I have a dual throw momentary switch. In the closed position (see below) it shows constant continuity thru an LED which I would also like to use as a counter. Or really an indicator that the switch has been thrown. The other half of the switch is not really in question. I show an optocoupler here.

So once I throw the switch I want the LED to turn off. And stay off. Even if the switch is pressed again. This would serve as an indicator that the switch has been already pushed once. When power to the circuit is turned off the flip is reset.

I thought of using an SR Flip Flop, but don't think this will work since I need to see continuity once I power the circuit. I would welcome any ideas and sketches. Thanks!

LED Flip Circuit http://sanantoniowebagent.com/images/flipflopLED.jpg

  • \$\begingroup\$ when the Switch is thrown led "off" and when switch is off(Other side) led "ON" is this the condition? \$\endgroup\$ – Sanjeev Kumar Jul 21 '15 at 4:57

This circuit will retain the memory of the switch closing. Q1 and Q2 have positive feedback, so once you feed base current into Q1, Q2's base is pulled low and base current is supplied via Q2 until the power is interrupted and the current drops to near zero. Q1/Q2 have a forward voltage of less than 1V, which shunts the LED and prevents it from illuminating.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Got parts in today and tested the circuit, it worked perfectly. Thanks Spehro. Can this be configured to work with 12v? \$\endgroup\$ – Johnny Holguin Jul 28 '15 at 4:41

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