Door alarm circuit with a time of recognition NE555

Posted on Dec 11, 2010

Door alarm here with a time recognition, the owner opened the door is normally open the door, open the door to complete the action within the general 30s, so it does not alarm,

Door alarm circuit with a time of recognition NE555
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tried to open the door to strangers or thieves breaking open the door is not normal, usually a longer time, Once more than 30s, it will sound an alarm, it has great practical value.Circuit shown in Figure 1, mainly by the 555 IC alarm IC components.Timebase circuit IC1 and R1, C1 composition transient time T monoflop 4min around, usually IC1 at steady state, the pin output low, VT1 off, VT2 conduction, the capacitor C2 is shorted VT2 It can not be charged when the base circuit IC2 threshold terminal feet high, pin output low, the alarm sound integrated circuit IC3 does not work without power supply, speaker B silent.M is a touch pads, connected with a metal door. When someone unlocking Chung Ning apricot µ brain hard µ mother shouted shin Xiang and C1 trigger end feet, IC1 into the transient, pin output high, then the transistor conduction VT1, VT2 off, power can charging of C2 through R4, with the extension of the charging time, feet IC2 potential declining after about 30s, IC2 flip it feet high output, so IC3 electrical work alarm. If the open time in less than 30s, then the IC2 pin is low, the circuit will not alarm. Alarm sound duration T1 determined by IC1 transient time, T1 1.1R1 C1 4min, 4min after, feet of IC1 output low, VT1 off, VT2 conduction, IC3 power failure, the alarm sound stops. If you touch door over 30s again, but again the alarm circuit.Battery power is available (four batteries in series) power supply, but must not use capacitor step-down power supply circuit to avoid electric shock!

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