The practical phone burglarproof alarm circuit

Published:2011/7/5 8:09:00 Author:Borg | Keyword: burglarproof alarm | From:SeekIC

The circuit principleThe practical phone burglarproof alarm circuit is shown as above. When the external wire of the phone is normal, there is a 48~60v DC voltage in the line, which triggers the dual-way triggering diode DV3 conducting by VD1, so the triode VT2 is saturated and conducting. Both VT3 and VT4 are blocked without the bias voltage on the basic pole, the LED is not glowing and alarming. When the external wire is used by others, the voltage is dropping to 10V or so, the dual-way triggering transistor VD3 is broken down.



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