[Q] How to calculate influence of metal object on dc biased coil?

500000 truns should be enough

You want to get the diameter of the coil up near scale of the events you're trying to measure. 500000 turns will be about 900mm diameter

you'll probably get better results from 200 turns on a 2m diameter frame.

like glass it's fragile :)

it'll be pretty useless for this application.

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VRS sensor technology is what he is looking for...

(Variable reluctance Sensor)..

Only good for moving objects of course. otherwise it should be done via ultrasonic or light..

Hmm, What are you going to measure? Change in inductance? Increase in current? How fast are the cars moving? Do you need to sense them when they are standing still? You might start by estimating the time the car will be there. How long does your signal last. Then it might help to make the L/R time constant of the coil 'match' the time signal you are looking for. I'd just wind a few and do some experiments. The magnetic fields in cities are really ugly. There is 60Hz AC stuff all over the place and the DC field jumps around as elevators move and trucks drive by. You'll want to shield your coil from eletrostatic stuff.

George H.

It won't work. A biggish loop sensor might make a tiny voltage blip, as the moving car distorts the earth's magnetic field. But DC excitation of your short dipole rod will produce negligable magnetic field three meters away.

Radar or sonar would be more sensible. Or a loop sensor in the roadway.

Why do you want to do this?

John

If I would like to attack that problem, I would use a bigger coil(~20 by 20 inch), air core. I would then put it in an oscillator, and measure changes in frequency. Any metal in the neighbourhood will change the frequency, just like commercial metal-detectors do. If your oscillator is at say 50Khz or higher, mix it with a stable 50khz or higher oscillator, and you get a nice zero-beat tone, easy to detect by ear. Your ferrite will make the local field of the coil smaller, and detection difficult. Dc measurement depends heavily on the residual magnetic field of the car, and will be very difficult to detect, and get mixed up with stray 60HZ fields, nearby radio transmitters, etc.

Sjouke Burry wrote in news:4cda02bc$0$8918$ snipped-for-privacy@textnews.kpn.nl:

I based my first thoughts on this article:

Then I started wondering how the author came to his coil design. It looks like a copy from a book.

I want to detect moving cars that pass at a distance of about 3 meters in front of my garden. I cannot use a loop detector in the road, because it is public property. I do not want to use optical detection because I want it to work in all weather conditions. Also I cannot install a reflector on the other side of the road so the cars should reflect the light, but they may not do that very well.

Jason

Utrasonic motion detection?

We have units at unit that mounts in a 1" Pipe hole that can sense up to ~75 Feet.. Much more than what you need.. They operate on low voltage and output a Analog signal.. You can get them also to do a logic output with adjustable settings.. Be prepared to use some of your hard earn money! Or Obama's free money!

Just an example.

So integrate a coil with your border decorations, It could be hidden behind a poster/billboard in the garden, etc. So it does not have to be small. The bigger it is, the greater its range. A simple way to make such a coil, is to use a 24 or 40 wire cable, and do some fancy soldering where the cable ends meet, to produce a 24(40) turn coil.

8918$ snipped-for-privacy@textnews.kpn.nl:

Hmm, not clear that who ever wrote that knew what they were doing. R3/ R4 look to be making a local 'ground' from the single power supply. Then I don't see that there will be any DC current flowing through the inductor. (Contrary to what the author claims.) What's the purpose of R1? And why is R5 tied back to the case ground and not the local 'ground' created by R3/R4?

I like Sjouke's idea. Look at how metal detectors work.

George H.

Weather shouldn't be that much of an issue. Even during the nastiest weather the atmosphere is still pretty transparent. Fog can be pretty dense, sure, but you should be able to shine through it at some wavelength. A camera and a little software could do the job (fog is an issue here). IIRC, someone here (or in SED) had a speed detector based on image recognition, a couple of years back.

...or you could give your neighbors a reflective driveway marker and bounce a beam off it.