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Last Updated on: Wednesday, October 18, 2017 05:04 AM

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Bad Light Bulb Indicator  --  April 17, 2009 
 

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 carís brakes are applied.

Click on Drawing Below to view PDF version of Schematic

CircuitsBad Light Bulb Indicator

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