Connecting PC to Transceiver

Hi all,

I want to connect my computer to my radio transceiver (transmitter/receiver). This means connecting the line-out from the sound card to the mic port on the radio, and one of the lines in a serial cable to the PTT (push to talk). (Connecting the received audio to the PC is done separately.) I could buy a commercial solution, but a) that would cost £40+, which is downright daylight robbery for what it is, and b) that's boring. The aim is to connect it to the mic port (which appears to be an RJ25 connection). This is the actual mic (schematic).

Now, I'm pretty much a complete novice at electronics. I did some at school a few years ago, but that's about it, so please bear with me.

From what I can tell, this circuit should work to switch the transmitter on and off. Obviously it's completely useless for any practical purpose (transmit what?), but I'm going to do this first just so I can get to grips with things and see that it works. Does it look correct?

**broken link removed**

From here there are two things to do: 1) activate the PTT from a serial cable, and 2) connect the mic to the line-out on the PC. For the PTT this means checking the RTS line. If it's on, close the PTT circuit, otherwise leave it broken. I would guess the check would be done with a transistor, but would the two circuits need to be separate? I'm not really sure what to do here.

As for the mic, I don't even know where to start. In the schematic the mic is in the top left with a bunch of capacitors and other bits and pieces, the purposes for which I don't know, or if they're even relevant. I would need to connect a 3.5mm jack in place of the mic.

Any advice would be greatly appreciated.

Chris.

I homebrewed one. Look at this. Write back if you have questions.

Thanks Mike. I've had a look and I need to build this circuit for the serial->PTT connection:

**broken link removed**

I have a some of questions about this circuit:

1. What is the line that goes from the ground and loops around the RTS/PTT wires?
2. Looking at the datasheet for the opto-isolator it says the reverse input voltage is 6V. Is this is the breakdown voltage for the LED inside the isolator? If so, the input will be up to 25V when the RTS line is off as the current flows from the ground to the RTS (if I'm understanding correctly). Would I need to add another diode to prevent this current from reaching the isolator, or will the diode already in there that is pointing upwards take care of this?
3. The circuit in the actual microphone unit has a 15kΩ resistor in the PTT circuit. Would it be necessary to add a resistor to this circuit, or would the resistance from the isolator be sufficient?

Thanks.