The "empty city meter" anti-theft alarm circuit, as shown in the figure, is designed to automatically flash lights when the house is unoccupied or during duty hours, and to trigger a delayed alarm upon intrusion. The system consists of a monostable delay circuit (IC1), a light-controlled multivibrator (IC2), a step-down rectifier power supply, and a thyristor triggering circuit.
The power supply for the delay circuit (IC1) is controlled by the thyristor SCR1. The platen switches K1 and K1 are mounted on doors and windows, and their contacts normally keep R1 and C1 connected, allowing C1 to charge to a certain voltage. When someone breaks in, the contact position changes, turning on the gate (G) of SCR1, which activates the delay circuit IC1.
At the moment SCR1 turns on, the voltage across C2 and C3 cannot change abruptly, so the trigger pin 2 of the 555 timer remains high, and no flip occurs. As C2 and C3 charge, the voltage at pin 2 gradually decreases. Once it drops below 1/3 VDD, the 555 timer flips, and its output pin 3 goes high. This energizes the delay circuit for approximately 20 seconds. At this point, D1 and D2 turn on, activating the buzzer and triggering the multivibrator IC2. The thyristor SCR2 then triggers intermittently, causing the light to flash.
After about 2 minutes, capacitor C2 continues to charge. When the voltage on C3 exceeds 2/3 VDD, IC1 resets, and pin 3 returns to low. This turns off D1 and D2, silencing the alarm and turning off the light.
In addition, the multivibrator IC2 is influenced by ambient light. R7 is a photoresistor that has low resistance during the day, resulting in a higher voltage across R8. If R7’s resistance is less than 500kΩ, the voltage at pin 6 of IC2 exceeds 2/3 VDD, keeping IC2 in reset with a low output. After dark, R7's resistance increases, reducing the voltage across R8. When the voltage drops below 1/3 VDD, IC2's pin 2 becomes low, causing its output (pin 3) to go high, which triggers SCR2 and turns on the bulb. When light returns, R7's resistance decreases again, raising the voltage at pin 2 above 2/3 VDD, which resets IC2, turning off SCR2 and the lamp. This cycle repeats, creating a flashing effect.
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