Daan said:
Hello Everyone,
We're students from Nijmegen, The Netherlands. In this final period we have to make a episode of "Klokhuis", a program that explains certain subjects to kids aged 10-13. As a subject we chose metal detectors. About half an hour ago we decided we'd try and make our own metal detector, but we failed, big time!
We used two coils, a big one and a small one, and put the small one in the big one. We hooked the big one onto a alternating current of 12V and the small one to a voltmeter with a capacity of 100 mV. We created the coils ourselves by spinning a (plastic coated) wire around cardboard toilet rolls. When we turned it on and put some metal near it, nothing happened, it only got really hot.
So, our question is: How does one make a simple metal detector, by just using coils?
We're using this simple metal detector to test out which metals can be detected.
Thanks in advance,
Daan, Alex, Hidde and Harm.
This is the pulse induction forum, and throwing something together to demonstrate pulse induction I don't think is possible. So in order to provide a useful reply, I'll have to reply "off-topic".
In order to demonstrate detecting coins, the frequency cannot be the 50 Hz you get from plugging into the wall, it will have to be in the several kHz range. 5 to 10 kHz works well for most coins. This can be generated by an ordinary inexpensive lab signal generator which you can probably borrow from an electronic technician or engineer friend. You will also need an ordinary inexpensive analog oscilloscope, and a large piece of electrical ferrite. An electronic tech will usually have both these items as well.
The signal generator will probably not put out enough power. So use it to drive a small audio amplifier. Wire the output of the audio amplifier to an ordinary 8 ohm speaker and adjust the volume (gain) for a voltage of about 5 volts RMS (14 volts P-P if using the 'scope to measure it).
All the experiments which follow must be done on a nonmetallic table or bench; otherwise the metal in the table/bench will keep it from working.
Wind a transmitter coil of about 50 turns of wire, diameter 10 to 15 cm. Wire this in series with the speaker. The speaker serves as an 8 ohm load to protect the amplifier in case you short out the transmitter. You'll hear the difference in the speaker between having the transmitter coil connected in series, and having it shorted.
Wind a receiver coil similar to the transmitter coil, and connect it to the oscilloscope. Put it on top of the transmitter coil. On the scope you should see several volts P-P, possibly more than 10 volts. Now slowly slide the receiver coil sideways. Shortly before the receiver coil no longer overlaps the transmitter coil, you will see the signal drop down into the millivolt range: as you move past that, the signal will shoot up again. The "signal null" position happens because the lines of magnetic flux coupling the interior of the transmitter coil through the receiver equal the number of the lines of flux coupling the exterior of the transmitter coil through the receiver coil. The "outside" lines of flux go in the opposite direction and so they cancel the effect of the "inside" lines of flux. This arrangement is known as
induction balance.
Glue the transmitter coil to a piece of wood or stiff plywood; then glue the receiver in the null position. Allow the glue to set. Epoxy is best: hot glue is faster but tends to move around.
No matter how careful you were to position the receiver carefully, it will move far enough that you lose that deep null. So with the oscilloscope hooked up, move the piece of ferrite around the coils and find a position where its distortion of the magnetic lines of flux brings the arrangement back into null (i.e., into induction balance). Increase the oscilloscope gain high enough that the residual signal looks a little ragged.
You now have an induction balance metal detector. If you bring a coin near the zone where the two coils overlap, you'll see the voltage change on the oscilloscope. This is because the transmitter induces current to flow in the coin, and because of that current, the coin radiates a magnetic field of its own. The receiver detects that magnetic field. (There are other ways to explain how voltage is induced in the receiver, but this explanation usually works the best.)
Induction balance metal detectors of this type date back to the late 1800's. Back then the AC transmitter voltage was usually produced by a commutator driven by a buzzer or electric motor (that was before vacuum tubes made electronics possible). Instead of an oscilloscope to detect the receiver voltage, the user listened to the audio frequency tone in headphones connected to the receiver coil.
If any of you other geeks on this forum would like to point out an easier way to do this or to supply information that I inadvertently left out, please jump in, but let's keep it to this thread since this is a bit off topic.
--Dave J.
