Art wrote:
> Hi Spehro,
>
> When I mentioned low mass, I was referring strictly to a compass like 
> device, that would indicate a slightly different direction as the 
> large steel vehicle interacted with the Earth's magnetic field. 
>
>   
Hall sensor? I've seen cheap electronic compass with LCD in local Lidl 
supermarket.

OR
two identical oscillators to avoid temperature issues. One oscillator 
uses coil under or beside road, the other a shielded coil. Use the NF 
radio freq (8.5MHz?). Mixer for difference.
Change in audio pitch as car goes past.

A range of  1m (coil in road) is easy enough. Traditionally this 
approach is used in Metal detectors using 455kHz (and an IFT for one 
oscillator coil).

Traffic light  sensors used to use tube/plate switches. they now seem to 
cut a slot around rectangle slightly smaller than a car with angle 
grinder, and I presume multiple turn loop of wire and then seal surface 
slot with tar or bitumen. I think.


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