DPELECTRONICS DOOR GUARD Educational Electronic Hobby Kit

Thieves often target locked houses. This door guard circuit detects any vibration in its vicinity when somebody tries to break open the door. It is very simple to build and can be fitted anywhere in the vicinity of the door.The circuit sounds the siren when someone tries to intrude into your home or office by knocking, pushing and hitting the door. Mount the piezo sensor element on the door and it will sense the vibration produced by knocking. The voltage developed by vibrations in the piezo sensor is fed to transistors T1 and T2, which fires MOSFET BS170 and the supply is connected to IC1. The output of IC1 is fed to the Darlington pair built around transistors T4 and T5 for sounding the siren through the loudspeaker.At the heart of the circuit is the piezo ransducer (PZ1), which is the sensor element. If anyone attempts to open the door, the piezo element vibrates and the developed voltage drives transistors T1 and T2 into conduction. Now the alarm sounds for 20 seconds, which is taken care of by MOSFET T3 along with capacitor C1 and resistor R4 wired as a monostable timer.Arming (activating) the circuit is possible with the door key, if switch S1 is mounted such that its contacts close when the door is locked.IC UM3561 (IC1) generates a tone simulating police siren. Resistor R6 controls its oscillation. Sensitivity of the circuit depends on the piezo transducer element. Therefore it should be mounted such that it receives vibrations properly. This circuit does not drain the battery as the standby current is zero. Assemble the circuit on a small general-purpose PCB and house it in a suitable cabinet. Connect the piezo element to the circuit by using a single-core shielded wire. Fix the piezo sensor on the front door and your door guard is ready for use, provided switch S1 is âon.âThe circuit works off a 9V battery.You can also use a stabilised 9V DC mains adaptor in place of the battery.