I’ll use a low-voltage rail-to-rail type next time.


Getting any MIDAS I2C display to work is a nightmare. They make nice, pretty and affordable displays. That’s why I buy them. And once you’ve managed to properly initialize them they are easy to work with. But getting them initialized with the right settings is always a lengthy trial-and-error process. I’ve ranted about that before but I feel the need to do it once again.

The entire initialization sequence needs to be written in one single I2C transmission. At least for me it didn’t work otherwise. The data sheet says absolutely nothing about that.


The data sheet doesn’t even mention the initialization sequence. Nor the timing of the reset singnal. The reset signal needs to be pulled low for 10ms. One then needs to wait for 5ms before the initialization sequence is sent. There is not one word about that it in the current datasheet. There is an older version in the net somewhere that mentions those timing issues and at least gives a sample initialization sequence.


There are various contrast settings that must be set properly via I2C commands but there is very little information on those commands in the data sheet. I wrote an email to their customer support asking if there is a list of commands the display supports. That was more than a month ago and I still didn’t get any answer. As so often, google was my friend and others have faced similar problems before so I managed to find some commands that make the display work properly. I’m sending 10 or so bytes to the display but for most of them I have not a clue what they actually mean or what they are supposed to do. And the commands for this black-on-white display are different to the (otherwise identical) white-on-blue display. Have fun…


I’ve devoted one of my very first posts to the much-neglected issue of debouncing. The logic is pretty much the same here except that I use different (non-inverting) gates here. The reason for that is the need for 3-state outputs as mentioned before.


Unfortunately the 74HC126 doesn’t have schmitt-triggered inputs so I need to implement the necessary feedback myself via the 680k resistors.

Debouncing something like a pushbutton is quite easy. Just use a relatively long time constant (i.e. large resistors and capacitors) and all will be fine. That will introduce some delay in the output signal but as a rule of thumb anything below 50ms is not noticable. And a time constant of a few milliseconds is totally sufficient so you might end up with something like 10ms of delay. No human user will ever notice it and it’s more than enough to perfectly debounce any switch that I’ve seen.


With rotary encoders things get much trickier. The basic circuit is exactly the same but choosing the right component values is more difficult. You can’t press a button more than a few times a second but a rotary encoder might produce pulses just milliseconds or even less apart.


So you face the challenge of debouncing the signal while still letting relatively fast transitions pass. If you use a time constant of a few milliseconds like with the push button you will not be able to detect when the encoder is turned quickly.


The data sheet of the Burns encoder I’m using here recommends using two 10k resistors together with a 10nF cap. I’ve increased the value of one resistor to 22k but have otherwise followed that recommendation.  So the time constant is now a few hunderd microseconds.


Toghether with the positive feedback via the 680k resistors that works very well. I expected to tweak those values a bit but found that they work so well that I just leave them as they are.


I’ve inserted plenty of scope screenshots here to give you an idea of what the undebounced and debounced signals look like. Even with the same encoder there’s plenty of variation as you can see. But with the hardware debouncing applied here we get a clean, digital output singal nevertheless.


Power consumption

Very low standby power consumption was one of the key design goals. When in low-power mode only the push button pull-up and the 74HC126 is supplied with power. Everything else, the display, the op-amp, the digipot and the pull-up for the rotary encoder are all completely powered off.

As long as the push button is not pressed, no power flows there so the only power consuming device is the 74HC126. Its datasheet specifies a maximum static current consumption of 8 microamps (worst) at room temperature.  There’s no typical figure so that’s all the information I had. When I measured it, this is what I got.


That’s 4 nanoamps  at 3.3 volts which is nothing really. Actually the measurement fluctuated between about 0 and 15 nanoamps. I thought something is not hooked up correctly but when I pushed the button the current jumped to 326 microamps which is just what I expected since there is a 10k pull-up resistor conected to that button.


So the measurement works and it seems that the 8 microamps given in the data sheet are just a very conservative worst case. I mean CMOS is close to zero-power when static and only a single tri-state gate is active so essentially no current flows. Cool.

Next steps

After the two hardware fixes this user interface works quite as intended. Using the spare gate to supply the rest of the circuit with power was not as good an idea than I thought. On the positive side the debouncing works just perfect with the chosen component values. And as mentioned, the op-amp was not a great choice for the job at hand.

But I like the look and feel of this universal user interface and think the overall concept is good. So there will be a Rev B of this board with those issues solved.

