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I want my car's DRL to go off when my turn lights flash. I'm assuming I need a P-channel MOSFET, Q1 to control the positive voltage. Then I need resistor R2 as a pull-down so its normally on. Then if I introduce a smoothed-out 12v from the turn signal, that should turn Q1 off, but it only dims it a bit. What am I doing wrong?enter image description here

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    \$\begingroup\$ What's the voltage of the turn signal? What's the switching threshold of the PFET What's the voltage of the gate input after R1? \$\endgroup\$
    – Puffafish
    Apr 11, 2019 at 10:14
  • \$\begingroup\$ 12v on/off approx 0.5secs on, 0.5 secs off \$\endgroup\$
    – nags
    Apr 11, 2019 at 18:04
  • \$\begingroup\$ I have a circuit that might work. What is a DRL? \$\endgroup\$
    – EinarA
    Apr 12, 2019 at 7:12
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    \$\begingroup\$ DRL = daytime running light (LED) \$\endgroup\$
    – nags
    Apr 12, 2019 at 8:05
  • \$\begingroup\$ My circuit will need a FET with a threshold voltage of at least 3V, preferably more. What do you have. \$\endgroup\$
    – EinarA
    Apr 12, 2019 at 8:23

2 Answers 2

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In order to turn off the P-channel MOSFET, you need to raise its gate all the way to the 12V supply rail. Your filter circuit is only bringing it to some intermediate voltage. You need additional transistors in order to accomplish that.

Instead, I would recommend turning a 555 into a retriggerable monostable, which will give a nice "snap action" in terms of turning off and on the DRLs. Something like the following.

schematic

simulate this circuit – Schematic created using CircuitLab

Adjust the time constant (1.1 × R1 × C1) so that it's slightly longer than the "off" time of the turn signal. The DRL will turn off as soon as the turn signal comes on, and the DRL will come back on one time constant after the last turn signal flash.

The diodes and R4 help make the circuit rugged enough for the automotive environment.

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  • \$\begingroup\$ Thanks, but that's quite a high component count. Is there a simpler solution? \$\endgroup\$
    – nags
    Apr 11, 2019 at 18:07
  • \$\begingroup\$ Plus one for intelligent use of D output. \$\endgroup\$
    – EinarA
    Apr 12, 2019 at 7:15
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I have mentioned a circuit elsewhere, but I think all you need to do is reduce R1 to 10 ohms and increase R2 to 1 meg. D1 should be a 1N4001 type.

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  • \$\begingroup\$ I just tried, that. It works with my sample MOSFET, thank you. Now I'm wondering what gate threshold voltage I need to specify for the FET. Mine is quoted as being -1.75v. Am I right in saying that anything numerically greater than 0.5 would be OK? I say numerically because some P channel MOSFETs are quoted as having negative gate threshold, and some as positive. Is this just because different manufacturers use different methods, or is there a real difference? \$\endgroup\$
    – nags
    Apr 14, 2019 at 7:45
  • \$\begingroup\$ Thank you for responding. A .5Vt would be too low. You can't expect C1 to get within about .7V of V+ and it discharges at 1V a second when the blinker is off. So 1.5 to 2 is good. An enhancement mode FET technically has a negative Vt, but many Mfgs (and myself) treat it as a magnitude and don't include the sign. This minimal circuit turns on fairly slowly and this could over heat a FET in a tiny SMP. \$\endgroup\$
    – EinarA
    Apr 15, 2019 at 0:28
  • \$\begingroup\$ Many thanks for your reply. I've found that when it turns back on, its at half brightness for a fraction of a second first, so I'm going to try a FET with a 4v threshold to see if that helps. Strange about the positive/negative business, as I've seen both quoted by the same manufacturer for P Channel MOSFETs. The same seems to apply to Drain Current specs too. \$\endgroup\$
    – nags
    Apr 15, 2019 at 9:41
  • \$\begingroup\$ If you do that you can reduce R2 so C1 discharges faster. \$\endgroup\$
    – EinarA
    Apr 15, 2019 at 21:54

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