At 08:59 PM 7/21/99 +0200, you wrote:
>hello,
>
>I'm working on a project to operate my driveway gate by using satellitedish
>motors. for the mechanical part it went ok, but for the electronical part i
>have some difficulties. I want to operate my gate by driving over an
>detection loop in my driveway, the kind they use by traffic lights. maybee
>one of you can give me a hint of how these detection loops are working.
>
>Please advise,
>Regards,
>Henk Smit.
>
>
The standard technique is to use a multi-turn loop buried in the roadway
and use the inductance as one component of a tuned circuit.  I vehicle
passing over it changes the inductance and hence the resonant frequency.
The problems are the standard ones: detecting a small change in frequency,
stability of the circuit under changes of temperature, voltage, etc.  I
have seen simple units advertised for a few hundred dollars, but it should
not be too hard to design one using a PIC to run things.  Typical frequencies
seem to be 40-100khz.  One could also put the inductor into a bridge circuit
and measure the actual inductance.  One of the drawbacks is the need to dig
up the drive to bury the sensor -- a real pain if the drive is already paved.

There are a number of other techniques in use also.  There are some magnetic
sensors that can be used to detect the presence of an automobile directly.

Optical methods are also used (beam across the road, etc) but are subject
to being tripped by humans, animals, rain.

Infrared detectors (for the heat of the engine) are also possible, but this
wouldn't detect a stopped car after it got cold.

Pressure sensors in the drive are often used in traffic counters, and this
is very simple, but not suitable for very high traffic areas (wears out, etc).

I have also seen an ultrasonic sensor employed for detecting vehicles, but
is also subject to certain false alarms, blah, blah, ...

I suppose there are probably another dozen on so physical phenomenon that
could also be used as well.  Whichever technique is used there are quite
a few engineering considerations if a solid reliable system is required.

Bob McClure