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.

Schematic

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Parts:

Part
Total Qty.
Description
Substitutions
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

Notes:

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.