From: Gordon Croft (terhi.victor@logonet.com)
Date: Mon Nov 08 2004 - 00:57:34 EET
Hello list,
I'm new to packet radio and am not as good at circuit design as I should
be!!:)
I've built a circuit to interface the serial port RTS signal to the
radio PTT line.
The radio is an old ICOM IC-20 and I checked to make sure it has a
positive voltage (+12V) on the PTT line . If I ground the PTT lead, it
keys the transmitter, so I think the radio is working correctly.
The circuit I built uses an opto Isolator (NTE3040).
I tried to send the circuit lay-out but the server rejected my message...
When I check the current flow on the PTT side of the opto isolator with
the computer trying to key the transmitter, I see about 14 mA and the
transmitter on the radio does not operate. If I just connect the PTT
line to ground through the ammeter, I see about 125 mA and the
transmitter does send. Is it possible that the opto isolator is
presenting too high a resistance and there's not enough current flow to
activate the transmitter? If so, how could I correct the problem?
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