I'm trying to design automotive-oriented circuit to limit ignition pulse duration from OEM ECU to an aftermarket ECU plug-in device. The device expects +12v pulses no longer than 9ms, while the car in question produces ignition pulses in 10...20ms range. The pulses can always be 9ms, so there is no need for them to be proportional to the input. I'm in a kind of brain-freeze situation trying to choose between an edge-triggered monostable multivibrator circuit based on 555 timer and some MCU contraption like opto-decoupled AVR + BJTs as 12v outputs. I understand that MCU will most likely produce more accurate and less temperature-dependant timings whereas analog circuit can be a bit fiddly to fine-tune the output. Right now I'm more into MCU approach but it feels kinda cumbersome to make all these input-output circuitry. Is there any go-to approach to the task like this? Can I user something simpler like an RC differentiator circuit?

  • \$\begingroup\$ Reprogram the ECU? \$\endgroup\$
    – Jeroen3
    Sep 1, 2020 at 14:02
  • \$\begingroup\$ Whats wrong with an RC-diode CMOS 1shot? \$\endgroup\$ Sep 2, 2020 at 14:27

1 Answer 1


If you just need the pulse, you could do something like this. Add an inverter if needed.


simulate this circuit – Schematic created using CircuitLab

The output can be tuned by adjusting the RC values R1, R2, C1.

enter image description here


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