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CAR_PRESENCE_DETECTOR

2017-01-18 21:55  
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CAR_PRESENCE_DETECTOR
Here's a circuit that will, if calibrated carefully, detect the presence of a large metal object. Coil L1 is made by winding 50 turns of 26-gauge wire on a 50-inch form. If L1 is buried in the ground, the circuit can be used to detect any car or truck that is driven over the coil. Transistor Q1 and its surrounding components make up a Coipitts oscillator, operating at 10 kHz. The output is coupled to a Maxwell bridge through the secondary of T1, which is a 600-Ω-to-600-Ω audio coupling transformer. With R1, R2, and R3 set properly, the bridge is balanced when no metal is close to L1, producing a near-zero ac signal at the bridge's output (the R5/L1 junction and ground). Should a large enough ferromagnetic object pass over L1, the bridge becomes unbalanced, producing a signal at the junction of R5 and L1. That signal is fed to the base of Q2, part of a common-emitter amplifier stage. Resistor R4 adjusts to the gain of that stage. The signal from the amplifier is fed to the base of Q3, which drives a voltage doubler that supplies current to the relay driver, Q4. The relay, K1, is a 1000-Ω, sensitive, 12-V unit that can drive a normally open or closed alarm circuit to indicate the presence of a vehicle. Use shielded cable to attach L1 to the circuit, and do not bury L1 deeper than 8 inches for best results. Remember to weatherproof all components that will reside outside.