Here you find the eagle files as well as PDFs of the schematic and board. The two errors are already corrected but I haven’t actually built them so check carefully yourself. I’ve also re-exported the gerbers so they, too, already include the fix.

MPPT Solar Charger Testing


In a previous post I have presented a design for an MPPT Solar Charger. In the mean time I have built a prototype and also wrote some software for it. So today I’ll go through my findings of what works well and what needs to be improved. And yes, there are some flaws in the design…

The software is far from final but with the notable exception of USB all the basic functionality has been implemented.

Power Supply


One of my main goals with this design is to archieve very low standby current, somewhere in the tens of microamps. The basis for this is a low-power buck on the basis of a Texas TPS62120 where the microcontroller can switch the output voltage between 2.2 and 3.3 volts nominally. This works as intended. With no load and the output voltage low, the supply consumes 12.9 microamps at 12V input voltage. With the high output voltage the idle current goes up to 14.3uA. Quite a bit of that current is due to the voltage divider that sets the output voltage. The regulator itself consumes about 9uA in both cases.



The PIC18F26J50 starts up using the primary oscillator’s 8MHz crystal with the internal PLL disabled. It can then switch to 48MHz operation by enabling the PLL or to the secondary oscillator running at 32.768kHz. The latter is always running since it also serves as the clock source to the real-time clock and calender (RTCC). Switching between clock sources as well as the RTCC have been implemented in software and work fine.

I2C Multiplexer and Port Expander


The microcontroller doesn’t have enough GPIO pins so a I2C port expander (Microchip MCP23008) is used to give us another 8 pins. The display is connected via I2C as well but since the display is entirely powered off when not in use, the display cannot be on the same bus. Otherwise it will pull the SCL and SDA lines low and block the bus. The NXP PCA9546 multiplexer takes care of that. Both devices, the mux as well as the port expander work as they should.

Voltage Reference, Temperature Sensors and Fan


There is a total of 3 temperature sensors, one on the board and 2 external ones. The temperature is measured by the PIC directly so it has a 2.5V voltage reference in order to get meaningful results. At room temperature all three temperature sensors deliver very similar results, typically within a few tenth of a degree. Better than expected. The fan output can also be enabled if the on-board temperature gets too high.

Voltage and Current Measurements


The more important measuements, namely the input and output voltages and currents are performed by a MCP3424 4-channel 18-bit ADC. I was already familiar with that type so I quickly got it to work. In this application I don’t use its internal PGA (programmable gain amplifier) or rather I leave the gain at 1. All the measurements are done with 14bit accuracy which is reasonably fast (23ms worst case) and sufficient here.

User Interface


The debouncing of the rotary encoder works ok but I’ll tweak the resistor and capacitor values a bit when I get time. The push button is uncritical but there is a bit too much debouncing for the rotary encoder itself making it miss some very fast turns. But it already performs reasonably well as it is.

The display module is the first real flaw in my design. It has 5 wire connections: GND, VCC, SDA, SCL and Backlight. The idea was to turn the display off by simply cutting off VCC when the display is not needed. When in use, the brightness of the backlight is then controlled by the Backlight PWM signal.


Since the display needs quite a few supporting components (mainly 1uF capacitors) and has no mounting holes I made a little one-sided PCB for it. When I first tried it nothing worked. A ground connection was missing and I didn’t notice that in Eagle because in Eagle it was a double-sided board. That was easily fixed by a short piece of blank wire soldered in place. So far so good.

Unfortunately the display (a Midas MCCOG42005A6W-BNMLWI) still didn’t work. Besides the data sheet being a nightmare it needs to be reset after having powered up. So you need to power it up, wait for a while and then do a reset by pulling its reset pin low. So just pulling the reset pin high with a resistor like I have done won’t work. My solution for now was to connect the backlight signal to the reset pin. So I can now use the display but the backlight brightness is permanently  at it’s maximum.


I already have a new and hopefully better version of this display unit in the making and there will be a separate post on that. My idea is to have a rather universal module that I can use in other projects as well.

USB Charging


The USB charging module works reasonably well. When pulling the full rated current of 2A it gets rather warm but doesn’t overheat. I was able to pull 2.5 amps for a prolonged period of time without any issues. I only had a Samsung A5 to try but at least that phone charged perfectly fast pulling around 1A of current. With that load the module only gets slightly warm.


Turning the USB charger on and off in software also works as planned. This can now even be done via the user interface – see above.

Power Outputs


The 4 power outputs can be controlled in software. In order to save power, the mosfet drivers can be powered off if none of the outputs is on . All that is already implemented in software and the outputs can be turned on and off individually via the user interface.


