Homemade empty city burglar alarm

RF cable can be customized for other specifications

The so-called empty city meter means that the indoors are unoccupied or on duty. After dark, the circuit can automatically make the lights flash intermittently, and when someone breaks into the door, a delay alarm can be issued.

In the above figure, the circuit consists of a monostable delay circuit (IC1), a light-controlled multivibrator (IC2), a step-down rectification power supply, and a thyristor trigger power supply. The power supply of IC1 is controlled by the thyristor SCR1, and the switching of the SCR1 is controlled by the contact position of the pressure plate type contact switch K1, K1". In normal times, the contacts of the platen switch mounted on the door and window will be R1. C1 is connected, C1 is charged with a certain voltage, and when someone breaks in, the active contact of K1, K1" hits the position shown, that is, it is connected with the G pole of SCR1, so that the thyristor is triggered to conduct. Then IC1 gets electricity. At the moment when SCR1 is just turned on, since the voltages on the capacitors C1 and C2 cannot be abruptly changed, the trigger terminal 2 of the 555 is at a high potential, and the 555 circuit does not flip. As C2 and C3 are charged separately, when the voltage of pin 2 is lower than 1/3Vdd, the 555 circuit is turned over and the 3 pin is at a high level. It takes about 20 seconds from power-on to flip. At this time, D1 and D2 are turned on, and the buzzer sounds an alarm. At the same time, the multivibrator IC2 starts to vibrate, and the thyristor SCR2 is intermittently triggered to be turned on, and the light is strobed. After IC1 flips the output high level, C2 continues charging for about 2 minutes. When the voltage on C3 is higher than the 2/3Vdd threshold level, the flip-flop resets and the output pin 3 goes low, D1. D2 cut off, the sound and the lights are off.

The IC2 multivibrator is also controlled by the lighting conditions. RG is a photoresistor device, and its resistance is small due to illumination during the day, so the partial pressure on R3 is large. As long as the resistance of RG is less than 500KΩ, the voltage of pin 6 is higher than 2/3Vdd, and the 555 circuit is still in reset state, that is, the output pin 3 is low. After dark, as the resistance of the photo resistor increases, when the voltage divider voltage is less than 1/3Vdd, that is, the level of the 2 pin of 555 is lower than 1/3Vdd, the output pin 3 becomes high level, and the SCR2 triggers the conduction. The light bulb is on. The photoresistor becomes smaller due to the light resistance value, so that the potential of the 2 pin is higher than 2/3 Vdd, the pin 3 of the output becomes low level again, the SCR2 is turned off, and the light is extinguished. So round and round, the lights are flickering. This is also the "empty city plan".