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.
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