R1 100K 1/2W 1% Resistor
R2, R4 10K 1/2W 1% Resistor
R3 1 Meg 1/2W 1% Resistor
C1, C3 0.1uF Ceramic Disc Capacitor
C2 0.01uF Ceramic Disc Capacitor
IC1 4001UBE Quad 2-i/p NOR Gate
Q1 MPSA14 Low Power NPN Transistor
SIREN Micro piezo siren 12V DC 150mA, 110dB @ 1M
LOOP See "Notes"
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.
The siren can be replaced with a relay to drive external loads.