The circuit is very simple and it only consists of few components. For the regulated output to the sensors we have the LM317 adjustable voltage regulator, we have two 1N4007 diodes to prevent any reverse current flow in the event of a power loss, a 1k Ohm resistor to limit the current output from and to the battery and 2 more resistors to set the correct voltage output to 9v.

In order to calculate the values for the resistors I used this handy calculator from Circuit Digest for which you can find the link below. You can play around with the values of R2 and R3 to find what works for you.

Additionally, there are 4 screw terminals on which all of the components attach to:

• J1 is for the input power source
• J2 is where the 12v battery is connected
• J3 is the 12v output for the central alarm unit and
• J4 is the 9v regulated output

Once I had the schematic ready. I’ve built it up on a perfboard, made sure to test everything on the bench and then proceeded to install it in the wall mounted box for the enclosure. There all of the sensor cables are converging so I connected everything up and made sure to isolate all of the connections as a safety measure. To power the entire system, I’m using a 12v LED power supply that’s been voltage adjusted to output 13.8v.