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Opto Coupler for Keying

With an opto-coupler component (4N25) and the circuit in FIG.1, we have excellent isolation between transceiver and computer, because there is not any undesirable "ground loop" between TX & PC. The operation is absolutely safe, without any RF feedback, without problems. That is a good protection for both machines and provide a clean signal "on the air".

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copyright: SV1BSX (Jun'98)

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 .

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The circuit is flexible to any modification... for example, if you need in the output a higher voltage rate, it is possible to add another transistor in Darlington configuration (FIG.2), because the internal transistor of 4N25 is very "low power". In this case, the breakdown voltage and the output current is increasing and that depends on from the Q1 voltage.
Another solution is to add a small fast-type Relay (ie reed relay ) in the output of the circuit to drive any kind of old transceivers, because some of them have unusual and dangerous (high-voltage 60-300 V) circuits in the keying circuit. Put as Q1 (relay driver) a small NPN transistor (BC237, 2N3904) and connect the relay between Q1 collector and +Vc (+12V). Don't forget to connect a small diode (1N4148) accross relay- pins in reverse polarity (cathode in +Vcc)

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)


if the "keying" does not works in the transceiver, JUST reverse the two cables in the output of 4N25 (pins 4,5)....

that's all folks !

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Makis SV1BSX (Jun'98)