Simple Metal Detector (Pinpointer)
Among the advantages of the device, we can mention very low power consumption (5 mA from a “Krona” battery), ease of adjustment, and the absence of problems with any interference. simple metal pinpointer detector .
Its sensitive element is the oscillatory circuit of the generator, assembled according to the classical scheme on the transistor VT1. At the same time, with the help of resistor R1, on which the depth of feedback depends, the generator is set to a special mode, very sensitive to the Q-factor of the oscillatory circuit. The latter, in turn, depends on the medium in which the circuit is located. The depth of the generator’s excitation determines the constant voltage at point “A”. Since this voltage depends not on the frequency but only on the depth of the generator’s excitation, unfortunately, it does not allow differentiating detected metals based on their magnetic properties. However, thanks to this, the coil does not have high requirements for stiffness and other parameters to achieve the required sensitivity. And if the coil is assembled conscientiously, the sensitivity of the device will be noticeably higher (up to 15-20 cm for a coin), and large objects made of non-ferrous metals can be detected at a distance of more than one meter.
The constant voltage taken from point “A” via a shielded wire (any brand) is supplied to a two-stage amplifier built on two operational amplifiers (DA1 chip).
Diodes VD1 and VD2 are silicon diodes with low reverse current.
On the operational amplifier DA1.3, a sound frequency generator is assembled, the excitation of which occurs when the potential difference at the inverting and non-inverting inputs decreases.
On element DA1.4, an inverter is assembled, serving to increase the volume of the sound emitter.
The generator is adjusted as follows. Instead of the constant resistor R1, a variable resistor with a resistance of 10 kOhms is installed, and its slider is moved to a position corresponding to maximum resistance. When its resistance decreases, the voltage at point “A” will also decrease.
At some point, it will stop decreasing and start increasing. It is necessary to fix the moment when the voltage at point “A” becomes minimum, measure the corresponding resistance of the variable resistor, and be sure to replace it with a constant one with the same resistance. The generator is located on a separate small board in close proximity to the coil. As for the coil, it is a 3-liter jar with 200 turns and a tap from the center of a wire with a diameter of 0.3 – 0.4 mm.
To ensure stable operation, the coil with the generator should be thermally insulated. (And SP-5 in the scheme and SP-3 in the sensor are not electrolytes. R1 = 7.5 kOhms plus SP-5 on 330 Ohms for precise adjustment.) It is advisable to stabilize the power supply.
Printed circuit board:
Compact metal detector.
Among the advantages of the scheme, we can mention the simplicity of making the sensor and the absence of problems with detuning from the ground. Sensitivity to a 5-kopeck coin is 9 cm.
The sensor elements form a measuring generator operating in a mode close to the breakdown of generation. A high level of feedback provides a “soft” start mode and a smooth characteristic of the dependence of the oscillation amplitude on the Q-factor of the circuit. The power consumption depends on the state of the generator, registering changes allows us to judge the presence of a metal object near the sensor.
Then the signal from the sensor is fed to an amplifier, an integrator is included in its feedback circuit. Such a coupling provides a dynamic operating mode of the device, compensating for slow drift and at the same time, when the sensor is brought close to the metal, a transition to a static mode due to the limitation of the integrator. Then, through the second stage of amplification, the signal is fed to the generator, voltage-controlled, and to the sound emitter.
The printed circuit board and the sensor form a single unit; the sensor is filled with epoxy resin to protect it from deformations and moisture. Coil L1 is made on four segments of a ferrite rod M400 NN with a length of 40 mm, the rods are glued to both sides of the board with super glue, then a coil of 200 + 200 turns of wire PEV 0.25 is wound on it, on top of it is wrapped an electrostatic screen made of metallized film (wrap from a floral bouquet), the contact is provided by a bare tinned wire pressed to the metallized side of the film with a thread along the entire length of the coil. The power button is glued to the board near the sensor on the side of the tracks through a resilient gasket (a piece of an old mouse pad), which allows you to easily change the button in case of failure. The button pusher is made from a 5 mm LED. A section of plastic water pipe with an internal diameter of 28.5 mm is used as the case, the end circles are turned from polystyrene on a lathe. A miniature loudspeaker with a diameter of 27 mm and a coil resistance of 100 ohms can be used as a sound emitter.