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Alarms & Security Related Schematics and Tutorials

Wire Loop Alarm Circuit Diagram

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 for tube amp diagram


PartTotal Qty.DescriptionSubstitutions
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 


  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.
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