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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 photo-darlington 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.
Click on Drawing Below to view PDF version of Schematic
