door knock alarm witth timer.pdf door knock alarm witth timer.pdf
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Browsing Archive: November, 2011

Dark Activated Terrace Lamp

Posted by demysteryboy Inusah on Friday, November 4, 2011, In : security circuits 

Compact circuit, Can be wired in parallel to existing switches
This device allows one or more lamps to illuminate at sunset and turn off at dawn.Q1 and Q2 form a trigger device for the SCR, providing short pulses at 100Hz frequency. Pulse duration is set by R2 and C1.When the light hits R1, the photo resistor assumes a very low resistance value, almost shorting C1 and preventing circuit operation. When R1 is in the dark, its resistance value becomes very high thus enabling circuit operati...


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Simple Knock Alarm With Piezo Sensor

Posted by demysteryboy Inusah on Friday, November 4, 2011, In : security circuits 

This circuit uses a thin piezoelectric sensor to sense the vibrations generated by knocking on a surface; eg, a door or table. Basically, it amplifies and processes the signal from the sensor and sounds an alarm for a preset period. In operation, the piezoelectric sensor converts mechanical vibration into an electrical signal. This sensor can be attached to a door, a cash box, cupboard, etc using adhesive. A 1-1.5m long shielded cable can then be connected between the sensor plate and the...


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