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This request came from a
Discover Circuits visitor. He wanted to turn on a LED indicator light whenever
a circuit detected a burned out tail/brake light on a car or truck. The
circuit shown below uses a transistor switch, to detect current flowing through a
low value resistor. If current is flowing to a light bulb, the circuit shunts
current around a LED light, keeping the LED turned off. If there is no current
detected, the LED turns on. The circuit should operate from 6v to 24v.
The
shunt resistor needs to be selected, so the current flowing to one light bulb generates
at least 0.1 volts across the resistor. Since most turn signals illuminate two
light bulbs at one time, this circuit may not detect a single burned out bulb, until the
tail lights are turned on or until the car’s brakes are applied.
Click on Drawing Below to view PDF version of Schematic
