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Last Updated on: Wednesday, June 02, 2021 01:17 PM

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More  Indicator Circuits  Automotive Circuits

Automotive 12v Electrical System Voltage Indicator
designed by David Johnson, P.E.

When troubleshooting the 12v electrical system of an automobile, it is nice to have a simple voltage indicator tool instead of a voltmeter.  This electronic circuit, hobby circuit is powered by two AA or even two N cells, which will provide enough energy for years of service.  The circuit has only two leads.
One lead connects to the chassis ground of the car. The second lead can be sharp tip probe. The circuit will indicate if an automotive electrical circuit has zero volts or 12 volts present.  If the voltage is near +12v, a green LED will turn on.  If the voltage is near chassis ground a red LED will turn on.  If the circuit is open, neither light will light.  The circuit draws so little current in the open standby mode that no on/off switch is required.

Click on Circuit Below to view PDF of Schematic

Automotive 12v Electrical System Voltage Indicator

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