Vehicle Detector

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VARISTORS
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Vehicle Detector

Post by VARISTORS » Thu Sep 27, 2007 9:17 pm

Needs to detect moving vehicles at night at a distance of 200 feet. Outputs to a 555 Schmitt trigger. (Houskhold motion detector shy
of 200 feet. LDR/photodiodes worked until the town put in street
lights - the farmer put in a flood light and the snow arrived.)
Just the detector is needed. (Ultimately it turns on xmas lights on a
large tree - a little pleasure for the mill's night shift.)

rshayes
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Post by rshayes » Thu Sep 27, 2007 10:19 pm

You might be able to improve the range on a passive infrared motion detector by using a better optical system. Edmund Optics (not Edmund Scientifics) sells a few infrared fresnel lenses that are about 2 inches in diameter in various focal lengths. Cost is 25 to 30 dollars.

This is several times the aperture normally found on PIR motion detectors and might give enough improvement in range for your purposes.

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Joseph
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Post by Joseph » Fri Sep 28, 2007 1:39 am

I built a photo detector which was sensitive to changes in light since it was supposed to indicate when a customer broke a light beam. It had to be fine tuned so that the sun brightening and dimming because of clouds would not set it off. You wouldn't have that problem at night, but you'd still have false triggerings from lightning or, in a worse case, lights turning on or off. You'd just really want it to go off from moving headlights.

The key is to use a long tube with a detector set way back inside. Point the tube where the cars pass. Amplify the light pick-up, but AC couple it to the detector through an RC circuit so that a quick change in light is only capable of triggering it.

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jwax
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Post by jwax » Fri Sep 28, 2007 3:35 am

Any phototransistor could be used at the eyepiece of a cheap telescope to detect light/movement changes in its narrow field of view.
Would that work for you?

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philba
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Post by philba » Fri Sep 28, 2007 6:51 am

If the vehicle is confined to a specific path, you could place a wire loop in the ground and detect the metal mass of the vehicle. basically a metal detector. the advantage is that it is pretty much impervious to everything except a shovel. I have a commercial on in my driveway and it works quite well.

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haklesup
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Post by haklesup » Fri Sep 28, 2007 10:36 am

I'm thinking of several ways to use a 35mm camera or lens with a standard off the shelf PIR or his original photodiode.

As we know, a camera with appropriate lens will allow you to zero in on a particular spot while blocking out various sources of visual distraction and light pollution. A wide range or distance and area coverage can be easily obtained with a zoom lens. Combined with a tube like Joseph suggested and you can block out even more peripheral distractions.

As a first proof of concept, I might simply point the camera on a tripod and tape the photodiode in the viewfinder. Or I might try the PIR at the back of the camera positioned near the focal plane with the shutter open on B. I'm not sure if you would need an intermediate screen to focus on and for the PIR to look at or if you could just focus the camera on the PIR front lens area. Once proven, I would go about finding a way to eliminate the camera body.

rshayes
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Post by rshayes » Fri Sep 28, 2007 3:05 pm

A camera lens combined with a photocell would probably help the range of a photocell type of motion detector. It probably won't work with a PIR (Passive InfraRed) type of motion sensor.

Glass lenses transmit light with wavelengths shorter than the two micron range (depending of the type of glass). This includes the visible range and the peak sensitivity of a silicon photocell at about .9 micron. Wavelengths longer than 2 microns are highly attenuated.

PIR detectors detect the thermal radiation given off by objects due to the temperature being above absolute zero. This radiation is mostly in the 10 micron range with a small amount of radiation at wavelengths in the 5 micron range. This radiation will not pass through any reasonable thickness of glass but it will pass through thin sections of some plastics. The PIRs use a plastic fresnel lens. This can be made thin enough to transmit about 75 percent of the incident radiation in the 10 micron region.

Normally the lens on a PIR detector is about 1/2 inch square. These lenses are available up to 2 inches square, which would collect 16 times the radiation. The focal length might be longer, which would change the field of view.

Focusing might be a problem, since you can't see the image formed by the lens.

If you have a detector that works at 50 feet, the simplest solution might be 150 feet of wire.

VARISTORS
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Vehicle Detector

Post by VARISTORS » Fri Sep 28, 2007 9:53 pm

The city graciously cut a six foot swath across the road removing the
pavement to install a pipe perfectly positioned for a coil to be placed
-- what luck! A miniture spoting scope sitting on a dusty shelf plus having
contacted Edmund Optics will easily handle the other direction.
THANKS! gentlemen, Jack

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dacflyer
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Post by dacflyer » Sat Sep 29, 2007 5:41 pm

i can get my hands on some vehicle detectors that are used by traffic signal systems. if you need something that the weather won't bother

a few turns of copper buried in the roadway and wallla !

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dacflyer
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Post by dacflyer » Tue Oct 02, 2007 3:35 am

varsistors > i am posting a reply here since your e-mail does not work.

we just use THHN wire 14Ga. typically

a 4'X 4' loop 4 turns
or a 6' X 40' or 60' loop in a figure 8 (2-4-2)winding
the loops are very forgiving and are adjustable sensitivity from a bike to a truck, and the loops are also frequency adjustable
and also have a fault diagnostic indicator..as well as a few other features.

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