user23840: 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.

user-1: @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

23840: <p>Given:</p>

<ul>
<li>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.</li>
<li>I have a Particle Photon (3.3V logic level) which will accept up to 5.6V on its VIN pin.</li>
<li>I have a 12V @ 2A switching power supply.</li>
</ul>

<p>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?</p>

<p>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.</p>
 -1: <p>@Majenko remarked:</p>

<p>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.</p>

<p>@loopforever added:</p>

<p>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.</p>

<p>@Dampmaskin added:</p>

<p>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</p>

<p>@SDsolar added:</p>

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