From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gordon Croft Subject: PTT problem Date: Sun, 07 Nov 2004 14:57:34 -0800 Message-ID: <418EA85E.6050706@telus.net> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: Sender: linux-hams-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: linux-ham 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?