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alarm circuit issue


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i made a alarm circuit with a fototransistor and ir led , when the door opens it turns ON and a siren HIGH sound activates and the only way to turn it off is manually…. anyway it works great!
but when the power supply ( a 3.5 v battery ) is On for about 7 hours, it automatically turns the alarm ON! and that’s bad.

my theory is that  when time goes it spends  current, so the 3.5 v battery doesn’t have the same amount of
current like 7 h before, so the “sensor” is like more or less sensitive and triggers the alarm….

how can I solve that problem? do I have to use some sort of HIGH PASS filter?

please help.

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