555 burglar alarm circuit diagram

As shown in the figure, the "empty city meter" anti-theft alarm circuit. The so-called empty city meter means that when the indoors are unoccupied or on duty, the lights can be automatically flashed after dark, and when someone breaks into the door, a delay alarm can be issued. The alarm consists of a monostable delay circuit (IC1), a light-controlled multivibrator (IC2), a step-down rectification power supply, and a thyristor trigger circuit. The power supply of the delay circuit (IC1) is controlled by the thyristor SCR1, and the contact position of the platen contact switches K1, K1. controls the on and off of the SCR1. Under normal circumstances, the contacts of the platen switch mounted on the door and window will turn R1 and C1 on, and C1 will be charged with a certain voltage. When someone breaks in, the active contact of K1 and K1. The position, that is, the G pole of the thyristor SCR, is turned on, and the SCR1 is turned on, and the delay circuit IC is applied with a voltage. At the moment when SCR1 is just turned on, the voltage on C2 and C3 cannot be abrupt, and at this time, the trigger terminal 2 of 555 is at a high level, and the circuit does not flip. As the capacitors C2 and C3 are charged, the voltage at pin 2 gradually decreases. When it falls below 1/3 VDD, the 555 circuit flips and the pin 3 outputs a high level. The delay circuit is energized to flip for about 20 seconds. At this time, the diodes D1 and D2 are turned on, the buzzer sounds an alarm, and the multi-vibrator IC2 is triggered to start, and the thyristor SCR2 is intermittently triggered to be turned on and the light is strobed. After IC1 flips the output high level, capacitor C2 continues to charge for about 2 minutes. When the voltage on C3 is higher than 2/3VDD, IC1 resets, pin 3 outputs low level, corresponding D1 and D2 are cut off, and the audio and electric lights are off. Extinguished.

In addition, the multivibrator IC2 is also subject to illumination conditions. The resistor R7 is a photosensitive device, and has a low resistance value due to illumination during the day, so the partial pressure on the R8 is large. If the resistance of R7 is less than 500kΩ, the voltage of pin 6 is higher than 2/3VDD, IC2 is still in reset state, and the output is low level (3 pins). After dark, the resistance of the photo resistor increases, and the voltage divider on R8 decreases. When the voltage divider is less than 1/3 VDD, the IC22 pin level is lower than 1/3VDD. Its 3-pin output is high, which causes the thyristor SCR2 to trigger conduction and the bulb is on. When the photoresistor is exposed to light, the resistance value becomes smaller, so that the potential of the 2 pin is raised. When it is higher than 2/3 VDD, the 3 pin outputs a low level, so that the thyristor SCR2 is turned off, and the corresponding lamp is off. So round and round, the lights are flickering.