Circuit diagram of disconnection alarm composed of 555

Principle analysis

The core part of the circuit is composed of NE555. R1, R2, C1 and NE555 form a multivibrator circuit with a frequency of about 3KHz. When the circuit is powered on, the oscillator starts to work and the buzzer LS1 sounds; when 1 and 2 When short-circuited, the operating conditions of the oscillator are destroyed and LS1 stops working.

circuit analysis

When the power is turned on, if the control switch is closed or the moving contact contacts the static contact, the alarm is in standby state;

When the control switch is open, the door opens and the alarm sounds; (on the contrary) the ideal working voltage of the alarm is 9V. When the working voltage is 9V, the standby current is about 3mA and the working current is about 8mA.

Instructions

As shown in the schematic diagram, fix the movable contact piece on the door, fix the static contact piece on the door frame above the door, connect the two movable contact pieces to 1 and 2, and place the control switch in a relatively hidden place. When the owner opens the door First close the switch to disable the alarm function and the alarm will not work; when leaving, turn off the switch to restore the alarm function.