The isolation voltage of 4N25 opto coupler device is 2,5 KV (peak) and the collector to emmiter breakdown voltage (pin 4,5) has a maximum of 30 Volts. The isolation resistance is 10E11 Ohms !
The circuits has also an external Led diode (LD 1) in order to be able to monitor the "key" pulses. That is very useful during setup configuration between com-ports. If you have selected the right com-port, the LED will be lighting, following the CW code (if you press F1, F2, F3 ... "hot-keys" on CT or SDI).
Please, put as LD1 a normal size RED Led (abt 6mm diam.) Sometimes the smaller size Leds need a very low current to illuminate and that is a problem.... maybe a small Led will always light .
In practice, the basic circuit in FIG.1 is the "heart"... FIG.3 is the theoretical connection of "reverse" polarity. The circuit does not have any problem to drive any kind of transceiver without any change in the
software, because the input section of 4N25 with the Leds and com-port connection is always the same and that is another advantage, except the isolation, specially for the SDI program. Always we change the output section of circuit to apply to our transmitter.
If you have the right setup for com-ports and the LD 1 lights during the key-pulses, the "input section" is working fine... for the circuit in FIG.1 and for the most commercial transceivers, the next "steps" are:
1)Put a Jack-cable into TX "key" socket.
2)Connect the other side of the cable in the output of the "keying circuit" (pin 4,5 of 4N25)
that's all folks !
Makis SV1BSX (Jun'98)