Vehicle Detector II

[VARISTORS]

A coil can be layed only parallel to the road. ( An old road shoulder has
been cut away and is about to be replaced.) 14 ga solid Cu wrapped
around a 3/4" length of PVC pipe comes to mind. How many turns?
Length of PVC ? Suggestions? Help?[/b]

[philba]

I suggest you experiment. quickly. The commercial one I have is fairly compact - 30 cm long by maybe 5 cm diameter. I have no idea how many turns.

The traffic sensor loops, from the asphalt cuts you can see, appear to be around 1x2 meters. Probably not that many turns either.

Phil

[zotdoc]

I can't help you with the # turns etc but you better be sure you talk to the road crew and tell them what you are doing. No need to have any visits from the dept of homeland security!

[haklesup]

Its basically a big metal detector optimized to find large objects like cars as opposed to false triggering on bicycles and pedestrians with too much change in their pockets.

As such the number of turns which determines the sensitivity of the detector is dependent on the sensitivity of the electronics attached. The shape and position of the coil also plays a part.

Even if you perfectly imitate the shape and turns of a typical traffic sensor, you still need a detector circuit to plug it into. I think you need to design both parts of the project together.

Do a little research on metal detector theory of operation and then extrapolate to a larger less sensitive detection coil.

There are commercial units designed for driveways (like automatic gate openers) you can adapt or imitate.

A coil can be layed only parallel to the road

A coil can be placed anywhere adjascent to the metal you want to detect as long as the plane of the coil faces the detection area. One could construct a loop detector that hung above the road or was attached to a pole beside the road but these would suffer from sensitivity (too far from car) and enviornmental threats (vandals, bad weather). Inside the road bed it is well protected and positioned.

[Externet]

Hi.
Use wire as fine as you can find, or ~#30 and wind as many turns as you can or until you get bored (1000, 2000...) on an iron construction 1/2" bar (rebar) of 3 or 4 feet length .
Put this coil inside a capped PVC pipe for protection and bury it parallel to the road.
Feed the coil ends to the +/- inputs of a differential amplifier chip; the passing of an automobile will rail its output.
The sensed signal will be the disturbance of the earth magnetic field by the automobile metal (magnetometer)
Miguel