The principle behind a metal detector is really very simple. Proof of this is the circuit that follows, in which it proves that the construction of a metal detector can be done in the minute, with few components that we find very easily everywhere. With the circuit of this metal detector, it is possible to detect a metal coin in depth of 90 to 100cm. In contrast to many other detectors, the design of this, is relatively stable in coordination and therefore quite easy to use.
Like many other detectors, this one uses the Beat Frequency Oscillator (BFO) principle. According to this principle we have the creation of a tone, the frequency of which is the difference between a variable oscillator and a reference oscillator. In the present circuit, we get the reference frequency from the second part of the circuit that equates to a medium wave receiver, while the oscillating oscillator grows around an integrated six-way Schmitt trigger inverter. The self-build detection coil is inserted between the input and output of one of the six gates of 40106. According to the theory, an induction is opposed to the rapid changes of each applied voltage. Thus, looking at the circuit diagram we have here, any change in the logic level at IC1 terminal 2 will be applied with delay to the input terminal 1. This delay causes oscillation of the circuit and the resulting signal is coupled inductively to the receiving circuit.
Although the receiving circuit is tuned to a frequency that is much higher than that generated at the gate 40106, the BFO startup works equally well with its harmonic oscillator.
In the case now that there is some metal near the coil region, the induction of L1 will increase (or decrease), causing a drop (or increase) of the oscillator frequency. This phenomenon is transformed into an audio signal, amplified by the LM386 on the headphones. With the potentiometer R3, the volume is adjusted.
Depending on the construction of the 40106 we use, the oscillator operates at 200-300 kHz. IC1 is also used as an isolator, ensuring that the oscillator will see a light, relatively constant load, and thus maintain the stability (within limits of course) of the output frequency.
The detection coil consists of 70 coils of insulated copper wire of 0.35 mm diameter, wrapped in a 120 mm diameter form. During wrapping, we can stabilize the spirals with adhesive tape. After the winding is completed, fully insulate the coil using an insulating tape.
Coil Shielding and connection
Then we will have to shield the coil, and for this we use some foil strips of aluminum foil. First we wrap around the insulated coil, naked copper wire, which will then be electrically connected to the shield. The metal strips are now secured around the coil, covering the naked copper wire. Although shielding should cover the entire perimeter of the coil, we should leave a small gap around 10mm, or in other words, the beginning and end of each metal strip should not come in contact. Shield the strips of the armor using an insulating tape. Then connect the ends of the coil to the circuit, where for better transport quality we use a shielded microphone signal cable. The whole presence of the detection coil can of course become more professional, for example by adding a disc and a suitable handle.
Finishing the construction, we power up the metal detector feed with a 9V battery, and turn C1 until a loud whistle sounds. Now the detector is ready and able to distinguish metallic from non-metallic objects. In the first case, the frequency is increased, while in the second drop. The truth is that there is enough room for experimentation about the oscillator output frequency, and coil designs. Happy searching ;)