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I have a setup where I have a 12V source and a PIR sensor like the one pictured (below) and some LED strips that are rated at 12V that I've mounted under the stair railings. The problem I've hit is that the pir sensor only outputs 3.3V when triggered.

In extreme simplicity (and horror) here's the setup I have:

The simple diagram

The PIR Sensor I have outputs only 3.3V when triggered The sensor - Here's the details page about it.

The infrared sensor operates on a voltage range from 5-20v, but only outputs 3.3V when triggered. The LED strips I have are rated at 12V (though I have tested them to work as low as 7.6V).

By searching around I've gathered that I need to turn up the voltage somehow or get more voltage running through the sensor. I've tried various step-up modules (this is one of them), but most require at least 3.5V and above to work correctly. Is there a way to have the sensor output more than 3.3V when triggered or some other way to make this setup work?

PS: I'm not an electrician, so I appreciate layman terms. I do understand logic though.

PS2: I've read this question, but didn't quite get what the accepted answer suggested.

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  • \$\begingroup\$ Were you going to share the datasheet of the PIR sensor? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Dec 6, 2016 at 22:37
  • \$\begingroup\$ I updated the question with a link to the sensor. \$\endgroup\$
    – Coreus
    Dec 6, 2016 at 22:38
  • \$\begingroup\$ Use a buffer or inverter from that 3.3 V signal with pullup to your 12 V rail? \$\endgroup\$
    – winny
    Dec 6, 2016 at 22:40
  • \$\begingroup\$ Winny: Are you talking about these? en.wikipedia.org/wiki/Inverter_(logic_gate) - How would you suggest the diagram should look then? \$\endgroup\$
    – Coreus
    Dec 6, 2016 at 22:47
  • \$\begingroup\$ Transistors can be your friends. Use one as a switch such that the 3.3V signal from the PIR controls the 12V power to your LEDs. There must be 10's, if not 100's, of questions here on this topic. \$\endgroup\$
    – brhans
    Dec 6, 2016 at 22:51

1 Answer 1

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The PIR sensor output is a signal. It is not intended to drive LEDs directly. Fortunately we have such things as transistors that can switch LEDs on and off in response to a signal like what the PIR sensor puts out.

This simplest is to use a N channel MOSFET as a low side switch. At the low voltage of 12 V, these are available that can be turned on/off quite nicely from either 0 V or 3.3 V. The IRLML2502 is one example.

Drive the gate of the transistor directly from the 0-3.3 V digital output, the source to ground, and the LED string goes between the drain and the 12 V supply.

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  • \$\begingroup\$ Thank you for the clear answer! Is this transistor easily obtainable on the interwebs? Edit: Is it one of these: ebay.com/itm/… ? \$\endgroup\$
    – Coreus
    Dec 8, 2016 at 1:56
  • \$\begingroup\$ @Cor: Pretty much any N channel MOSFET that is rated for a bit more than 12 V g-s and is intended to be driven from 3.3 V logic will be good enough. The IRLML2502 is just one example. Use the part finder features on most distributor web sites to find others. \$\endgroup\$
    – Olin Lathrop
    Dec 8, 2016 at 11:47

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