The BITX40 PTT is a simple circuit: A switch grounds out the PTT Relay and the transceiver goes into transmit. As a stripped down SSB radio, this works fine and is simple and uses few parts. Why modify it?
The challenge is that the PTT circuit is not solid state. The PTT circuit is just a relay connected to a switch. Most digital interfaces use an opto-isolator which does not conduct enough current to energize the coils in the PTT relay.
BITX40 PTT: A Single Transistor Solution
I came across a solution to the BITX40 PTT relay issue while reading through Allard’s modifications where he’s got a simple circuit for controlling PTT with the Raduino for CW. His solution for CW is the same solution for controlling with an opto-isolator.
One of my goals was that I wouldn’t have to modify the BITX40 circuit board itself. I made a modular circuit that plugs in between the BITX40 PTT switch plug and the actual PTT switch. Here’s the schematic:
It’s embarrassingly simple, but solves the problem nicely. +5v is provided to the PTT switch. The PTT switch closes the circuit and provides +5v to the base on a NPN transistor when engaged. The emitter is connected to PTT+ and the collector to PTT-. Allards circuit uses +5v from the Raduino. I accidentally used a 47KΩ resistor rather than a 4.7KΩ and so I had to power it with +12v. You will be able to use 5v if you build it correctly, unlike me!
Building for simplicity
I built the circuit on prototyping board that I cut down to size after building, and then used some heat shrink tubing to tidy it up. A quick note about that prototyping board– it rocks! Couple it with some 24ga tinned bus wire and some Hakko wire cutters to cut it to the size you want and you can build a lot of things! I’ve used these exact parts to build the pluggable filters on the Multiband BITX40 conversion as well as several other small boards including this one.
As you can see, the PTT amplifier plugs in between the PTT from the microphone and the BITX40 PTT connector. 5v is provided by the the LM7805 that powers the QRP Labs VFO at the front of the radio.
When in operation with a standard microphone, there’s no difference in BITX40 PTT operation. The PTT circuit is now sensitive enough to be triggered by the opto-isolator in most digital interfaces. I’m using the Easy-Digi interface that’s sold on Ebay, and a USB to Serial adapter as outlined on this previous post. I’ve have confirmed that PTT now works as controlled by the PC. You’ll need to make sure that your digital interface does not share the PTT ground with audio ground, however.
I hope you find this useful for your BITX40 PTT woes!
Edit 7/12/2021: I finally got around to fixing this, and I’ve edited the article: Use a 4.7KΩ resistor and power it with 5v, it works great. I also realized that if the PTT device doesn’t work at first, reverse the PTT leads coming from the opto-isolator. The opto-isolator output is polarized, since it is a photo-transistor. You can read more about opto-isolators at the Wikipedia page on the subject: https://en.wikipedia.org/wiki/Opto-isolator