RX/TX Sequencer

Much like the beacon keyer presented here earlier, this RX/TX sequencer is a simple but useful little device. Its typical use is in ham radio applications when a separate power amplifier (PA) and/or a sensitive low-noise pre-amplifier (LNA) is used. Care has then to be take to safely transition between RX and TX states – and that’s where this sequencer comes in.

Why a sequencer?

There are many sequencer designs on the web, both analog (http://www.ifwtech.co.uk/g3sek/dx-book/sequencer/) and digital (http://www.mancuso.net.au/sequencer.html) There’s also a beautifully made but somewhat more involved design here: http://www.w6pql.com/relay_sequencer.htm. The links above (and many more) explain the purpose of (and need for) such a sequencer in quite some detail so I won’t repeat it all here.

soldernerd.com on YouTube

Maybe the best way to understand what this sequencer does is to see it in action. I’ve just yesterday started a YouTube channel to share that kind of videos. Here’s the link: https://www.youtube.com/watch?v=Ctxm0VxeM4g.

This is already Rev D

I have designed and built a number of sequencers over the years so this is already revision D. Unfortunately, I don’t seem to have photos of the first two versions.

Above is a photo of the previous (Rev C) version. Its design was very similar but quite a bit larger as the side-by-side comparison shows.

Input and Output

The sequencer presented here is of the digital variety, uses a 12V power supply and is fully reverse-polarity protected. It is controlled via a PPT input which is pulled high (to +5V) with a 10k resistor and active when pulled to ground.

That PPT input is both debounced as well as reasonably well protected. You can short it to the 12V supply without any adverse effects. Actually, anything in the plus/minus 50 volts range is fine.

Its outputs consist of 3  relay outputs. I’ve used DR-12V types. Those are single-pole, double throw (SPDT) reed relays. They are not inexpensive but both reliable and fast as well as nearly bounce-free.  Each relay has an LED next to it indicating its state and all three relevant relay contacts are accessible via the orange 200mil header.

PIC Microcontroller

The actual behaviour of the relays is controlled by a PIC16F18325 whose programming pins are accessible via the 5-pin, 100mil ICSP header.  Its pinout matches that of the popular PICKit3 programmer distributed by Microchip.

Speed control

How much delay should there be between switching the individual relays? The answer obviously depends on your lineup and that’s why you can control the speed via the on-board pot. The way I have it programmed the delay can be varied from 70 to 325 milliseconds.

Undervoltage lockout

 There were still some pins left on the microcontroller and the PIC comes with a 10bit ADC and voltage reference module. So I decided enable the PIC to measure the input voltage of nominally 12 volts. As it is now programmed, the sequencer will only start operation after the input voltage has consistently been above 11.1 volts for 1.6 seconds.


As usual, all the files are on github: