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I have a project where I have an existing 12v parking light and I want to also have the turn signal use the same light. I've constructed a circuit using 2x NMOS and 1x PMOS. I think it will work and will basically function as such:

  • When parking light is on and signal is off, the LED will light for the parking light
  • When parking light if off and signal is on, the LED will flash with the signal
  • When parking light is on and signal is on, the parking light will be on, and it will cut out (turn off) each time the turn signal 'flashes'.

Does this circuit look like it will work? The mosfet model numbers and ratings of the resistors, etc. are just dummy values as a proof of concept. Im pretty new to circuit involving MOSFETs so im hoping ive got this right.enter image description here

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  • \$\begingroup\$ This schematic is really difficult to follow. I suggest redrawing it in a cleaner manner. I'm pretty sure this won't work as intended though. \$\endgroup\$ – Hearth Oct 15 at 4:27
  • \$\begingroup\$ Are you sure you want the signal light to override the parking light when both are on? Typically you want the brake light to always have priority. \$\endgroup\$ – Ben Watson Oct 15 at 7:36
  • \$\begingroup\$ Yes, this is a light on a front mirror, no relation to the brake light \$\endgroup\$ – mikemaat Oct 15 at 14:13
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As the schematic is drawn:

  • When parking light is on and signal is off

Q2 will conduct through its body diode. Since there is no safe current limiting component, the weakest one of Q2, D1, and LED will blow.

  • When parking light if off and signal is on

Q3 will turn on. Since there is no safe current limiting component in that path, the weakest one of Q3, D2, and LED will blow.

  • When parking light is on and signal is on

A combination of the above will occur.

This is a bit tricky due to the third bullet item requirement. My suggestion would be to create a truth table. Could this be an XOR? If so, it could be handled by one gate, one FET, and one resistor.

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  • \$\begingroup\$ This could definitely be a XOR and probably accomplished using a CMOS 4030 (I think?) I'm doing it this way more as an exercise in learning. I'm missing a resistor for the LED for sure, but I think I also have a few issues with how the MOSFETs are wired up that I need to do some more reading on. I'll post back with a new circuit diagram when I think I have them sorted out. \$\endgroup\$ – mikemaat Oct 15 at 22:10

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