Having completed construction of the alarm unit, it’s time for the test procedure. At this stage, microcontroller IC1 should be out of its socket and the SLA battery should be disconnected.

It’s also a good idea to disconnect the display board from the main control board until the power supply has been checked out.

Here’s the step-by-step test procedure:

Step 1: apply 16VAC power to the main controller board (but leave SLA battery disconnected).

Step 2: set your DMM to read DC volts and check the output from the 12V regulator (REG1). Anywhere from 11.5-12.5V is acceptable. Now, do the same for the 5V regulator (REG2) – its output should be very close to 5V.

If either of these readings is drastically low, switch off immediately and check for shorts on the PC board and for polarised components that may be the wrong way around.