The cheap module I received from China had no instruction manual or circuit diagram, so I had to do a lengthy Google search in order to get its basic schematic. In this concise primer you will have the schematic and all other details you need to know if you want to make a simple metal detector on your own. So, let’s start!
The clever part of the design is the single rectangular coil made by etching one continuous track on the printed circuit board. If you look very carefully at the printed circuit board, you can see that the rectangular coil is tapped at one turn i.e. a single coil with a precise tapping at one turn. The coil starts at the top side of the double-sided printed circuit board and makes square loops towards the center, A via in the center of the printed circuit board takes the coil to the other side of the printed circuit board and the same set of tracks are on the bottom side. Pretty nice!
A handful of commonly available discrete components remain including three bipolar junction transistors, a few resistors, and capacitors. The sounder in the module is a standard active piezo-buzzer, and there’s also a small trimpot to fine-tune the circuit. Here is the basic schematic of the metal detector module. I borrowed it from the seller’s webpage.
Next is the schematic of another version of the metal detector module that has a circular coil etched on the printed circuit board. In this near-identical design, you can see an annunciator LED too.
This is the photograph of the second version (I don’t have one handy yet):
The working principle of the non-contact metal detector module is not very hard to understand. Following is a very concise explanation, so this session is best read along with the tutorial . Both items together will help your understanding of the metal detector circuits.
As pointed out earlier, the sensor-head of the circuit is a 50 turns (25T top + 25T bottom) ‘printed’ coil tapped at first turn. The single turn (1T) feedback coil (L1) is routed to the base of Q1 via C2, and base of Q1 is biased by R1 while end of the coil (50T) wired directly to the collector of Q1.
The tuned circuit (a coil plus a capacitor across it) formed here generates a smooth sine wave signal. Keep note, the signal produced here comes from the natural ability of the coil and the capacitor across it, and hardly needs any other components. But Q1 helps the tuned circuit to generate the sine wave signal as Q1 supplies the essential pulse of energy at the right moment in each cycle (it turns on at the start of each cycle and delivers the pulse of energy and then turns off).
How does this setup keep running? It’s a bit of a complex process that calls for a lengthy explanation, but going through the above mentioned tutorial link will simplify it so that you are no longer in the fog.
Note that the signal coming out from L1 is out-of-phase with that of L2. Here, L1 will deliver a signal that increases the noise produced by Q1 to create an oscillator that has a certain amplitude. We can control the amplitude by the trimpot VR1. This signal is passed to Q2 where it gets an uplift and prevents C4 being charged via R2, thus disables Q3 and the piezo-sounder SP1 does not make a noise in idle state.
On the other hand, if a metal object is placed in the close vicinity of the coil (sensor-head), amplitude of the magnetic waves from L2 is reduced somewhat together with the amplitude of the oscillator. This effectively disables Q2 but puts a small voltage across the base and emitter of Q3 to fire it slightly. The piezo-sounder then makes a noticeable tone because a small dc voltage is available across its terminals.
The major advantage of this type of metal detector circuitry is simple construction of both the device and its search-head (coil), but one drawback is its poor sensitivity (very short detection range). This is an ingenious design idea for learning how simple metal detectors/locators work. That’s really all there’s to it.
Oh, that’s enough theory talk for now! Let’s get on with testing the $5 Chinese metal detector module. Well, coming to the module’s key specifications, the module is designed to be operated by any low-current 3V–5VDC power supply, but a 3V battery (1.5V x 2 AA or AAA) operation is recommended.
According to the accompanied user guide, standby current consumption of the module is under 3mA (at 3VDC) which climbs to around 30mA when in active state. Anticipated detection distance of the metal detector module is 60mm.
The 3V battery pack powered experiment showed me a ridiculous detection range that’s just below 10mm, when tested with a 20mm diameter coin as the object.
Repeated tests proved that the metal detector module under test is useful only for detecting large metal objects at a very short distance. It’s also observed that the module hiccups often, especially in case of even small input voltage level fluctuations. Further, the trimpot needs to be retuned often after every power cycling – a bit awkward task to carry out!
As observed, oscillator frequency of the circuitry (in idle state) is about 300 kHz. If you hook your oscilloscope’s probe at the collector lead of Q1, you can see a nifty sine wave produced by the resonant circuit.
A side note: If you’re the proud owner of a professional metal detector/locator, you can find a ‘frequency’ range in its user manual. What does frequency mean when metal detecting? See, in a metal detector its frequency is the number of electronic waves sent into the ground to detect metal objects. For example, 8 kHz frequency denotes your metal detector will send and receive signals 8000 times per second. Commercial metal detector frequencies range between 3 – 100 kHz as a general rule. Low frequency has longer wavelengths and gets greater depth as long waves penetrate the ground more easily. High frequency has shorter wavelengths (less depth) but is better for detection of low conductivity targets. It’s noticed that many hobby metal detectors prefer 6 – 8 kHz for best depth and sensitivity trade off.
Moreover, there’re a few proven techniques used for metal detection. Beat Frequency Oscillator (BFO) is one of the simplest and most erstwhile metal detection methods. Second is the Induction Balance (IB) method – another good old approach still in use today. Next one, the Pulse Induction (PI) metal detection works on an entirely different (and a bit complex) principle, although it’s based on the same principle of the changing inductance of a search coil when metal is near to it. Other two popular methods are Off-Resonance (OR), and Transmit/Receive (T/R).
Going Further
Now you need to know to get started with do it yourself metal detector projects. It’s great to have an extremely cheap and compact metal detector module ready to go for funny applications. Funny? Yes, you can try the module to make a fabulous science fair model for your kids, all you need is a battery pack and of course a bit of crafty know-how.
The only trouble is that the metal detector module has an insensitive search coil on board. If I feel inspired, I may use my soldering iron to lift the discrete components off the board and incorporate into a future, more efficient design, of course with an improved search-head (search coil). I’ll save that project for another sleepless summer night!
Finally, if you enjoyed this short article, then stay tuned to find an extension of this write up that also includes a slightly advanced, microcontroller-based metal detector project.
Hello, my name is Jack Sparrow. I'm a 50 year old self-employed Pirate from the Caribbean.
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