DIY Induction loop car detector

[dog80]

I want to build an induction loop system that will sound a buzzer whenever a car passes over the loop.

I've seen the documentation of such a system and apparenty the circuit resonates at a frequency of around 25-30 KHz Whenever a big enough chunk of metal passes over the loop it will cause the resonant frequency to increase a few KHz. The system monitors the frequency and whenever it detects a rise it assumes that a car has passed and will do the desired action.

I have absolutely no idea on how to build this.

What type of wire should I use for the loop and how many turns?

Any ideas on how to implement the frequency detector?

[kayvee]

Front end consists of a Colpitts oscillator where your roadway loop forms the inductive component.

A 2 X 1 m loop with 3 turns of wire is a good size.

Expect a lot less than a few KHz change with a typical vehicle passing, more like 50 to 500Hz change typically.

0.01 to 0.02% delta L/L is normally the maximum sensitivity of a loop detector.

[dfmischler]

Start here: Traffic Detector Handbook

In practice, real world detector loops are self-tuning (perhaps with a PLL).  This is necessary because temperature affects the inductance and capacitance of the components. So eventually, the presence detected will be tuned out.  Ideally, a very gradual change in loop frequency will just be accommodated without ever indicating a presence.

You might find that you can get a commercially built loop detector on ebay for a low price.

Other interesting links:

How Loop Detectors Work

http://www.dkaccess.com/English/Gate_Operators/Loop_Info-P-3-10.pdf?

[traxonja]

Hi,

I know this is an old thread, but here is a DIY version of Inductive Loop Vehicle Detector built around PIC microcontroller: http://www.elektronika.ba/843/inductive-loop-vehicle-detector-v2-1/
You can put a buzzer to relay output or even remove the relay and power your buzzer directly from a transistor.

Regards!