At 08:59 AM 8/28/97 +1000, you wrote:
>Thanks to all who replied on this. I managed to get a working one yesterday
>to play with. I made a detector coil (around 40cm diameter) from some 24/020
>cable - seems that 4 turns works well but 6 is more sensitive. When observed
>on the CRO, the coil voltage seems to drop by 50% near metal. The frequency
>also changed from around 85 KHz to 90 KHz. Different metals made if change in
>different direction. So... I still not sure whether it's a frequency or level
>*change* that triggers the unit. The micro (?) seems to drive the coil via a
>small coupling transformer - possibly to balance input for less interference.
>The waveform on coil is a nice clean sinewave. Maybe the micro is really a
>hybrid chip - just happens to use a 4.00MHz crystal ! (no xtal caps though)
>Two of the chips have 'Detector Systems' stamped on them - hard to say if they
>are 'house numbered' for DS or are actually custom chips. I tried a search on
>that name,etc but drew a blank. I'm not sure how to implement the HF side of
>things but maybe a PIC with A/D could process a F/V converter ?
>Regards...
>__________
>Dave Duffy
>
>Hi Dave:
I am reading your E-Mail message with great interest as I have buildt a "CAR
DETECTOR"
for my driveway. I buryed 4 turns of #14 Non-Metallic Sheathed Cable ( House
hold wire )
under my driveway actually I pounded a plastic pipe under the drive ... the
driveway is 19
feet across and I spaced the coils 10 feet apart under the drive. I ran 75
feet of wire to the
house. I made up a Osc. circuit that runs at about 60K Hz. Right now that
feeds a frequency
counter and the counter feeds a comparator. the Circuit changes about 600 Hz
when a Car
drives over it the time base for the counter circuit is 1 second. The only
problem I have is that
the frequency drifts over time and I can't figure out why it does not seem
to have anything to
do with temperature or time of day or moisture.