I plan to implement a low-frequency (say, 100Hz) PWM functionality for these outputs in order to e.g. control some LED lighting but that has not been done yet.



I haven’t implemented any data logging functionality yet but I have written a few library functions for the EEPROM. When the date and time is set via the user interface those values are saved to EEPROM and restored after a reset. Reading from and writing to the EEPROM therefore works.


Solar Charger

Yes, I’ve left the main thing for last. Don’t know why.


When I first assembled the board I noticed that I had ordered the wrong mosfet driver – a MIC4605-2 instead of a MIC4605-1. They are near-identical except the fact that the -1 has two independant inputs while the -2only has one PWM input plus an enable signal. That also works but doesn’t give us the option to run the converter in asynchronous mode. So I’ve unsoldered the chip again and replaced it with the MIC4605-1 that I had in mind when I designed this thing.


As you can see on the photos above, I’m now using a 22uH coil, not a 68uH coil as shown in the schematic. The reason for that is that I’m using a relatively high switching frequency of 48MHz / 256 or 187.5kHz. That high switching frequency enables me to use a lower value inductor which massively increases its current rating while pyhsically being the same size. Since the inductor was the limiting element in this design before this significantly increases the power rating of this solar charger. The previous version was (very conservatively) rated at 30 watts only. With the new 22uH inductor this very similar design can handle something like 75 watts. I need a more powerful power supply to do some serious testing but at 35 watts it’s getting slightly warm at the most. I’ve only done some quick and dirty testing but efficiency at 35 watts is around 97%. That means we are only losing about 1 watt which is the reason the charger barely gets warm.


There will likely be another post focussing only on the charger itself so I won’t go into more details here. The code regarding the buck is extremely basic and potentially dangerous at this point. I’ve killed the bottom mosfet twice due to the not very graceful way the buck turns off. If it turns off while the bottom fet is on, that fet will stay on, effectively shorting the battery to ground. At least that’s my explanation at the moment.


Oh yes, there’s another issue with the buck as well. With the battery at its output there will always be the battery voltage minus a diode drop at its input where the panel is connected. So when there is no sun, the battery is powering the panel. I’ve tried that with a small 30W panel and the panel draws 8mA at 12 volts which several magnitudes more than we are willing to use when the charger is sitting idle. So the next version will have to disconnect the panel when the charger is off. And we will also need to disconnect the voltage divider at the charger’s input when we’re not measuring the voltage, just like we’re doing at the battery.


There are a lot of features on this board and getting them all to work properly requires quite a bit of work. But most of the heavy lifting has been done so I can now focus on improving the software.


First and foremost I need to work on the way the buck is controlled. First there needs to be a strict and safe procedure how this thing is turned on and off. I’m confident that there won’t be any blown-up mosfets once that’s done. Then I also need to improve the algorithm of how the maximum point of power is tracked. And I also want to implement asynchronous operation at low loads in order to improve efficiency. I’ll spend some time on all these issues next, looking at the signals on a scope and trying to improve things.

But all-in-all I’m quite happy with how how this design has performed so far. Yes, there are some issues but nothing that can’t be fixed with relative ease.

For those of you interested, the code can be found under downloads on the overview page. In my next post we will take a closer look at the display unit / user interface.

Ultrasonic Anemometer Part 28: New hardware tested


I last time proudly presented the new RevB board and got a lot of feedback from people who want one, too. As mentioned I have all the components here to ship up to 10 kits but I was reluctant to send anything until I had the chance to do some hardware testing. Not much had changed since the last revision but I don’t like taking chances on things like this.


In the mean time I managed to do some rudimentary testing and now feel confident to take orders. These tests concern the hardware only. What I said last time about the state of the software still applies. But let me tell you what I’ve been able to test so far.


Tests performed so far

  • The PIC32 can be programmed from a PICKit3 via the ICSP header without any issues.
  • Power consumption is as expected. Like the previous version it draws 45mA@12V (programmed) and the other two rails come up with +3.310 and -3.279V, respectively. Also as expected. Regulator stays cool.
  • With the PIC controlling the AXIS, DIRECTION and SIGNAL pins, the transducers receive the 12V signal from the mosfet drivers. HOWEVER: the signals AXIS and DIRECTION are incorrectly labeled both in Eagle as well as on the silk screen. Electrically everything is fine but the names have been confused.
  • The signal from the transducers is properly received (Rec pin on the board) and amplified by a factor of 11 by the first stage of the amplifier (S1 pin on the board).
  • The PIC is able to control the digipot over the internal I2C bus and the second amplifier stage also performs as expected.
  • The zero-crossing detector (ZCD) works.
  • The input to the PIC’s ADC (ADC+ and ADC-) look fine, too. HOWEVER: the labeling on the silk screen is wrong. Eagle is correct, it’s just the silk screen. Plus should be minus and vice-versa. Again, electrically everything is fine.
  • The PIC can communicate (as a slave) with an Arduino UNO connected to the external I2C bus.
  • Communication over USB to a YAT-terminal under Win 7 works.



The following has not been tested so far

  • For my tests I still used the transducers already used previously. I believe the ones I ordered last week are of the same type but I haven’t tested them.
  • EEPROM. I haven’t tried the newly added memory yet. I have confirmed that it has power and is connected to the same bus as the digipot so I have no reasons to assume there are issues with it. But testing it would require some software first.
  • The external SPI bus has never been tested, neither on the Rev A board nor with the new one. I don’t expect any problems but I haven’t done any testing so far.


Is there anything important that I forgot to mention? In that case just ask.  A lot still needs to be done but at this point I’m confident that the board has no major flaws and performs much like the prototype.  Want to give it a try? I have some kits left for you.


Continue here to the next post of this series.

Ultrasonic Anemometer Part 27: Ready to take pre-orders

20160903_StandaloneAnemometer_001Good news: the boards from dirtypcbs.com have arrived and look great. I also got all the components for the 11 boards. Why 11? I ordered about 10 (they call it a protopack) and was lucky enough to get 11. Thats dirtypcbs.

Last week I also upgraded my hobbyist Eagle license to a proper Premium LS license which means I can now legally start selling stuff. So I’m basically ready to ship the first kits.


Today I assembled one of the boards and it at least looks great. All the footprints are correct and it was a pleasure to solder. Now what I want to do is to run some tests with it just to make sure it works as intended. I didn’t change much since the last version but I want to be sure first.


About the kit

There’s one thing I want to be absolutely clear about. At this stage of development the wind meter is not yet ready to be deployed. While I think the hardware is final now, a lot more work is needed to get the software ready. So for the time being this kit is intended for people who want to join the development and testing. I’ve done most of the low-level, register-fiddling stuff but much remains to be done at a higher level. I know there are a lot of people out there with much more experience in signal processing than me and I’m looking forward to work on this challenge together. And a challenge it is. But the PIC32 still has plenty of RAM, Flash and CPU time left to try out new ideas and approaches until we find one that works well.


The kit contains the board and all the necessary components. Details can be found in the BOM linked on the overview page. Once assembled it should look precisely like on the photos on this page. But as the name suggests, it comes as a kit, i.e. as components that you have to solder yourself. Most components come in relatively large SOIC packages but there are a few smaller MSOP and SOT-23 packages as well. They can all be soldered with a conventional soldering iron and strain solder just like I’ve done today.


The microcontroller is not yet programmed so you will need a suitable programmer. Microchip’s PICKit3 (USD47.95) is the obvious choice here. This is also what I’m using and matches the board’s pinout. All the software (MPLAB X IDE and XC32 compiler) are available for download from Microchip free of charge.

Taking pre-orders now

I’ll start taking pre-orders now but as mentioned I won’t ship the kits until I’ve done some tests with my own board. Once that’s done I’ll let you know and if you’re still interested by then I’ll give you my PayPal details and ship the kits.

Some have mentioned that they already have some ultrasonic tranducers and/or want to try some specific model so you are free to order your kit with or without the transducers.


There’s no online shop or anything like that so just use the contact form on the about me page.


Now it starts getting interesting. I’ll quote all prices in USD, EUR and CHF.  Choose what’s cheapest or most convenient for you.

  • Kit without transducers: USD 70 / EUR 63 / CHF 69
  • Kit with transducers: USD 95 / EUR 85 / CHF 93

The prices above include worldwide shipping. The kits ship by Swiss Post Priority Mail in a padded envelope.  I’ve used this service before to locations like Brazil or India and never had problems. However, there are no tracking numbers.

Any other questions? Just ask.



MPPT Solar Charger Design

I’m currently waiting for the boards for my Ultrasonic Anemometer Rev B to arrive from Hong Kong and this gives me some time to write about the MPPT Solar Charger design that I did quite some time ago. I published a series of posts on a Arduino MPPT Solar Charger Shield and got a lot of encouraging feedback. But that shield was more of a proof-of-concept than a finished product.  While it generally performed well it drew way too much current when idle to actually be deployed unless you can count on plenty of sunshine every day.


