There are times when you would like to transmit a signal from one LED indicator light to second LED at another location.  The circuit below works well for this application.  It takes advantage of the fact that the internal infrared LED inside an opto-isolator has a lower voltage drop than the visible LED being tapped into.  Using a darlington type opto-isolator also means very little current needs to be diverted to the isolator.  The photodarlington side of the isolator can be used to turn on the remote LED using any convenient DC source.  In automotive applications, this is often 12v. You can also use the output of the opto-isolator to drive a low power beeper.  This might be handy for something like a “check engine” or a “windshield washer fluid” light.

LEDs     Indicator

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