In article ,
TheOldFellow writes:
On 25 May 2009 09:41:26 GMT
(Andrew Gabriel) wrote:

In article ,
jim writes:

inductive loops eh... could you describe how they work please? - are

A sort of up-side down metal detector, i.e. the metal detector
is buried in the ground and detects metal passing overhead.
Google is you want to know more.

they DIY-able?

Yes, but possibly not legally. I don't know what the radio licence
requirements are, but whatever, you're unlikely to have the tools
necessary to prove they meet the radio licence requirements (as
they are radio transmitters).


Not really, they are inductors. The inductance of a loop changes when
a piece of iron (and some other) materials pass close by. If the
inductor (the coil buried in the road) is in a tuned circuit then some
characteristic of that circuit can be used to detect when the
inductance changes. There are no radio emissions, it's just
electromagnetism.

Um, that's an aerial.
Providing you can show your device lays within the power ratings of
table 3.12 of
http://www1.bsc.org.uk/radiocomms/if...R2030final.pdf
then it's exempt from licensing.

See, for instance:
http://www3.telus.net/chemelec/Proje...p-Detector.htm

They used to be sensitive enough to detect a bicycle, but with all the
Carbon-Fibre these days I suspect not.

R.
(who used to work for GEC-Elliott Traffic Automation, back before the
flood)

Snap! It was called GEC Traffic Automation when I was subcontracted
to them from GEC Computers. Many years later, I was working back in
the same building, and GEC Computers and GEC Traffic Automation had
a bowling league each Monday evening, until the Traffic guys suddenly
won the Pools, stuck up 2 fingers to their management, and left!

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]