Metal detecting is a fun activity that gets people out into the great outdoors, as well as providing a thrilling experience while prospecting for the next bit of trash or treasure. Pretty much all prospecting metal detectors work using a similar technique of manipulating electromagnetism, which was first discovered By Professor Maxwell in the 1800s. In order to understand how to build a metal detector, it is useful to understand a short and simple physics lesson so that you can understand the technology of how one works.
Magnetism vs. Electricity
Magnetism and electricity are intertwined in their relationship, hence the term electromagnetism. Simply put, you can make electricity using magnetism, and you can create magnetism using electricity, as they exist in a symbiotic relationship. Let’s explain this in a little more detail.
To generate electricity, you need a generator. A generator works straightforwardly by rotating copper wire at a high speed in a magnetic field. When this happens, electricity is “magically” generated, and we can use that electricity to drive motors that run washing machines and vacuum cleaners around the home. A motor is essential the reverse of a generator that is used to power a device. As electricity flows into the motor, it generates a changing magnetic field in a coil of wire, and this pushes against a permanent magnet which causes a motor to spin.
James Clerk Maxwell created a series of formulas known as Maxwell’s equations, which spell out this symbiotic relationship between electricity and magnetism. One of the formulas very simply implies that when there is a changing magnetic field, you get a changing electric field and this is the basis of how a metal detector detects the object. The head of a metal detector contains a metal coil known as the transmitter coil. When electricity flows through this coil it creates a magnetic field. When that magnetic field passes over a metallic object, it reacts with the electrons in the metal and changes the magnetic field of the source metal.
As we know from Maxwell’s equations, if there is a change in magnetism, this creates a change in the electric current, And similarly, this change in electrical current also creates a further change in its magnetic field. It is this change in the magnetic field that the receiver coil in a metal detector picks up and transfers into a reading, usually an audible signal.
Basic Construction Steps
As we can see from the physics of electromagnetism, all we really need to do to create a detector is to create a transmitter coil that emits a magnetic field and a receiver coil that detects a change in a magnetic field and convert it back to electricity. Simple! Well simple if you know how! It is possible to make a simple metal detector using some basic materials that you will have lying around the house and a few technical components that you can easily buy online or from your local electronics store.
Step 1 – Make an oscillating circuit. The circuit in this metal detector is a basic 555 integrated circuit that is used for time delays and can be used as an oscillator or a flip-flop element. There are thousands of how-to and schematic diagrams on the internet that show you how to create a simple 555 circuit, so I won’t go into that detail here. If you’re really stuck you can buy one online!
Step 2 – Form the coil. Understanding the requirements for making the coil is probably the most complicated part of this project. Again, there are many useful calculators available online that will tell you the amount of copper wire you will need, based on the size of the coil and the width of the wire. It is also possible to buy copper wire coils online.
As a guide, if you wanted to create a 9 cm coil, you would need 250 winding turns using a copper wire of 90 mm or 290 turns using 70 mm. A simple way to do this is to wind it around a cardboard coil about an inch in thickness and then solder both ends into position.
Step 3 – Connect it up! Connect the coil to the circuit board and power source. Before constructing the nuts and bolts of the metal detector, it is worth testing that your circuitry works by connecting the coil to the circuit board and power source. Assuming this all works, you are ready to build your metal detector. If you create the circuit on a breadboard and the test was a success you may want to transfer this onto a PCB and solder the circuit, for a more permanent solution.
Step 4 – Make the handle and shaft Depending on what materials you use, you can be as creative as you like at this stage. A handle could be easily constructed using the center of a kitchen paper roll, or a piece of plastic tubing. Just feed the power cables through the center tube and attach the PCB board securely in the stem. Connect the wire from the PCB to the coil and firmly secure the coil to the stem. Similarly, at the other end attach whatever you choose to use as a handle. And away you go.
Fun Using Household Electrics
As a fun activity, it is also possible to create a metal detector using an old AM radio, some duct tape and a calculator. Just tune your AM radio (with the volume on loud) to the high end of the frequency range. Ensure it is not on a broadcast station, so you can hear the static. Next, turn the calculator on and position the two items close together until you hear a loud tone. Duct tape the two pieces together and voila you have a DIY metal detector.
Pass this over any metal object, and the loud sound created from the radio and calculator combination produces a radio frequency signal which bounces off the metal, and is heard through the radio speakers.