Both useful and fun, this metal detector is easy to build and practical to use.

BUILD TIME: 2 HOURS
DIFFICULTY RATING: BEGINNER

Metal detectors come in all shapes and sizes. Most of us are familiar with the walk-through variety at airports and large venues, or the hand-held variety used by security staff. Some designs of stud and pipe finders for walls use a metal detector. Until fairly recently, most manufactured food passed through a metal detector before packaging to check for contamination. Today, most factories use X-ray imagers fed to a computer image-recognition system, which also detects non-metallic objects.

Of course, the classic metal detector mental image is of the treasure hunter or prospector sweeping the beach, ground, or stream with headphones on, looking for lost rings, gold, or pirate treasure. Our metal detector isn’t going to find gold nuggets under the ground, as the circuit required is far beyond the complexity limit we have set for Kids’ Basics. Similarly, it won’t detect the car keys that got lost down the side of the lounge, unless they’re close to the top. It will, however, allow you to explore the world around you, or find coins if they’re only just buried in the sand. With practise, if your hearing is good enough, you may well even be able to find an earring retainer lost in the grass.

The Build:

The build begins by making the search coil, which itself begins by making a former. You could wind the coil around a cylindrical object, slide it off when done, and tape it together. Some internet designs do this, however, we don’t recommend it. We found that it was too hard to keep the coil compact and together, and too hard to remove it once complete. Instead, we’re going to use found objects to make our own coil former. We used the top of a mailing tube for ours, something many of us have around with the popularity of online shopping.

You could use anything non-metallic and larger than about 80mm. It needs to be less than 15mm thick, but many materials can be cut down with care.

While you’re preparing, we used a drawing compass in our prototype to draw our circles. However, not everyone has one, although most Primary school-aged people will. If you don’t, look at the images ahead to see how we made our own for the demonstration build, using a drawing pin, string, and a pencil.

Parts Required:	Jaycar
1 x Solderless Breadboard	PB8820
1 x Pack of Breadboard Wire Links	PB8850
2 x Pin to Pin Jumper Leads	WC6024
1 x 51kΩ Resistor *	RR0613
1 x 1kΩ Logarithmic Potentiometer	RP7604
1 x 100nF Capacitor #	RM7125
1 x 2.2μF Capacitor	RE6042
1 x 4.7μF Capacitor	RE6060
1 x 100μF Capacitor	RE6130
1 x 555 Timer IC	ZL3555
57m 0.5mm Enamelled Copper Wire	WW4016
1m Light Duty Speaker Wire	WB1702
1 x Small Speaker	AS3000
1 x 9V Battery Snap	PH9232
1 x 9V Battery	SB2423

* Quantity used, may be sold in packs.

# We used an MKT capacitor, but you can use any type with the correct value.

% Linear type. Will work but volume response will be different.

Step 1:

Draw a line on both your pieces of cardboard, down the middle of the sheets, and mark roughly the middle of each line.

Step 2:

Measure the diameter of your round object, and mark half this distance either side of the middle mark on the lines, so it adds up to the full diameter of your former on both pieces.

Step 3:

Use a compass to make a circle at least 30mm bigger than your former, with its centre on the same mark. Do this for both pieces of cardboard.

Step 4:

Cut out both circles carefully. Corrugated cardboard is harder to cut with scissors than poster card, so take care and have an adult supervise.

Step 5:

Use hot-melt glue to bond the former to one cardboard circle, then glue the other on top so that they line up.

Step 6:

Cut a slot in one side between the outer edge and the former, then push the enamelled copper wire into it so that around 10cm is hanging from the outside of the assembly.

Step 7:

Wind the enamelled copper wire from its spool onto the former, taking care to make the layers fairly even. Also note that the spool may want to unravel itself and make a tangled mess.

Step 8:

When all the wire is on the former, slide the free end into the slot where the beginning of the wire was placed. Trim the wire to be a similar length as the beginning.

Step 9:

Use sandpaper or a small, fine file to scrape the enamel from the ends of the wire. Note that it may be colourless. The kind of emery boards used on fingernails work well here, but ask before you take one from someone!

Step 10:

Double-check the ends of the copper wire are free of enamel. Bare the ends of the speaker wire, and twist them tightly to the ends of the enamelled wire, before taping over the joins.

Step 11:

Bare the other ends of the speaker wire, then cut a plug-to-plug jumper wire in half and join the halves to the speaker wires. Twist firmly and tape over the joins.

Step 12:

Place the breadboard in front of you with outer red (+) rail away from you and the outer blue (-) rail closest to you. Install the wire links to join the two red (+) rails and the two blue (-) rails.

Step 13:

Insert the 555 IC and the six wire links. Count the rows to get everything in the right place. The wire colours on these links indicate length, not function like ground or power. Also install the 51kΩ resistor (green-brown-orange-gold or green-red-black-red-brown).

