I have an interesting question, which I really don't know how to do it or it is even possible.

Problem: I want to start a computational unit (ex: raspberry pi) when the engine starts. If the engine goes down, it should close itself but a soft close from the software.

Need: I should detect the status of the engine.

Constraints: I can not attach anything to the engine or ignition key circuit. I only have the power line from the battery. The power line does not close even if the engine is not working.

Question: Can I detect the status of the engine from the power line? How?


  • \$\begingroup\$ How about adding a small microphone or looking for alternator pulses on the supply? \$\endgroup\$ – Andy aka Feb 18 at 13:10

If the "engine" is a typical automotive internal-combustion engine then you can look for an increase in the supply voltage as the alternator is charging the battery. Measure the battery voltage with the engine running, then with the engine stopped. Use a comparator to look for that change, and have the comparator signal your computational unit to shut down, or start up.

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  • \$\begingroup\$ Exactly. I'd say a cutoff point of around 13.5V (but determine by experiment). Also some hysteresis and a low-pass filter might be a good idea. \$\endgroup\$ – StarCat Feb 18 at 13:05
  • \$\begingroup\$ 13V should be fine - a fully charged 12V battery sits around 12.4 - 12.6 V, generator output should be around 13.5 - 14.4 V. A simple option would be the use of a 13 V Z diode, triggering a transistor. \$\endgroup\$ – towe Feb 18 at 13:11
  • \$\begingroup\$ I will consider your solution and if it really works, it will be very clean. \$\endgroup\$ – HVK Feb 18 at 13:17
  • \$\begingroup\$ It works Thanks!! :) \$\endgroup\$ – HVK Feb 21 at 8:04

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