I have an old and simple car (Renault Express gasoline engine 1990). I bought it because it has almost no electronics and is simple to repair. I discovered one of the drawbacks with the simple technology which is that it will not warn me if I have left the headlights on when parking the car. I sometimes drive with headlights on as I live in a forested area where the light is often poor. I intend to get LED running lights but until then I need to detect:

  1. Are the headlights switched on
  2. The engine is not running.
  3. If 1 and 2 are true then I want to sound a warning. I would like to do this using passive components such as diodes and relays and possibly a Hall effect sensor but avoid using microprocessor. The car has 12volt electrics with negative earth. I had thought to power the sensor circuit from the courtesy light circuit which switches on interior light when the door is opened.

Question 1. Can I wrap a few turns of wire round the headlight supply line and use a Hall effect sensor? What kind of output will the sensor produce? Can I connect the output to a relay to sound the buzzer?

All suggestions gratefully received. Today I had to call the breakdown service for the fourth time to start the car. I know I can do better!

I have the usual boxes of various components just waiting to be put to good use.

Best wishes to any reader.

  • \$\begingroup\$ "If 1 and 2 are true then I want to sound a warning." Um, I don't think so! \$\endgroup\$
    – Transistor
    Commented Feb 21, 2021 at 16:07
  • \$\begingroup\$ Yes, only sound buzzer if engine is not running AND headlights are switched on. \$\endgroup\$ Commented Feb 21, 2021 at 16:10
  • 1
    \$\begingroup\$ Why not replace the headlight relay to a switched power line? Or add a second relay in a row from a switched source. \$\endgroup\$
    – Passerby
    Commented Feb 21, 2021 at 16:39

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. A possible solution that uses a simple buzzer getting its ground return through the various loads connected to the ignition circuit.

D1 prevents the buzzer sounding if the ignition is on and the lights are off. A buzzer with integrated electronics is unlikely to like reverse polarity anyway and the diode protects against that too.

  • \$\begingroup\$ Rapid and excellent reply thankyou. Now I will start to get to grips with the wiring. Reminds me of fitting a buzzer to my motorcycle to let me know when indicators on. \$\endgroup\$ Commented Feb 21, 2021 at 16:29

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