This is a test version of a simple metal detector in the ground, which is assembled on a universal circuit board and does not look aesthetically pleasing, but it works quite well.

BFO METAL DETECTOR CIRCUIT

Just in case, let me remind you that a BFO metal detector is a beat detector . The BFO detector works on the principle of detecting a change in the frequency of an oscillator whose coil is a measuring sensor. Since detectors typically operate above audio frequencies (in this case 17 kHz) and frequency changes are less than 1%, the signal from the pickup oscillator is mixed with the signal from the fixed oscillator so that beats at about 100 Hz can be heard. Here the frequency change is clearly audible.

The problem with such devices is their rather low sensitivity to small objects, but this can be improved by increasing the frequency (reducing the capacitors C2, C3, C6, C7) and stabilizing the frequency drift with temperature changes. The detector is based on two three-point generators in a common base configuration (transistors Q1 and Q2). The oscillator coil on transistor Q1 is a measuring sensor. Its diameter is 15 cm, but it can naturally be changed while maintaining the same inductance.

• L1 is the sensor (the search coil itself) which has 6.2mH of inductance.
• L2 – two tuning coils from a car radio connected in series and 3 1 mH chokes, similar to a resistor. I had to combine this because there was no other way to match the inductance.

The signal coming from this oscillator goes to the LM358 op-amp in the same way as the signal from the second oscillator, so the rest of the circuit does not significantly affect the signal, the two signals drive the same amplitude. The signals after leaving the operational amplifiers are mixed through two resistors and then amplified. There is sound in this signal, which is the difference between the two input frequencies.

The two diodes in the circuit pull these beat frequencies (up and down envelopes), which are then subtracted from each other at the next op-amp element.

METAL DETECTOR CONTROLS

• R2 – used to set a good sine wave and the largest amplitude
• R10 – controls the signal amplitude
• R11 – controls direct current
• R9 – signal amplification

Of course, PI pulse detectors are simpler and more efficient. But even such a scheme will go to search for large metal structures. The metal detector is completely built on the parts from the soldering of the boards, the batteries from the non-working laptop, so the cost turned out to be zero.