When leaving a parking garage have you ever wondered how the gate knows how to close only after you have pulled out? The magic lies below the concrete, there is an inductive loop in the ground under the gate arm. It sensed the presence of your car and lets the gate close only once your car can no longer be detected. You can also see these loops used in modern intersections to control the traffic lights as efficiently as possible, for example when there are no cars in the turn lane there is no need to waste 30 seconds flashing the turn arrow while holding back the through traffic.

Thanks to our friend Muris for creating a DIY Inductive Loop Vehicle Present Sensor and providing all the details so you can build your own or modify the design for your own needs. It has a PIC16F877A to control the show and has a ton of adjustability so it can be used for almost any situation.

" Specifications

• Tuning: Automatic
• Detection type: Presence/Pulse
• Presence time: Adjustable in 4 steps (default: 11 min, 33 min, 55 min, infinity)
• Pulse duration: 100 ms / 500 ms
• Signal filtering: Adjustable in 4 steps (LOW, MED-LOW, MED-HI, HI)
• Coil/loop inductance: 20 uH - 1000 uH
• Frequency range: 40 kHz - 140 kHz
• Frequency adjustment: 4 steps (LOW, MED-LOW, MED-HI, HI)
• Sensitivity: 0.001\% - 0.5\% digital in 8 configurable steps
• Detection speed: <10 ms (with LOW filtering stage and loop frequency of 40 kHz)
• Startup time: ~8 sec
• Power supply: 7-40 V DC / 5-28 V AC
• Current consumption: ~0.035 A
• Protection: Galvanic isolation + gas discharge tube for lightning protection"