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Last Updated on: Thursday, November 16, 2017 06:32 AM

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More  Automotive CircuitsSwitches

Car Ignition 12v Switch

A Discover Circuits visitor needed a way to redirect +12v DC in his car to some auxiliary loads whenever the carís engine was running.  Many cars have cigarette lighter outlet jacks, which are either live all the time or become live when the carís engine is running.

The visitor wanted to plug a device into a full time live outlet but only switch on when the carís engine was running. The circuit below performs this function.  It relies on the fact that the carís battery voltage increases from about 12.5v in standby mode to 14 volts whenever the engine is running.  When the voltage is above 13.5v, the circuit switches on a p-channel MOSFET.  The circuit uses a common LM393 dual voltage comparator and a 10v 5% zener diode as a voltage reference.

Click on Drawing Below to view PDF version of Schematic

Car Ignition 12v Switch designed by David Johnson

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