Metal detector circuit, low power

[Higgler]

Does anyone know of a low power metal detection device. We'd like to put this in a parking space mounted in a metal box on the ground, and when a car moves into the space, sense that a car is present. It has to be located outside in the rain and snow, sleet and oil (from cars leaking).

It's a battery powered detection mechanism, so low power is key.
One thought is RF transmit and receive, short burst every minute.

Any options or hints are appreciated.
Thanks,

kch

 

[VEBill]

Traffic sensing loops for traffic lights are pretty common around here. They cut a small groove in the pavement (at the stop light) and put in a loop of wire. The off-the-shelf electronics (basically a metal detector) would not be low power, but using duty-cycle and clever techniques it could be redesigned to be much lower power.

But more recently, the traffic folks have started to use cameras and vision systems to detect cars.

Another option might the old break the IR beam trick.

 

[itsmoked]

Burying something in ass-fault is not the thing to do anymore. Cities hate it. Those loops break so often that they actually put 5 or 10 in and then they have some to hunt thru every year after a break.

VE1BLL is right the latest is to use cameras. You will see them mounted on top of the signal light arms. They are crummy cameras who's only point is to note something the size of a motorcycle being there or not. Actually they notice a sudden change of view only.

You could actually do the same or you could use a high quality camera to actually see a large area and then use software. You snap a view empty then the SW compares the empty shot, parking spot, by parking spot, to see if there is a difference.

What kind of parking lot and why are you monitoring it? This can vary the methods too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

What's the time response requirement? That'll dictate a sizable chunk of the power requirements. If you only run the check once avery 5 minutes, that'll allow for a more practically powered sensor.

http://www.eistraffic.com/rtms_features.html

looks promising. Got that from

http://www.roadtraffic-technology.com/contractors/detection/

TTFN

FAQ731-376

 

[Higgler]

We talked about the inductive loop, that's a possible option, but we wouldn't bury it, probably put it atop our metal box. Camera and IR sensors can be blocked by mud and dirty ice.
The response time can be a few seconds up to 10 seconds possibly.

I was hoping for a low power chip measuring inductance or Capacitive, i.e. low frequency sensor that uses little power. Maybe just an RL or RC circuit with oscillator and detector at low enough frequency that a car atop the inductor can change the value enough to show a large metal presence. Silly? ?????

Sounds like maybe RF at 433
Mhz, with tx and rx antennas that are somewhat isolated is the way we are proceding. Possibly the 2.4 Ghz band, which we are already using.

any more thoughts?

kch

 

[analogkid2digitalman]

How about GMR devices as the sensor?
See pg6 of the pdf.

http://www.nve.com/Downloads/expo00.pdf

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"It's the questions that drive us"
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[VEBill]

Such high frequencies would normally imply small (even tiny)coils. The EM fields surrounding small coils are themselves small, and thus the practical range tends to be low. For more range, use much lower frequencies and larger coils.

I expect that traditional traffic-sensing loops (maybe 6 x 12 feet) would be in the xx kilohertz range (this is a guess).

If you wanted to use high frequencies, then you'd be looking at more of a radar (refelction) approach. And then IR reflections become easier. Think bathroom hands-free faucets, towel dispensers and hot air hand dryers. Not to mention auto-flushing urinals. Range of up to several feet should be easy.

 

[benta]

How about using PIR sensors like the ones you use for alarm systems?
Dirt cheap, wireless, battery supplied and they'd pick up the heat from the car enigine.

Benta.

 

[GonzaloEE]

Using ultrasound, you don't need to put your device at floor level or right under the car, so mud, oil and rain water aren't a problem anymore.
Any cheap microcontroller can measure echo delay times, waiting for a big difference lasting long enough, to assume there's a parked vehicle in front of it.