Aiming for very low power consumption

While I like the Arduino platform I had to admit that it’s probably not ideal for a low-power design. Yes there are some things you can do to reduce the 50mA or the Arduino draws but it will never be truely low power. So I designed a stand alone version with plenty of extra features that I hope to draw only a small fraction of the current. I particularly care about the idle current, i.e. the current it pulls when the solar panel is not producing any energy. In winter, the panel might be covered by snow for weeks and you don’t want your charger to drain the battery during that time. With this new design I’m aiming for an idle current in the tens of microamps. Even if it ends up being 100 microamps that is still 500 times less than an Arduino uses just by itself. And that means it draws less than 1Ah (ampere-hour) per year so you will never drain the battery no matter how little sunshine there is.


PIC18 series microcontroller

At the core of the design is a PIC18F26J50 in a 28 pin SOIC package. It’s capable of running at down to 2.15 volts and consumes extremely little power when running at lower clock speeds. And apart from that it features USB so we can have all the benefits of USB without any external components except, of course, a USB socket.

The PIC has two crystals at its disposal. A 8MHz crystal which will be boosted up to 32MHz by its internal PLL. That’s what the PIC will run on when there is work to do. And then there is a 32.768kHz crystal that will be used to run its real-time clock (RTC). When there is little to no work to do this low-frequency clock will also be used to run the CPU which will greatly reduce power consumption. Power consumption is approximately linear in frequency so this should cut power consumption by a factor of about 1000 compared to full-speed operation.


Switch mode power supply

Now we can run the microcontroller at only a few volts but our power supply is a 12 volt battery. So one of the most straight forward things to do in order to save power was to use a switch mode step-down regulator aka buck.

I looked around and found the Texas TPS62120. It’s only capable of providing 75mA but that’s more than enough for us in this case. It works at a switching frequency of 800kHz and only consumes a bit more than 10 microamps with no load at its output.

It needs a 18uH inductor as well as some ceramic capacitors to work. The output voltage is set via a pair of resistors acting as a voltage divider. I’ve added a n-channel mosfet that allows the PIC to increase the output voltage from 2.2 volts to 3.3 volts when needed. Because while the PIC can run on down to 2.15 volts the display can’t. And even the PIC needs 3.0 to 3.6 volts for USB operation.

Synchronous / asynchronous operation

The actual MPPT converter has changed only little. I’ve changed the mosfet driver to a MIC4605.  Unlike the IR2104 used last time, this model has adaptive dead-time (as opposed to the longish 540ns fixed) and separate inputs for each mosfet. This will allow us to either operate in synchronous (using both mosfets) or asynchronous (only using the upper fet and relying on the diode) mode. Asynchronous operation has a certain efficiency advantage at low power levels so this might come in handy.

The MIC4605 consumes quite little quiescent current for a mosfet driver, only 100uA typical. But that’s still too much for our purpose. So the PIC can power the whole thing off via a NPN transistor and a p-channel mosfet. That BUCK_ON signal also serves as the supply voltage for the INA213 current sensors already used in the last version. So the entire converter can be powered off and should consume precisely zero current when not in use.

What about the voltage divider necessary to measure the battery voltage? That’s been taken care of, too. That divider is interrupted by a (low threshold voltage) n-channel mosfet unless a signal from the PIC turns on the mosfet and closes the circuit.


Port expansion

I soon ran out of GPIO pins with that PIC so I had to add an I2C port expander (MCP23008) to gain another 8 I/O pins. I’ve also added a PCA9546 I2C switch in order to translate between different voltage levels.

It’s probably not the most elegant solution and a future version might trade these 3 chips for a higher pin-count PIC. But I had all components here already so that’s what I’ll use for now.

Precise measurements

In order to precisely measure input and output voltages and currents there is now a MCP3424 4-channel 18-bit ADC. To be sure, we don’t need 18 bits of precision here but we’ll trade some of that precision for speed and work with 14 or 16 bits.

The ADC has its on-board voltage reference and PGA with gains up to 8.  Since it only consumes a maximum of 1uA in standby mode it is always powered on.

There are also a total of 3  LMT86 temperature sensors, one on the board and 2 external ones. The external ones are intended to measure the temperature of the panel and battery, respectively. The maximum voltage for a battery is quite dependent on temperature so that’s a useful information to have. The inputs from the temperature sensors are measured directly by the PIC.  For that purpose there’s also a ADR361 2.5V voltage reference.

