I built a sound card interface for my Icom 746Pro and my computer and it works fine.
Now, I want to add a PTT circuit using a 4N35 Opto-isolator.
Using my PSK31 program, I want to key the rig to transmit. I am using the computer's RS-232 socket, which has the RTS output on Pin #7 to the input of the opto-isolator and the output of the isolator to short the PTT circuit on the rig to gound.
I question if my simple circuit will work....without
damaging my rig. Why am I in doubt, you ask?
Well, there is a +7 volts on the PTT line on the rig.
There is a constant -11 volts coming out of pin #7 on the RS-232 plug, Pin #7.
When I select "send" or "tranmit" on my PSK31 program, the output on pin #7 goes to a +11.75 volts....which goes to the input of the 4N35.
That's quite a range in voltages and I don't understand why this is.
I don't know how an opto-isolator works, but it appears to me something is not right.
Question:: does the output of the opto-isolator short to ground when pin #7 goes to 11.75 volts positive?
What is the output voltage when the input is a minus 11 volts?? I wouldn't want a plus voltage there.
I thought this was going to be an easy project, but obviously, it's not to me...
For reference of what I'm building, here is the web site with the interface and PTT circuit:http://www.geocities.com/n2uhc/interface.html