12v Battery Backup (UPS)

I’ve recently purchased a wireless alarm system for my home that uses 9v batteries for the sensors. However while building the house, I’ve already installed the wiring for the wired alarm so I decided to centralize the power for the alarm, and power the sensors from there.

This way I will not need to replace the batteries every few months and the entire system can be powered for quite some time in an event that power is cut to the house.

The battery that I used is a 12v lead acid battery, that is specifically designed for such applications. It can be trickle charged to a specific voltage of around 13v without losing too much capacity over time. Mine is 7 AHrs so in theory it can power the system for more than 48 hours. Depending on your system you can choose to go up or down depending on your requirements.

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.

I’ve been running the circuit for over a few months now and it has run without issues. It is easily adjustable to work for a lot more voltages and you can add indicator LEDs or additional regulated power outputs if you choose so.

If you have any suggestions on how to improve the circuit then feel free to leave them down in the comments and if you liked this Instructable be sure to follow me.

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