Recently, I got my amateur radio license with the call sign OE1VQS and spent quiet some time at the “A1 Amateurfunk Club”. They have set-up a repeater for the digital mode called TETRA, which isn’t that widespread in the ham community in Austria. They thought about implementing EchoLink support on the repeater for quite some time. But after someone brought a Raspberry PI Model B+ to the club, we’ve decided to use it with svxlink as EchoLink gateway and let people which do not own a TETRA compatible radio join the conversation.
There was already a Motorola MTH800 which we could use.
So we took the headset of the Motorola device and broke open the talk-button assembly, which was by the way very good sealed and took quiet some force before it finally gave up. Inside of it was a small PCB which had nicely marked pads for the soldered-on cables. We carefully removed the microphone and the cables which led to the earpiece and connected the pads where the microphone was originally to the speaker output of the USB sound card. Also the speaker ports were connected to the microphone input of the sound card. As the levels do not match and would lead to distortion of the voice signal, we used voltage dividers.
As in the most radios the PTT pin has to be connected to ground to activate it. So I built a quick circuit around an NPN transistor which is connected to the GPIO1 pin of the CM108 sound card which pulls the pin to ground.
As the audio part was working great I wrote a little test program to test the PTT functionality. I used some code borrowed from the hamlib which already has support for the CM108 chip. From playing with the PTT circuit and my Baofeng UV5-R I noticed that sometimes the push-to-talk does get stuck. This most likely happens as the transmitting radio is not isolated from the circuit and there is some voltage induced on the transistor which is sufficient to keep the PTT active. I tried to fix this by using a ferrite from an USB cable, which seems to work as expected.
The repeater has the call sign OE1XTR and there are some monitoring graphs available at http://afu.wq.lc.