Measuring temperatures requires the board voltage to be high, 3.3 volts nominally. So both the voltage reference and the temperature sensors are only powered on when then VCC_HIGH signal is set.

User interface

To communicate with a human user there is a 4×20 LCD display that is controlled via I2C in order to save some I/O pins. There are quite a few external components needed to run it at 3.3 volts so it has its own PCB and connects via a 5-pole wire. There is 3.3 volts and ground for power, SCL and SCD for I2C communication and a PWM singal in order to control the display brightness. Because that’s a rather universal board I will document that as a separate post.

As an input device there’s a rotary encoder with push button. Together with the display (and some decent software) that should allow for a pleasant user experience.

The inputs from the encoder are debounced in hardware via a 74HC126 that serves a double purpose. With its 3-state outputs it allows us to use the pins that are otherwise used for in-circuit programming of the PIC.

Both the rotary encoder (except the push button) and the entire display unit can be powered off when not in use. The push button is always powered on so the user can wake up the user interface at any time by pressing the button. When the user interface is not actively powered on the 74HC126’s outputs are in high-impedance mode and the PIC can be programmed without being affected by the rotary encoder.


Data logging

One might be interested how much energy has been harvested by the solar charger over the last hours, days, weeks or months. So there’s some non-volatile storage as well. The 24FC256 connects via I2C and offers 256kBit of memory. It consumes only 100nA in standby so it can stay powered on at all times.

Fan control

As mentioned, there’s a temperature sensor on the board. If it gets too hot, a fan can be powered on via the FAN_ON signal controlling an n-channel mosfet.


There are 4 power outputs, each controlled by a separate signal from the PIC. There should be some PWM modules left on the PIC so some (two I think) can be PWM controlled. A typical application would be LED lighting which can be controlled and dimmed directly from the solar charger via the rotary encoder. Each output channel has a beefy  mosfet of the same type as the MPPT switcher (IPB136N08N3).

Each has its own FAN3111E mosfet driver. They have a separate input for the reference voltage so the PIC can easily control them when running at only 2.2 volts. The FAN3111 should only draw 5uA (10uA max) when idle but there are 4 of them so they can be powered off (all togeher, not individually) when not in use.


USB charging ports

One of the first thing one wants to do when there is power is to charge one’s cell phone or similar device. So there are two USB charging ports capable of delivering some 2 amps of current at 5 volts in total.

They are powered from the 12V batter via a TPS54231 buck converter. So there are a total of 3 buck converters on this board… It is always connected to the 12V rail but the PIC can turn it off when not in use. The regulator’s shutdown current consumption is only 1uA typical (4uA max) so that should be adequate.

Manufacturers of mobile devices have all come up with nasty ways of making their devices incompatible with other manufacturer’s chargers. They basically abuse the otherwise unused (for charging purposes) data lines to recognize their own chargers and discriminate against any others. That might mean they won’t charge at all or only at a slow rate.  So there are two charging ports on this board, one emulating an iPad charger and the other one a popular Samsung scheme. So pretty much any device should be able to charge on at least one of these ports at a reasonable speed.


As you can see, there’s a lot of functionality in this new design just waiting to be unlocked by some clever software.  I’ve already milled a board and I’m looking forward to populating it and bringing it to life. I guess that will be quite a bit of work but I think its well worth it. Thanks to all of you who have kept asking about this project – I very much appreciate all your feedback.

There are only Eagle files (including PDFs of the layout and schematics) at this point. They can be downloaded on the overview page. There’s now also a prototype. First test results are presented here.

Ultrasonic Anemometer Part 26: Rev B Board ordered

I recently ordered my first PCB at dirtypcbs.com and the result was promising. So there was nothing stopping me from finalizing the Rev B of my standalone Ultrasonic Anemometer and ordering a protopack. I’ve placed the order a few days ago and expect the boards to arrive here in 2 to 3 weeks. This should be good news for all those of you who have been asking for kits and want to contribute to the further developement of this project. I’ll build up one or two boards as soon as they get here and do some testing. If everything works as planned I can order some more components and ship some kits soon after that.


So today I’ll go through the changes I’ve made compared to the previous version. All in all the changes are quite minor and only require minimal changes in software. But let’s go through them one by one.

