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I'm trying to connect an led light strip in my car. The light strip is powered via USB, however, just connecting it to the 12 socket with an adapter or a USB port would leave the lights on all the time. So, I was wondering if it was possible to splice the USB cable and put a mosfet in to be able to automatically turn the leds off. The source and drain be in between the 5 V USB power or ground and then connect the gate to a 12 V accessory wire that is only hot when the key is in the ignition, thus turning the LED strip on and off with the key being in the ignition.

Wondering if this is feasible or if anyone has any other suggestions for how to do this.

Thanks

Like so:

enter image description here

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    \$\begingroup\$ Would it not be easier to power the adapter from a source switched by the ignition switch accessory circuit? I know on my car that not all of the cigarette lighter outputs are permanently live, one is on the accessory circuit, but you could just tap into that circuit for the adapter. \$\endgroup\$
    – Phil G
    Mar 2, 2020 at 23:08

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Your original schematic isn't clear but I think you're trying to use the N-Channel MOSFET as a switch. Since I don't know the current required by your LED strip you should spec one out yourself. This video gives a great introduction to MOSFETs. Below is an explanation for each of the components used.

R1 - current limiting resistor to limit the gate current to reduce voltage oscillations

R2 - pull down resistor to avoid spurious activations by creating a defined voltage level at all times

R3 - load resistor that represents the position of your LED strip in the whole schematic.

schematic

simulate this circuit – Schematic created using CircuitLab

As others have mentioned, it may be easier to just power the LED strip from a source that's controlled by the ignition. If it's a 12V supply you'll have to use a voltage regulator like the LM7805 to reduce the voltage. It's only rated for about 1A though. If your strip needs more consider a buck converter.

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  • \$\begingroup\$ I was thinking about tapping into a remote wire for my amp as the 12 V signal. I do not believe that any of the cigarette lighters are on the switched accessory line. Do you think I could tap directly into the 28 AWG remote wire and it could supply enough current to power to all the LEDs? There is 60 LEDs in total, so assuming 10 mA per LED, around 600 mA total. I have read varying current rating for the wire, I believe the amp draws about 200 mA from the remote wire. Or could I use a relay to power it? I don't have a lot of experience with using relays though. \$\endgroup\$
    – Joel Mercier
    Mar 3, 2020 at 14:40
  • \$\begingroup\$ @JoelMercier Are your LEDs RGB LEDs or just pure white LEDs? \$\endgroup\$
    – JoeyB
    Mar 3, 2020 at 19:07
  • \$\begingroup\$ They are RGB LEDs. \$\endgroup\$
    – Joel Mercier
    Mar 3, 2020 at 19:38
  • \$\begingroup\$ @JoelMercier Please note each RGB LED is furthur broken down into three other LEDs namely red LED, blue LED and green LED and each red, blue and green requires about 20mA of power. Therefore if you light up the green and red you pull 40mA. If you want the colour white then all three namely the red, blue and green will be lit to produce white then this consumes 60mA. Therefore your worse case is each RGB led pulls 60mA NOT 10mA as in your comment. \$\endgroup\$
    – JoeyB
    Mar 3, 2020 at 19:55
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    \$\begingroup\$ @JoelMercier not a problem that's why this platform exists to help people. 3.6A is correct. You may need to tap power directly from you ignition power source. I dont know what vehicle you have so I can help much.A relay is a switch that isolates a high voltage i.e. AC from a low voltage i.e. DC. It can still be used for DC to DC applications. \$\endgroup\$
    – JoeyB
    Mar 3, 2020 at 20:16

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