DIY pulse metal detector.Finds a coin at a distance of 20cm

Pulse metal detector. With its help, you can find small coins in the ground at a distance of 10-20 centimeters from the coin. You can find a sheet of iron at a distance 1 meter. The principle of operation is based on the radiation of a signal by the search coil, which induces eddy currents in the search metal. The search coil receives these eddy currents when there is no radiation of pulses from the coil.

The metal detector generator is made on a 555 timer. At the timer output, a rectangular signal is down, the frequency should be 120-150 Hz. The pulse duration is 120-150 µs.An inverting cascade is assembled on transistor T1, the signal is amplified, it is also needed as a buffer between the microcircuit and the field-effect transistor.The key is assembled on the field-effect transistor. The load is connected to the transistor drain - the search coil.The search coil has a diameter of 24 cm and contains 27 turns of 0.35 mm wire. The cable going to the coil is 1 meter long.

Checking the operation of the search coil

Resistor R13 adjusts the sensitivity of the metal detector. How to set up the metal detector: to check the cascade on the TL072 chip, disconnect capacitor C3 from resistor R8 and touch pin 2 of the chip with your finger. A hum will appear in the speaker. Checking the operation of the generator, made on 555 and two transistors, comes down to listening to the hum if you put a speaker instead of a search coil. If the fully assembled metal detector constantly hums and is not regulated in any way, increase or decrease the resistance of resistor R7. Capacitor C3 is a ceramic smd.Capacitor C8 should be installed closer to the cascade on the field-effect transistor. Power supply - three 18650 batteries connected in series.A properly assembled metal detector starts working immediately. It is better to do the setup outdoors, away from metal objects.

Briefly, how this circuit works. The square-wave generator is assembled on a 555 timer. Transistor T1 is an inverting stage, and transistor T2 is a key transistor. T2 must be high-voltage. The coil emits high-voltage pulses. The metal we want to find delays the decay of these pulses. It is this decay that must be registered.Through resistor R8, the signal is fed to the diode pulse limiter. The limiter serves to protect the input of the microcircuit amplifier from high voltage and limits the pulse to approximately 1V.Through C3 the signal (EMF) is fed to the inverting input of the amplifier, where it is amplified and inverted, "flipped".The second "half" of the microcircuit is assembled according to the comparator scheme. From pin 13, the amplified and inverted pulse goes through the integrated circuit R12 R13 C5 to the inverting comparison pin 5. At pin 6, a chain is formed on the resistor R14 R15 C6, which commutates the voltage relative to the signal formed by the chain 5. In the absence of metal, the comparator does not switch, when metal appears, the comparator switches and produces an amplified positive signal at pin 9. transistor VT3. A short pulse means the absence of metal, a long one means its presence. Resistor R16 closes the transistor.

Well, something like that)).I am writing through a translator, there may be incorrect terms. In the video I show on the oscilloscope how the signal on the coil changes when metal is brought to the coil.