Non-volatile memory

This is the biggest change from a functional point of view. The PIC32MX250 doesn’t have any EEPROM memory of its own. So in order to be able to save some settings an the like I’ve added a Microchip 24AA16 I2C EEPROM providing 16kbit (2kB) of non-volatile memory. That should be more than enough to store any settings and calibrations one might want to make. For example, this gives the user the possibility to calibrate the filter kernels to the transducers used. Of course, the software first needs to make use of that memory but I think it’s great to have the possibility to durably  store a reasonably large amount of data.


Support for 5V I2C communication

I’ve hinted at this in a previous post. On the Rev A board the I2C signals were pulled up to the 3.3 volt rail. This was great as long as one didn’t want to interface to a 5V device such as an Arduino. The microcontroller pins are 5V compatible so you want to be able to pull those lines up to 5V whenever you interface to a 5V device. So I’ve added a diode to allow the SDA and SCL lines to be pulled higher than 3.3 volts. The I2C reference voltage of 3.3 volts minus a schottky diode drop or about 3.1 volts is accessible from the I2C header at the bottom. So just connect that to the external device’s 5V operating voltage and you have a fully compliant 5V I2C bus.


Physical layout and connectors

There was a rather large 8-pin connector on the last version to connect to the transducers. Now all the connectors along the edges of the board are standard 100-mil headers. This also allowed me to slightly shrink the physical size of the board to 60x70mm.

The pinout has also slightly changed. The board is now powered from a header on the right-upper side and all three voltage rails are now externally accessible from a newly added header on the right side. The pin order on the I2C and SPI headers on the bottom side of the board has changed, mainly to accomodate an exteral I2C reference voltage.


Power supply

The tiny (SOT23-5) 3.3 volt linear regulator on the last version worked well but got rather hot when providing close to 50mA from a 12V input voltage. I never had any issues with it at room temperature but decided to be cautious and upgrade to a LD1117 regulator in a much larger SOT223 package. This should be more than sufficient any reasonable ambient temperature..


Miscellaneous changes

I changed the digipot used to set the amplifier gain to a Microchip MCP4531. This model only has 128 steps but this is still more than sufficient for its task and it’s quite a bit cheaper than the 256-step version.

I also had to change the crystal because the model previously used became unavailable.


That’s it for now. I’ll let you know as soon as the boards get here.

Bi-Directional Voltage Level Translator – Board House Test


While most of my microcontroller designs run on 3.3 volts there is still the ocasional 5 volt design. Or I do something with an Arduino. So the need may arise to interface between logic working at different voltage levels. There are several ways of doing this, depending on your needs. Things are relatively simple as long as you know in advance which side is transmitting and which side is receiving. It gets more difficult if the communication is bi-directional or with busses such as I2C that are bi-directional by nature. I did a search on farnell.com and identified two chips that can translate between almost any two voltage levels bi-directionally. The Texas Instruments TXB0106 works with up to 6 CMOS (i.e. actively driven high or low) signals for protocols such as SPI. The PCA9306 (also from TI) is intended for protocols such as I2C that rely on pull-up resistors and where a line must never be actively driven high.


I ordered a few of these chips and laid out a simple board that contains the two chips together with the pull-up resistors and some decoupling caps. I also added two larger 10uF ceramic capacitors for a generous amount of bulk capacitance. The result is a tiny little board (45x30mm) that can universally be used to translate between almost any two logic voltages for almost any protocol.

Giving dirtypcbs.com a try

I can think of a lot of situations where it could be useful. But the main reason for this project was to gain some experience with getting a PCB professionally manufactured by a board house. For several years I have done my own designs but I always milled and drilled them myself. This had pros and cons. I never had to worry about silk screens or solder masks because my boards never had any. On the other hand I suffered from the lack of plated-through holes. Vias were always a pain in the arse because I had to manually solder in pieces of wire to connect the two sides. And it was very difficult if not impossible to put a via below a component which made the layout challenging when working with ICs with many and/or tightly spaced pins.


So this simple project seemed perfect to gain some experience in getting a design made by a board house. I chose dirtypcbs.com for their unbeatable prices and read their requirements and restrictions.


