Driveway metal detector ?

Looking to build one to detect incoming traffic, like the traffic signals have. Any ideas to keep this simple?

Reply to
cessna
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A medium-to small yappy dog with a view of the driveway?

Reply to
Richard Crowley

You could try the same method that's used to control traffic signals, a detector loop embedded in your driveway. The electronics of these are simple, but installion of the loop in the driveway is a real bitch, plus could end up damaging your driveway!

Were it me, I'd simply purchase a simple weatherproof ultrasonic or r.f. based proximity detector switch and mount it next to the driveway. Check what's available to control outdoor lighting at Home Depot or Lowes.

Harry C.

Reply to
Harry Conover

Reply to
cessna

I'd embed a "hula hoop" (circular plastic tube, popular toy from the

1960s) with an access box and a plastic conduit to the edge of the driveway. Reason: All the loops I've seen have been re-installed more than once (not sure why). Either make it "re-enterable" or be prepared to have it quit operating at some point without any possibility of repair.

The professionals install these with a gigantic "hole saw" on the bottom of a truck. The diamond tips cut a large (2-3 foot) diameter circular slot into which several turns of wire are laid. Then the slot is sealed back up (with epoxy or such?)

The circuit is almost certainly just an oscillator that uses the large loop (inductor) as part of the frequency-determining LC tank. Same principal as any other metal detector (like the ones used for detecting nails/studs in your walls, etc.)

Reply to
Richard Crowley

The traffic loops I've seen in use consist of a relatively few turns, IIRC 4 to 8, usually in the shape of a rectangle (or more recently a diamond shape, presumably to enhance the detection of motorcycles). I've never measured one, but they're roughly 3x6 feet in size. (Try a Google search on 'traffic loops'.)

These urls may be helpful:

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One type detector consists of an LC oscillator with the loop functioning as the inductive component. This is combined with an active notch filter and a detector circuit driving a relay.

I've never built one of the loops, but I'd begin by driving 4 nails in a sheet of plywood to define the sized and shape of the loop. Then wind it with 8 turns of #16 wire going around the nail rectangle and test it with the detector electronics for resonant frequency and sensitivity.

Once it works to my safisfaction, I'd solder its leads to end of a length of #12 UF-B to connect with the detector, coating the entire assembly with multiple coats of RTV silicon rubber potting compound. To embed it into the concrete, you can support it it position with four small stakes before the pour.

I would caution you that both rebar and steel mesh (often used to reinforce concrete) could pose a problem for the loop, but I have no idea to what extent.

Harry C.

Reply to
Harry Conover

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