Wire Loop Alarm

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Wire Loop Alarm

By Alex Meaden: 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

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. Email Alex Meaden with questions, comments, etc.

2. 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.

3. The siren can be replaced with a relay to drive external loads.

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