# Powering 3.3V Arduino and 12V peripherals from single 12V power supply?

Given:

• I have some home alarm peripherals (motion detectors, glass break sensors) which operate at 12V and draw a nominal current of 20mA when in alarm state.
• I have a Particle Photon (3.3V logic level) which will accept up to 5.6V on its VIN pin.
• I have a 12V @ 2A switching power supply.

Can someone describe to me an efficient circuit which would allow me to power all of the peripherals @ 12V and the Arduino (Particle Photon) at 3.3ishV?

I have on hand an LM7805 linear regulator, which I believe will work, but will be inefficient. I also have on hand a DC-DC step down converter (4.5V-28V IN, 0.8V-20V OUT @ 2A) but I am a novice and don't know how to use it for the scenario described here.

• "I am a novice and don't know how to use it for the scenario described here." - Put 12V on the IN, connect the OUT to a volt meter, twiddle the knob till it reads 5V. Disconnect the volt meter. Connect it to the VIN of your photon. Jul 12, 2016 at 20:11
• You could use the arduino's built in regulator, no? Jul 12, 2016 at 20:34
• How inefficient will the 7805 be, in the big picture? Depends on the current draw. How much current will be drawn? Jul 12, 2016 at 21:23
• @Dampmaskin For the 3.3V Arduino (Photon) current draw will not exceed 500mA. For the 4x 12V peripherals, if all were in an "alarm" state simultaneously, current draw will not exceed 110mA. Jul 12, 2016 at 21:51
• Ok, dropping 8.7V at 500mA wastes 4.35W so OK you're right, that is inefficient. A buck converter is probably the best approach then. Jul 13, 2016 at 10:04

@Majenko remarked:

Put 12V on the IN, connect the OUT to a volt meter, twiddle the knob till it reads 5V. Disconnect the volt meter. Connect it to the VIN of your photon.