"Watchdog" anti-theft alarm circuit

How it works

The whole machine is powered by four No. 1 batteries connected in series, without the worry of power outages. With the power-saving design, the battery only needs to be replaced after more than two years. K1 is the main power switch and should be closed during normal operation. (1) On-duty state: When the door is closed, the door magnetic switch K2 is closed, the base of V1 is grounded, and V1, V2, V3, V4, and V5 are all cut off. At this time, there is no buzzer sound and no alarm call is made. The power consumption of the whole machine is only 3μA (VCC/R1).

(2) Alarm state: When the door is opened by the thief, the door magnetic switch K2 is disconnected, V1 changes from cutoff to conduction, V2 and V3 are saturated and conducted, the collector of V3 outputs the working power supply, the light-emitting tube (LED1) lights up, the buzzer is energized and emits a buzzing sound, and at the same time the power supply provides a large base current to V4 through C1, which is amplified to make the relay J1 energized and the normally open contact of J1 closed, which is equivalent to manually pressing the hands-free button (SP) of the telephone. After about 0.5 seconds, the charging current of C1 drops rapidly, the collector current of V4 is greatly reduced, and the relay J1 operates again and returns to the normal state (disconnecting the normally open contact). Since a large capacitor C3 is connected in parallel to the emitter junction of V5, the closure of relay J2 lags behind the closure of J1 by about 1 second, so that the hands-free button is pressed first (J1 is closure) and then the redial button (J2 is closure), and the owner's mobile phone number stored in the phone is dialed quickly (no more than 3 seconds). As the charging current of C2 decreases, J2 will return to normal (disconnect the normally open contact) after a few seconds. The resistors R5, R7, and R8 connected in parallel with capacitors C1, C2, and C3 are all discharge resistors, and D1 and D2 respectively protect transistors V4 and V5. When the whole machine is in alarm dialing, the power consumption is only tens of milliamperes.

(1) The selection and installation of the door magnetic switch in this circuit are key. Since the reed switch is easy to break, it should be soldered on a small piece of printed circuit board and two connecting wires should be led out, and then it should be pasted on the door frame with wide tape; the permanent magnet in the waste earplug is better to be used as the magnet, and it should be installed on the door. The relative position of the permanent magnet and the reed switch should be adjusted so that the door magnetic switch K2 is disconnected when the door is opened more than 10 cm, and K2 is closed when the door is closed. Finally, the permanent magnet is fixed with wide tape. (2) Since the battery needs to be replaced once every two years or more, the positive and negative poles of the battery can be tinned and connected with wires, which not only saves the battery holder, but also avoids the fault of poor contact between the battery and the reed. (3) This circuit uses few components. It is recommended to use a universal PCB board for wiring. As long as you are careful, it can generally be installed in one go; the component parameters are shown in the figure. (4) Purchase a telephone with hands-free and redial functions (specially used for alarm), connect the telephone line (in parallel with the existing landline at home), connect the hands-free button and redial button to the normally open contacts of J1 and J2 respectively, and then use the phone to call the owner's mobile phone once to store the owner's mobile phone number. (5) When leaving home, close the main power switch K1, first press the hands-free button of the alarm phone to make the hands-free indicator light up, then open the door (at this time, the hands-free indicator of the phone turns from light to off, and the buzzer sounds), and finally close the door (the buzzer stops sounding); after returning from outside and entering the house, remember to press the hands-free button of the alarm phone to turn off the hands-free indicator so as not to affect the normal use of the landline at home.