I wanted a circuit to power on and off the front parking sensors of a car with a single button. After searching the internet for a bit I came across a soft latch circuit by eevblog and after changing the resistor values for 13V I re-created it on a breadboard which worked fine. This is the circuit I built.


simulate this circuit – Schematic created using CircuitLab

Push button switch (SW) was already in the car (one end connected to ground) so in order to use that I had to use RLY1 relay.

When connected to the car it functioned as normal but when the circuit was in off state (when sensor is off) and I turned on my turn signal or low beam, the circuit got auto-triggered and moved to the on state turning on the sensor automatically and it kept on until I press the SW to switch it off again.

I thought this was because of the voltage fluctuations when the lights were turning on so tried putting 0.1uF, 4.7uF, 100uF capacitors parallel to the power lines but this didn't solve the problem.

Can I know why this happens and if it is happening because of the sudden voltage drop, why does the circuit gets auto-triggered when a sudden voltage drop happens and what can I do to prevent this?

(Please note that my car has incandescent turn signal lights and HID low beam lights)

PS : I found another circuit which consist of CD4017 IC and it rarely (about 1/10 times) gets auto triggered and it only happens when turning on the HID low beams. I am also inserting the schematic of that here in case someone wants to have a look on it.


simulate this circuit

  • \$\begingroup\$ I don’t think your schematic is correct. R8 seems way too low. Yes, i’d suggest the problem is noise on the power supply. A capacitor across R5 might go some way to solve the issue. \$\endgroup\$
    – Kartman
    Commented Sep 29, 2021 at 7:40
  • \$\begingroup\$ Anything electromechanical, that is the switch and the relay, will cause the signal to dance around a lot regardless of how stable your voltage is. Likely the relay is too slow to keep up with signal bounces of the switch, but Q3 can keep up with the bouncing of the relay just fine. Both of these need filters, or use a MOSFET instead of a relay. You should actually measure these signals with your scope to see how much of an impact the various bouncing has on the circuit. \$\endgroup\$
    – Lundin
    Commented Sep 29, 2021 at 10:07
  • \$\begingroup\$ @Kartman sorry for that. I edited the schematic. \$\endgroup\$
    – Ravindu
    Commented Oct 3, 2021 at 3:27
  • \$\begingroup\$ @Lundin Thanks for the idea. I will have a look at it. \$\endgroup\$
    – Ravindu
    Commented Oct 3, 2021 at 3:29

1 Answer 1


According to my opinion, the 4017 solution is better. To avoid sensitivity to voltage drops use connection below. The 4017 will be supplied from 100u cap when voltage of battery suddenly falls.

Also consider button RC debouncing for reliable operation.

Also RCD startup reset for proper initialization.

enter image description here

  • \$\begingroup\$ Thanks for the idea. I think this will work for my purpose. \$\endgroup\$
    – Ravindu
    Commented Oct 3, 2021 at 3:31

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