BFO metal detector

BFO metal detector uses two generators. One generator uses a search coil, the second a smaller coil that is used to pattern the signal. By tuning the two coils so that their frequencies nearly match, the difference between the two frequencies begins to be heard through the speaker. The sound frequency changes slightly as the search coil approaches the metal. During the tests, it was found that the best efficiency was obtained when the coils worked at a frequency of about 100 Hz. The sound disappears when the frequencies of both generators are equal. This type of metal detector is most sensitive when the sound frequency is around 5 Hz.

Scheme

Components

Power source:
9 V battery.

Capacitors:
2 pcs. 220 uF/16 V
5 pcs. 10 nF
5 pcs. 100 nF

Resistance:
6 pcs. 10k
1 pc. 1K0
1 pc. 2M2
2 pcs. 39 k

Transistors:
All BC337, Q6 can be a bit more powerful because when the frequencies equalize the current rises to 100-110mA, so Q6 starts to heat up.

Speaker:
2.5 inch diameter 8 ohm computer speaker, but headphones are better because they are more sensitive to higher frequencies.

Coils:
The search coil was made from the cardboard of a computer box. We cut out two circles with a diameter of 16 cm and one circle with a diameter of 15 cm. We place a circle with a diameter of 15 centimeters in the middle and glue all three circles together. We take a lacquered copper wire with a diameter of 0.25 mm and wrap 10 turns around the coil. We leave a little bit at the ends of the wire to reach the motherboard. The search coil is connected to the Coil A contacts. If you have an oscilloscope, this coil should be inducing about 104 Hz.

The template coil should be wound on a plastic or wooden tube or stick with a diameter of 10-12 mm and a length of 5 cm. We used a hot glue stick, its diameter is about 12mm. We use the same lacquered copper wire with a diameter of 0.25 mm. The exact number of turns depends on the diameter of the core. To begin with, we wind about 125 windings. We leave the long end of the copper wire. If you have an oscilloscope, measure the frequency at which the coil operates. The frequency of the coil can be changed by winding or unwinding the wire. Tune the coil for ~104 Hz. This coil is connected to Coil B contacts. If all is well, you should hear a beep through the speaker. If there is no sound, try unwinding or winding the wire on the pattern coil. When the frequencies of both coils are the same, the buzzing disappears (the metal detector is ready for work).

Assembly diagram

Assembly diagram made with the program “Eagle 4.13”:

You can download all (Eagle 4.13) schematic and assembly files .

Realization

This is what the assembled and assembled scheme looks like.

This metal detector (when matched) can find coins at a distance of about 10-12 cm. The bigger the metal object, the higher the frequency the speaker starts to scream.