How to build a scalable thyristor-based anti-theft alarm

November 17, 2020

This is a simple anti-theft alarm circuit thyristor. Its functions include automatic entry and exit delays-Bell is regularly shut off and reset. It is designed to be used with normally closed input device types such as reed contacts-microswitches-aluminum foil tape-and PIRs.

The basic alarm has an "exit / enter" delayed single zone. This will be sufficient in many cases. But-bigger and better buildings are divided into districts. The modular design means you can add any number of system zones. These "instant" areas may trigger normally open and normally closed input devices.

schematic diagram

It is easy to use. When you turn on the alarm switch-you have about 30 seconds to leave the building. When you return and open the door-the buzzer will sound. You have turned off the alarm for about 30 seconds. If you do not do this-Siren will sound.

After about 10 minutes-the alarm will try to reset itself. If the trigger circuit has been restored-the attempt will be successful. But-if the loop is still open-the attempt will fail-and the alarm will restart. Of course-you can turn off the alarm at any time to turn off the alarm.

A traditional clock uses up to about 400mA. The general electronic alarm uses less. If you plan to redraw regardless of the current from the buzzer, the alarm terminal-the problematic thyristor will need to rush to the metal heat sink-and the relay contacts may need to be upgraded.

If you don't want the timing "cut-off and reset" function-gave D5, D6, R11, R12, Q3, Q4, C6 and relays.

Due to manufacturing tolerances-the exact length of any delay depends on the characteristics of the components in the actual circuit you are using. But-to some extent-by changing the value of R2 to R7 and R11, you can adjust the exit, entry and reset time to meet your requirements. Increased value increases time-and vice versa.

Stripboard layout