Re: [AFMUG] underground marker balls

Sat, 06 Oct 2018 11:44:53 -0700

That's what a lot of sensor loops use for traffic lights, automatic gates, ans so forth. Granted, the L part of the tank circuit is a rather large coil of wire (~~ 4' in diameter), but it's still an LC tank circuit. When a vehicle passes over the loop, the Q of the L changes; hence vehicle=true.


bp
<part15sbs{at}gmail{dot}com>
On 10/6/2018 11:31 AM, [email protected] wrote:
There are these round plastic balls that you can throw in a ditch and later come back and use a locator to find the ball.  Always wondered how they work so I bought one and took it apart.
It was partially full of some liquid,  I presume antifreeze.  Probably to keep it from floating.
 
Inside there was a hockey puck type of plastic enclosure. 
 
Inside that was a coil and capacitor.  Nothing else.  No RFID chip or transponder or anything else.  Just a simple LC resonant tank circuit.  A bit of a let down. 
 
Then I realized I probably did not need to go to all this effort:






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