Friend of mine (Rich, N0PQU) owns IC251a. It is a nice reliable 144MHz all mode transceiver. Rich wants to use it as IF radio for his 10GHz transverter. Problem is that this radio doesn't have PTT output.
We have found nice article (by Scott N0EDV) about installing a sequencer inside this transceiver. However, his technical solution wasn't exactly what we needed, so we designed one of our own.
The IC251a has a RX-TX switch on its front panel. Our first idea was to use that. This switch has +9V on it in RX mode and 0V in TX .. apparently the 9V comes from a switching logic inside the radio and the RX-TX switch grounds that when transmitting.
We needed something with high impedance and we also needed the logic reversed (switch relay in TX mode), so we designed following circuit:
First half of the OP-AMP, the "O1a", is basically a "unity gain amplifier" with very high input impedance and the second half, the "O1b", is a voltage comparator. Should the input voltage on the PTT switch drop below ~8V (RX-TX is switched), then the comparator energizes its output, "LED1" comes ON and the optical insulator "OP1" switches its (galvanically isolated) output ON. Exactly what we needed.
Connection between "PTT" input and the RX-TX switch must be made with a 100k resistor in series, otherwise the "unity gain" amplifier doesn't copy the "0" voltage when switched to TX. This is because of the way how this OP-AMP is connected to the power supply - no negative voltage applied. In the real world, the "-" (inverting) input always has a little lower voltage than the "+" (non-inverting) input, so if you ground the "+" input by PTT directly, the "-" input has nowhere to go and "O1a" goes to output >10V.
We have taken several pictures of the connection points inside the radio to share the idea:
Everything works well on a test bench. We will connect it to the transverter soon and update this Posting with results of that.
This radio was successfully used for 10GHz contacts on 09/29/2012, details here.