Building an Inductive Loop Vehicle Detector

[Trax] was asked by a friend to build a device that could detect the presence of a car in front of his garage gate for it to open automatically. After searching the web for such a project and trying many of them, he decided to build his own detector based on an induction loop. As you may have guessed, this kind of detector works by detecting an inductance change in a wire loop (aka coil) buried in the road. Having a car pass several inches on top of it produces such an effect.

[Trax]‘s write-up shows a very well thought and professional design. All the detector parameters can be adjusted using DIP switches and buttons: detection type (presence/pulse), signal filtering, main frequency and sensitivity. The wire loop is isolated from the main sensor electronics using a 1:1 isolation transformer and a Colpitts oscillator is used to drive the latter. Moreover, gas discharge tubes are also used for lightning protection.

The change in inductance translates to a change in resonant frequency which is later detected by the main microcontroller. The board is 24V AC powered and a diode bridge + LM2596 SMPS step-down converter are in charge of generating the required +5V in an efficient way.

As if this was not enough, [Trax] also made a PC-based tool that can change other platform settings using a serial connection. All the resources can be downloaded from his website and a few videos are embedded after the break.

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