simple metal detector circuit

pashnoy

Hi guys, Im new at this so sorry if i get the format wrong.

Im trying to build a basic metal detector that will respond when a steel pendulum passes nearby. A simple response is all that is needed right now.

I thought of this:
Building a RL circuit with a copper coil air core. placing a m/meter over the resistor and reading that its (the resistors) voltage drop should lower as the inductance of the coil changes with the pendulum passing near to it. (and the flux having a low impedance to pass through )

this didnt work well enough as the change over the resistor drop was minimal!
(the source was set at 10Vp-p and a high frequency)

I then thought to make an RLC circuit in resonance and using the same logic only that the drop should change dramatically (what i need!) with a slight change in impedance.
this too did not yield a clear enough result.

any tips for improving this design or a new simple design all together?
thanx

waht

A pretty sensitive metal detector topology is having two oscillators at around 250KHz. One oscillator is a reference oscillator and it's shielded. The other one is an LC oscillator like the Colpitts with the inductor being the main pick up coil used to detect metal.

When a metal is brought close, the LC oscillator will slightly change frequency. Then subtract the reference oscillator frequency from the LC oscillator frequency using a mixer. The difference will be in audio range which can be heard as a pitch or tone.

schip666!

I (sorta) invented a little metal detector -- more likely I stole most of it from some unspecified online source -- for my robots using a 5" diameter 40 turn coil that calcs out to being about 50uH. The coil makes an oscillator osculate and that is feed into a 4046 phase lock loop which tracks the slight changes in frequency caused by nearby metal thingies and puts out a voltage that changes with frequency:

http://www.etantdonnes.com/ROBOCAR/d...oOther_sch.png

YMMV on sensitivity and such, but it's a place to start. I found that the 4046 is _extremely_ sensitive to supply voltage, so add a good regulator to the power -- the built-in Zener diode is just not up to the task when the supply is near the cutoff.

berkeman

Can you use optical detection instead?

pashnoy

I am restained to using a copper wire coil in the detection of the metal

jim hardy

if you're willing to use 1970-ish technology, try the circuit on page 68-69 of this old book (PDF page 70).
Back before microcomputers I built one and it worked as well as store bought detectors of the day.
http://bitsavers.trailing-edge.com/p...plications.pdf

the book is an excellent introduction to phase locked loops. I'd recommend you download and print a copy.

old jim