Step 14:

Insert the four capacitors. The 4.7μF is between pin 2 of the 555 and the lower blue (-) rail. The 100μF goes in negative to pin 3, positive the right. The 2.2μF goes above, negative to pin 6, positive to the right. The 100nF is unpolarised and goes between pin 5 and the upper blue (-) rail.

Step 15:

Insert the two wire links which connect pin 3 to the top half of the board. Insert the plugs attached to the speaker wire, one at the junction of the wire link and resistor, the other to the end of the capacitor.

Step 16:

Insert the potentiometer into the board so that one pin lines up with the negative end 100μF capacitor. Insert a wire link to join the remaining two terminals.

Step 17:

Cut a plug-to-plug jumper wire in half, bare then ends, and twist through the terminals of the speaker. Tape the joins. You may have this made up from previous projects. Plug one end into the lower blue (-) rail, and the other into the joined end of the potentiometer at the wire link.

Step 18:

Insert the red wire from the battery snap into the upper red (+) rail and the black wire into the upper blue (-) rail. Connect the battery, and you should hear a sharp sound. You may need to adjust the potentiometer. If you don’t hear anything, check all connections, starting with the enamelled copper to speaker wire junctions.

TESTING AND USING YOUR DETECTOR

Your new metal detector is unlikely to find you the world’s biggest gold nugget (unless it’s close to the surface). However, there’s plenty it can do. Start by placing a few metal objects, such as coins, a drink can, and a nail, in known locations, then listen for the change in sound from the speaker as you move the search coil around the objects.

Is direction important? What kind of distance can you hear a change in sound from? Is the pitch different for different metals? One the workbench, we were even able to get a tiny difference in sound off a single resistor, albeit at 1cm distance. It took a while to learn the sound response well enough to tell that it was indeed picking up the resistor.

After you’ve familarised yourself with the detector, you can move on to unknown environments. Items lost in leaves or grass should be easily found. If you have plaster lined walls, you may be able to find the nails holding it on. This gives a line showing where the stud is. Modern building practice is to just use glue, however, so this may or may not work for your house.

You probably won’t find a buried treasure chest, but you probably will find rocks that are high in metal. Some road base materials have rocks high in iron, which will react with the search coil. Depending on where you live, there may be other natural metallic minerals in the area. Some metal oxides will react, others will not. Use your new detector to have fun and explore.

HOW IT WORKS

The part of this circuit that makes it work as a metal detector is the group of components formed by the 2.2μF and 4.7μF capacitors, the coil of wire, and the 51kΩ resistor. In particular, the 2.2μF capacitor and the inductor formed by the coil of wire will oscillate as current flows into the capacitor. As it charges, current flows in the coil, which builds a magnetic field. When the capacitor has charged, the current stops. The collapsing magnetic field in the coil now causes current to flow in the opposite direction, charging the capacitor, and the cycle continues. Of course, there is no such thing as perpetual motion in the real world, and likewise, the inductor-capacitor circuit, called an LC circuit, needs energy input into it to overcome internal resistance losses. L is the letter used to denote inductance, which is measured in Henries. C is of course for Capacitance.

When the circuit is first powered up, the voltage at pin 2 of the 555 is below ⅓ the supply voltage, and so the device triggers. With the output now high, current flows from pin 3. The 100μF capacitor charges, causing a current to flow in the speaker until the capacitor is charged. Current also flows to the junction of the inductor and 51kΩ resistor. Because of the polarity of the capacitor, current only flows through the resistor to charge the capacitor from its positive side. This starts the oscillation of the LC circuit. As the voltage of the oscillation reaches the ⅔ supply voltage point, the Threshold input of the 555, pin 6, is triggered. This turns the output off, allowing the LC circuit and the 4.7μF timing capacitor to discharge through the Trigger input, pin 2.

Once the voltage has fallen to ⅓ supply voltage again, the process repeats. It repeats at an audible frequency, which is why we get sound from the speaker. However, the key part is this: The value of inductance in the coil is not fixed. Air cored inductors, which ours is, are easily influenced by metal objects in the area of the coil. The biggest effect is to place an iron object right in the middle of the coil, but we can’t because of how we physically constructed ours. Any metal object near the coil will change its inductance, and therefore the frequency that the LC circuit oscillates at. The size of the object and the type of metal dictate the amount of change. The resistor we used in testing only produced such a small result that we had to learn the sound very carefully to spot the detection at all. Any movement of the jumper wire connections will result in a greater change.

WHERE TO FROM HERE?

You could mount your detector in a more comfortable housing. The circuit can go in a plastic or cardboard box with a hole for the speaker and volume dial, and the box mount to a wooden or plastic (not metal) broom handle (please use a spare or old one and ask first!). If you do have an old broom, you could cut off the bristles and use the angled plastic head to glue the search coil onto. You can increase the sensitivity of the detector by increasing the inductance, which means increasing the length of the wire. Remember to sand and twist firmly if you’re joining a new spool of enamelled copper wire to the existing one. Additionally, the size and shape of the coil also play a part. Does a flatter coil change the response, or a larger or smaller diameter coil?