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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


Schematic for tube amp

Parts:


Part
Total Qty.
Description
Substitutions
R11100K 1/2W 1% Resistor
R2, R4210K 1/2W 1% Resistor
R311 Meg 1/2W 1% Resistor
C1, C320.1uF Ceramic Disc Capacitor
C210.01uF Ceramic Disc Capacitor
IC114001UBE Quad 2-i/p NOR Gate
Q11MPSA14 Low Power NPN Transistor
SIREN1Micro piezo siren 12V DC 150mA, 110dB @ 1M
LOOP1See "Notes"
MISC1Board, 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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