Here are some things to watch out for:

  • If you’re working with Eagle you can download a design rule file to automatically check if your design fulfills their requirements. This is great for checking things like minimum via size or trace width/spacing.
  • Make your own gerber files. Yes, you can just send dirtypcbs.com your Eagle files and they will take care of it. But I highly recommend you do it yourself so you know what you’re getting. It’s not difficult. Just download their .cam file and run it. I modified it somewhat so the file names reflect what the file contains. The file types themselves are rather cryptic.
  • View the gerbers online: There are several online gerber viewers available for free. I particularly like the online viewer at mayhewlabs.com. Just drag and drop the gerber files produced in Eagle and get a beautiful 3D view of your design. Check it carefully to be sure the gerbers represent what you are after. My workflow was rather iterative: I produced the gerbers, looked at them online, changed something in Eagle, produced some new gerbers… Again and again until I was happy.
  • Control what’s on your silk screen. A lot of the silk screen comes directly from the component libraries. Typically, the libraries print the outline, orientation, part name and part value on the silk screen. Unless your layout has a loooot of space between components the silk screen will soon be cluttered with all that information. I recommend you make your own libraries. You can then change the components  to only contain the information you want and how you want it. I ended up only putting the outline and orientation of the components on the silk screen.
  • Be aware that the design rule check will not look at your silk screen. You need to make sure your silk screen meets the minimum width of 0.15mm. If you’re using vector fonts (as you should) the math is simple. Just multiply the font size by the font ratio to calculate the line width. Example: A 70mils font size with a 10% ratio gives a line width of 7mils or 0.178mm.
  • Make sure your silk screen doesn’t overlap any of the pads. Again, he design rule check will not check for this. The board house might avoid printing the silk screen over an exposed pad but I wouldn’t bet on it.
  • Eagle seems to do quite a good job with the solder mask without any manual intervention but check the result carefully using the gerber viewer. Pay particular attention to the vias. You generally want your vias to be covered with solder stop – there’s no reason to have them exposed. But you might have some vias that you use as pads to access some signal for testing and debugging. Make sure they are not covered by the solder mask. An easy way to distinguish by the two types of vias is by drill size. You can set this parameter under DRC -> Mask -> Limit.


It took me quite some time until I had my files ready for manufacturing. Especially given the simple nature of this project. I then ordered a proto-pack from dirtypcbs.com. I went for a industry-standard 1.6mm (they default to 1.2mm) FR4 with a cool-looking (and non-standard) blue solder mask at no extra cost. Some 3 weeks later I found 10 of those nice little boards in my mailbox. They are quite precisely how I expected from the gerber viewer. When you look at them close enough you’ll notice that the solder mask and silk screen are not perfectly aligned. The silk screen could also be somewhat cleaner. But don’t get me wrong – I’m not complaining. The quality is absolutely ok, these are nice little boards. And at this price – I paid 14$ including everything – they are simply amazing. Thank you, dirtypcb.


So the board house test was a success but how about the rest? So far I’ve only populated a single board. And while I haven’t actually used it yet I have performed some tests with a signal generator and a scope.

PCA9306 for I2C

So let’s look at the PCA9306′ performance in some more detail.

I2C: 5V to 1.8V at 1MHz

The chip performs excellently when translating from a high voltage to a lower voltage. In the  example above it is translating from 5 volts to 1.8 volts at a very high 1MHz frequency and the wave forms still look nice.

I2C: 1.8V to 5.0V at 100kHz

Translating in the opposite direction (1.8 to 5 volts) everything is fine at 100kHz.

I2C: 1.8V to 5.0V at 400kHz

At 400kHz the output waveform starts to look very jagged, however.

I2C: 3.3V to 5V at 400kHz

But of course, translating from 1.8 to 5 volts is a rather extreme example. In the much more likely scenario of translating from 3.3 to 5 volts the chip performs reasonably well, even at 400kHz. And I2C doesn’t usually happen at frequencies higher than that. An Arduino, for example, will usually work at 100kHz. So I’m very happy with the performance so far.

TXB0106 for SPI and the like

Now let’s look at the TXB0106.

5V to 1.8V at 100kHz

Translating from 5V to 1.8V at a modest 100kHz is rather undemanding and the waveforms look nothing but perfect.

5V to 1.8V at 1MHz

Even up at 1MHz this translation works totally fine.

1.8V to 5V at 100kHz

Translating the opposite way, from 1.8V to 5V is more demanding but works very well at 100kHz.

1.8V to 5V at 1MHz

However, at 1Mhz the wave forms start to look pretty uggly. And with a protocol like SPI it is much more likely to actually operate at 1MHz. Note that the glitches might be an artefact of my primitive test setup.

3.3V to 5V at 1MHz

As with the other chip, the output signal starts to better when only translating from 3.3 volts to 5 volts. As mentioned above this is a much more likely application and the chip does a decent job even at 1Mhz.

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