12V alarm zone circuits

Making our zone circuits use 12V instead of 3.3V is as simple as changing the power supply, and in fact all of sensors we used so far can handle 12V power passed through their switches.

However, if we were to present the 12V circuit to the inputs on our GPIO port on the Raspberry Pi or our port expander, we would expect to see some magic smoke and smell something burning. So, we need to add some circuitry that allows us to use 12V alarm circuits as well as protect our control board inputs.

Alarm circuit protection

An effective way to protect our zone inputs from 12V alarm inputs is to use a little low-cost device called an opto-isolator. As the name suggests, this isolates the alarm circuit from the digital inputs of the control board using light.

Inside an opto-isolator (also called an opto-coupler) is an infrared LED, which transmits light to a photo-transistor when a current is passed through it, thus switching it on. The circuits are electrically isolated as they...