Toggle to Momentary
Using opto-couplers
Circuit to convert toggle switch action to momentary switch action using relays

Many sim cockpit controls are activated by a key-stroke. This means that the keyboard switch is closed momentary, for about 0.1….0.2 seconds. If you connect a real toggle (tumbler) switch across the key-board switch, the constant ON of the toggle switch will result in a constant repeat of the key function due to the auto repeat feature of all PC keyboards.

To convert the toggle switch constant ON action into a momentary ON action you need some extra circuit in between the toggle switch and the key-board switch input.

The below circuit shows a simple way to achieve this. It makes use of a relay and an electrolytic capacitor.



In the switch position shown, the electrolytic capacitor is discharged. The relay is not active.

When the switch is flipped to the +12V position, the electrolytic capacitor is charged to +12V via the relay winding resistance. The pulse charge current through the capacitor and the relay will close the relay switch momentarily. When the switch is flipped back to ground, the capacitor will discharge, again activating the relay switch momentarily. Thus the toggle switch action can be transferred to the keyboard as if a key was pressed momentarily. A 330uF (micro-Farad) capacitor and 12V relay (260 Ohm resistance) will give about 0.2 seconds ON-time. The time is not really critical. You can also use a 5V relay and supply, but you may need to experiment with the capacitor value. (100uF ….470uF). Remember that electrolytic capacitors have polarity. The positive side should be connected to the switch side.
According another cockpit builder, reed relays can also be used: a 5V / 500 Ohm reed relay seems to work best with 5V supply and a 470uF capacitor.

Note that you don’t need the diode across the relay for blocking the inductive spike, as the current through the relay has a slow decaying current (via the capacitor) in both directions.

The above circuit is for cockpit functions that use the same key for activating / disabling, like Parking brake, Gear Up / Down, etc.

If you want to control a function that uses different keys for activate / disable with one toggle switch, you can use the below circuit where charge and discharge currents are directed to different relays via two diodes, see circuit below. (Used in FU-III engine on/off via car key-switch)


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