An opto-coupler is a device that can be used to electrically isolate two circuits, so that a voltage spike or other problem on one side will not destroy the circuit on the other side. A common use for them is when you want to interface a computer to an AC-powered device, such as a light or a motor. Usually, the opto-coupler will not be used to control the device directly, and instead will just transfer a signal from one circuit to another.
So, how does it work? You can think of an optocoupler as a combination of an LED and a phototransistor. To send a signal, the transmitting side power the internal LED, just as you would power a regular LED. This lights up and causes the phototransistor on the other to start conducting current. You can think of it as kind of a switch at this point, and use it to turn on a low-power device directly, or turn on a relay to turn on a higher-powered device. The above circuit diagram should work for an automatic keyer. Choose the resistor based on your microcontroller voltage and the current draw of the optocoupler chip that you use. One thing to remember is that the output is polarized, so you have to make sure to connect it up so that the high voltage side is on the collector, and the low voltage side on the emitter. Good luck with your keyer!
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