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Forget door alarm circuit

2016-02-18 18:42  
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This circuit uses to warn when forget shut the door. By use switch stick at the door, if do not shut the door switch will off as a result will have a voice warn from a loudspeaker hear immediately.

The work of the circuit , when stick switch S1, at the door which in usualtime switchstill pressed the circuit will build the frequency upwards with Q1,Q2 C1. When the circuit will work to have current flow through R1 reach C1. By C1 do something charge enough then have current flow get along well with B of Q1. It make Q1 work cause Q2 lead current flow reach a loudspeaker.

At the same time C1 will discharge change R1 , when discharge be finished. It make voltage at a pin B of Q1 be valuable a little make Q1 and Q2 stop work. And have current flow through R1 reach C1 again. The work in this character will cause the loud sound goes up. But when shut the door make switch S1 pressed. Q1 can not can work. Because voltage at a pin B be 0V make the circuit doesn’t work and have no the sound comes out from a loudspeaker.

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