metal detector


Haven't been on in ages. I've got a lot of catching up to do!

I need a simple metal detector to interface with a Picaxe that can detect a metal ball (probably steel) as it approaches to within a couple inches. Suggestions? I would prefer if not operate on light or IR or sound but rather on its affect on a magnetic field. Thanks.


Senior Member
I've had a lot of success working with the "MicroMag3" magnetometer from It uses SPI, but it can definitely detect a metal object an inch or two away, especially if it's got any type of iron in it. I'm actually doing a project where I have 3-4 of those talking to an 18X, and then feeding data back to a computer (it's a magnetic anomaly detector).


Senior Member
Passing the ball in front of a cheap hall effect sensor with a permanent magnet behind it will give a detectable analogue voltage change.


A "Hall Effect Sensor" should detect distrubances in magnetics fields and output a proportional voltage for ADC detection by the Picaxe. I have little experience with these, but they look to be inexpensive and readily available. Perhaps someone who has interfaced one with a Picaxe can comment further.


Senior Member
HES can be remarkably sensitive- Andrew &quot;BrightSpark&quot; was involved in a project here in NZ, with a Picaxe setup that could detect a passing farm motorbike several meters away. Part of the beauty of the HES approach(I recall he used a<b> UGN3503 </b> with Picaxe READADC) relates to it's ability to detect just ferrous objects &amp; not similarly sized biological ones (cows in this case!) Stan.

Edited by - manuka on 23/04/2007 23:48:54
If you want to magnetise the ball slightly this should work...

On a course in Christchurch we made a super sensitive pickup loop for a basketball hoop. We set up a single supply 741 op amp with open loop gain as a comparator with one input pin mid supply biased with a pair of resistors. Connected the other IP pin via a few dozen turns of wire which makes the detector loop.

Set up the offset null of the op amp so it &quot;just&quot; rests in the low state with no activity. A short pulse should come out that could be caught or conditioned for PICAXE use with diode, C, R if needed. (Bias offset null to give a high if you want a normal high state) The offset null is the &quot;fine zero&quot; OP adjust of an OP amp and can be set to give fantastic comparator sensitivity.

We found that even a slightly magnetized metal object falling through the hoop induced enough volts for the &quot;hair trigger&quot; &quot;Op Amp Magnetometer Comparator&quot; to trigger. In our case it triggered a series of cascaded 555's for various sound, noise and lighting effects.

We found you could even trigger it by flipping the coil with respect to earths magnetic flux.

(Can just remember someone talking about a PICK-AXE ??? thing on the way back from the course... )


Well, here I am, back 2 months later. Thanks for the ideas. It will be a steel ball about an inch in diameter. I need to pick it up anywhere from about 10 to 0 cm. It will be moving no faster than 20 cm/s.