Wire Loop Alarm

This circuit is a simple wire loop alarm that can be used in doorways, hallways, or any other place the tripwire will be broken by intruders. The circuit has a built in siren, but it can be replaced by a relay to drive an external siren, commercial alarm, etc.

R1           1              100K 1/2W 1% Resistor 

R2, R4    2              10K 1/2W 1% Resistor   

R3           1              1 Meg 1/2W 1% Resistor              

C1, C3    2              0.1uF Ceramic Disc Capacitor      

C2           1              0.01uF Ceramic Disc Capacitor   

IC1          1              4001UBE Quad 2-i/p NOR Gate 

Q1          1              MPSA14 Low Power NPN Transistor       

SIREN    1              Micro piezo siren 12V DC 150mA, 110dB @ 1M  

LOOP     1              See "Notes"      

MISC     1              Board, Wire, Socket For IC1       

1. The loop can be any type of hookup wire, with a maximum resistance of about 90K. Using very thin wire (40AWG, for example) will make a very sensitive trip wire, but will shorten the distance it can be strung due to the high resistance.
2. The siren can be replaced with a relay to drive external loads.

ref: aaroncake.net/Circuits/alarm1.asp