 Homemade magnetometer based on 555
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Conundrum
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Sun Mar 27 2011, 10:41PM
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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
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Hi all.
I have been busy duplicating Todd R Johnson's entry for the 555 contest,
This circuit uses a special Metglas strip taken from an anti-theft tag which has been wrapped in wire, normally about 2K turns.
As this proved difficult to do reliably I used a slightly different approach mentioned on an online forum,
and wrapped the Metglas into a tight spiral which was then installed into the coil from a reed relay.
These can be obtained from old answering machines, normally the microcassette type use them.
This module was then connected between pins 7 and 6 of a 555 timer with 10K between pins 7 and 8, 0.022uF ceramic as the timing capacitor and pins 2 and 6 linked which yielded a nice audio tone at around 8 kHz on the piezo.
I found that it could detect magnet axial rotation from up to 6" away with a small neodymium magnet which is pretty impressive.
I also noticed that rolling up a second strip and placing it inside the first increased the sensitivity by about 15%.
Interestingly the circuit still worked even down to 20 Hz so the inductance change seems to be quite large.
I would speculate that if the Metglas was wound into a spiral, and installed into a small drinking straw then wrapped in wire this would make a very compact lightweight assembly while somewhat increasing sensitivity.
Another interesting experiment would be to build a negative resistance oscillator with this setup, as it is simple to test.
This might boost the effective gain of the system as the inductance change would substantially affect the resonant frequency.
-A
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