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