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Version:

June 19, 2010:
Revised: v2.0

ZL2PD Mini Metal Detector

This very simple metal detector uses a coil wound on a small chunk of ferrite rod. A cheap piezo speaker allows the changing tone in the presence of metal to be clearly heard.

Introduction

When I'm at home, Saturdays are often spent fixing things around the house, with the occasional bit of redecoration thrown in for good measure. A typical morning some time back found me facing a wall, my hammer in one hand and a large, and heavy, picture frame in the other. My mission - To hang a picture on the wall.

The wall was a good example of what some artists might call a blank canvas. It was a featureless and dead flat, painted with matt pale cream paint, and made from  plaster board fixed over a standard wooden frame. Most times, you can look at these walls at certain angles against the light and spot where the plaster board has been nailed down. Or you can tap at it with a screwdriver handle or a rubber hammer handle and hear the sound change as you tap on a hollow section or right on a wooden stud. If I could find one of those studs, I'd stand a better chance of nailing the picture hook in a spot which could reliably support the heavy picture frame. 

Well, I stared at that wall for a bit, first one way, then the other. I tapped at it here and there trying to listen for the sweet spot - the place to place the picture hanger. But that wall was pretty much perfect. Heavy plasterboard, well fastened to the frame. Somehow, I had to find those wooden studs. Maybe I could locate the galvanized nails fastening that wallboard using a metal detector!

This little mini metal detector was the solution I came up with in less than an hour.

Design

Three basic types of metal detectors exist - Beat frequency oscillator (BFO), field balance, and pulse induction. Other more esoteric types also exist, including capacitive field and 'radar' detectors, but those three are the ones most commonly build by enthusiasts.

This circuit is one of the BFO variety. A fixed oscillator is mixed with a second oscillator whose frequency varies when metal objects approach this oscillator's search coil. The mixing process results in a difference tone, or "beat" frequency, being heard. The unit is built around a single CMOS IC operated from a 9V battery. It draws about 8mA during use, but since that is limited to the odd 5 minutes three or four times a year, I've yet to replace the battery.

The fixed oscillator in this metal detector uses a cheap 455 kHz ceramic resonator recycled from an old TV remote control. This forms the feedback path around one of the four gates in the CMOS CD4001 chip which is configured as an inverter.

Note: Click on the diagram (shown on the left) with your right hand mouse button to see the circuit diagram at full scale

The variable oscillator uses a similar circuit, but the feedback path this time includes the search coil. It is made from 120 turns of fine 34 SWG enamelled copper wire wound directly onto a 15mm length of ferrite rod. The rod is about 10mm in diameter. I used a small piece cut from a broken broadcast (AM) radio ferrite rod.

To cut off a piece like this, score around the ferrite rod using a small triangular file. It needs to be more than a scratch - probably about 0.5mm deep will do the trick. Then, while holding the larger end of the ferrite rod in a vise, lightly tap down on the end of the rod with a small hammer. The rod will break (almost always) quite cleanly at the scored line.

The two oscillators are mixed in the third gate, and the resulting mixture of output frequencies are passed through a low pass RC filter (R1 and C9) to the last gate. This gate is configured as a high gain amplifier, and drives a small piezo speaker. These speakers are about 25mm in diameter, less than 0.5 mm thick. (The picture to the left shows two of these - One has a white plastic cover, the other is just a bare piezo element)

I find (recover) most of mine from musical greeting cards, although quite a few of mine also were used as ringers in old cordless phone handsets.    

The audio level from this circuit and the piezo speaker is more than adequately loud. You won't want to leave it on any longer than necessary!

Construction

I built my version of this on a scrap of PCB measuring 20mm wide by 50mm long, in "ugly bug" style with the chip soldered "pins-up" onto the PCB. The other parts were soldered into place using the chip and the PCB to hold things together. (This really was a "quick and cheap" project!) No parts are at all critical - The values shown are just those which I had right on hand at the time. All capacitors are disc ceramic types, although C9 and C10 can be mylar ('greencap') types.

The battery was temporarily mounted onto the back of the PCB with a nylon ziptie. I used a pushbutton for the on/off switch. It must be continually pressed to keep power on the circuit. That way, I can't accidentally leave it turned on.

The search coil was left to dangle by its leads from one end of the PCB scrap. I've never bothered to build it into any better or tidier fashion. I just don't use it often enough to be bothered, but for those making greater use of the device, it can be built into a small plastic box.

Operation

Once built, the variable frequency oscillator is adjusted using the 60pF trimmer capacitor. This is very sensitive, and a wide range of "tones" (I use this word with some care since it describes a set of audible sounds which musicians would not usually classify as tones) can be produced as this trimmer is adjusted. I found it easiest to adjust using a plastic screwdriver. Just adjust it for a reasonably loud audible tone. Moving a metal object (such as a small nail or the end of a metal screwdriver) near to the search coil will cause this tone to change wildly.

In service, the search coil is moved across the painted wall, about 10mm from the surface. The tone will abruptly change as the coil passes close to a hidden nail. The small diameter coil allows the location of the nail to be found with satisfactory precision.    




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