# Vehicle detector

Discussion in 'Electronic Basics' started by Roger Dewhurst, Sep 12, 2006.

1. ### Roger DewhurstGuest

Would a multi turn coil, a metre or two in diameter, buried beneath the
surfce of a driveway, with the ends of the coil connected to the input pins
of a operational amplifier serve as a reliable vehicle detector?

R

2. ### petrus bitbyterGuest

Maybe. These type of coil detectors are often used near traffic lights. They
are connected to an oscillator to produce a magnetic field. When a vehicle
approaches the coil, it absorbs some of that field which lowers the voltage
on the detector coil. These change of voltage is measured and interpreted.
Frequency, coil dimensions, available power are amongst the things you need
to find out to build a reliable system.

petrus bitbyter

3. ### John PopelishGuest

I seriously doubt it, if the opamp is connected as a simple gain block.

Most vehicle detectors based on coils operate on the principle of
exciting the coil with some frequency around 10kHz to 100kHz, (higher
frequency for a single or double turn, lower frequency for higher
number of turns) and react to either the change in inductance or
change in loss of the coil when a vehicle gets inside the AC magnetic
field. This takes a bit more than a gain block.

4. ### Roger DewhurstGuest

I was thinking that the vehicle would be magnetic enough to induce a current
in the coil and that the op-amp would amplify the voltage difference between
the two inputs.

R

This seems to be much like a metal detector.

R

5. ### Sjouke BurryGuest

Ah!! you never heard about a moving magnet(the car)
over a coil??
A bit of amplifier and a Schmitt trigger plus a oneshot
for a dead time window ought to show interesting
things.
Things might improve,if 1 or two amps dc flowed in the
coil,that way the car needs no remanent field of its own.
Of course there are about @#\$% other ways of doing it,
but dont underestimate rogers idea.

6. ### petrus bitbyterGuest

Hmm... The idea is not very new. Magnetic mines used the magnetic field (or
better disruption of the earthmagnetic field) caused by a ship. But a ship
is a very big thing compared to a car. Ships (iron ships) were often more or
less magnetic which could be decreased by degausing. Navy vessels still have
a degausing system installed. Most cars are lousy magnets. To induce
something measurable the car has to pass the coil with pretty high speed. I
guess you will not drive that fast in a driveway. (Which I suppose to be the
place it is meant to be used.) Pushing a DC-current through the coil will
not do any good. It's only wasting energy.

petrus bitbyter

7. ### Roger DewhurstGuest

I suppose if one makes the loop part of a frequency generator any change in
the inductance caused by the vehicle will change the frequency in the loop.
But that is a bit more fiddly.

R

8. ### jasenGuest

that'd detect the motion of the car, but not its presence,

the active loop ("metal-detector") way triggers on the presence of a vehicle
and is more sensitive. (a loop of steel packing strap or a bicycle wheel is
enough to trigger many)
you could sneak up on it and it'd not trigger.

Bye.
Jasen

9. ### Roger DewhurstGuest

It is only movement over the coil, onto it or off it, which are of interest.

R

10. ### Rich GriseGuest

No, unless the car is magnetized, then you'd get one long, slow, not very
strong, pulse.

But if you use the coil in the tuned circuit of an oscillator, and some
kind of frequency discriminator, then yes, certainly, although depending
on the sensitivity, it might not detect your bike.

Good Luck!
Rich

Exactly.

Cheers!
Rich