Security alarm USING 555 timer and logic gates

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Joined Feb 2, 2010
Just for hobby purpose I designed this alarm circuit and tested it on a breadboard.
When the activation switch is open the circuit does not work, because the Q3 and Q4 transistors are disabled by the collector of the Q12.
By closing that switch the security alarm starts working with a delay depending on the C5 capacitor.
The EXOR inputs 1 and 2 of the CD4030 are controlled by the S1 sensor, through the Q1 and Q2 transistors.
Since the first one features a small delay (due to the C1 capacitor), when the sensor changes condition the EXOR output 3 gets high for a fraction of second, sufficient to trigger the IC1 timer through the Q3 transistor.
The siren is supplied by the IC3 timer with a delay depending on the C3 capacitor. The IC4 turns off the alarm sound and the circuit backs to the start condition.
By using the values indicated in the schematic (C3 and C4) the circuit should work with a time delay of 10-15 seconds. The circcuit in the video works with smaller capacitors.

This circuit may be used in combination with an electronic key that I designed some years ago. The K3 relay of that circuit activates if four buttons are pressed in the correct order and backs off by pressing any button.

555 timers (4)
CD4030 (1)
capacitors: 3300uF (2); 220uF; 22uF
NPN transistors BC547 (10)
PNP transistor BC327 (2)
resistors: 120K (4); 4,7K (2); 12K (10); 470K (1); 47K (2); 2,2K (3); 330 (1)


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Joined Jun 26, 2012
Which diodes?
D1 and D2 on 'security alarm' can be any small signal diode like 1n914 or 1n4148 but the lower Vf the better.
The ones across the relay coils K1-K3 on 'electronic-key' can be 1N4001 or better (1N4002, 1N4004 etc.)
Good luck and welcome to AAC!
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