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.
@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.

@loopforever added:

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.

@Dampmaskin added:

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

@SDsolar added:

If you use a screw-terminal barrier block you can have both 12V and your lower voltages available at the same time