Quartz beat metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors
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Schematic diagram
Schematic diagram of a metal detector on beats is shown in fig. 2.49.
Rice. 2.49. Schematic diagram of a quartz metal detector on beats
The scheme consists of the following nodes:
- quartz generator;
- measuring generator;
- synchronous detector;
- Schmidt trigger;
- display device.
The crystal oscillator is implemented on inverters D1.1-D1.3. The oscillator frequency is stabilized by a quartz or piezoceramic resonator with a resonant frequency of 32768 kHz (clock quartz). The R1C2 chain prevents the excitation of the generator at higher harmonics. Through the resistor R2, the OOS circuit is closed, through the resonator Q1, the POS circuit is closed.
The generator is characterized by simplicity, low current consumption from the power source, reliable operation at a supply voltage of 3-15 V, does not contain trimmers and overly high-resistance resistors. An additional counting trigger D2.1 is needed to generate a signal with a duty cycle exactly equal to 2, which is required for the subsequent synchronous detector circuit.
The measuring generator is implemented on a differential stage on transistors VT1, VT2. The POS circuit is implemented galvanically, which simplifies the circuit. The load of the differential stage is the oscillatory circuit L1C1. The generation frequency depends on the resonant frequency of the oscillatory circuit and, to some extent, on the mode current of the differential stage. This current is set by resistor R3.
To convert the low-voltage output signal of the differential stage to the standard logic levels of digital CMOS microcircuits, a cascade is used according to a common-emitter circuit on a transistor VT3. The shaper with a Schmidt trigger on the D3.1 element provides steep pulse edges for the normal operation of the subsequent counting trigger.
An additional counting trigger D2.2 is needed to generate a signal with a duty cycle exactly equal to 2, which is required for the subsequent synchronous detector circuit. The synchronous detector consists of a multiplier implemented on the D4.1 "XOR" element and an integrating chain R6C4. Its output signal is close in shape to a sawtooth, and the frequency of this signal is equal to the difference between the frequencies of the quartz oscillator and the search oscillator. The Schmidt trigger is implemented on the D3.2 element and generates rectangular pulses from the sawtooth voltage of the synchronous detector.
The display device is simply a powerful buffer inverter, implemented on the three remaining inverters D1.4-D1.6, connected in parallel to increase the load capacity. The load of the display device is the LED and the piezo emitter.
Coil making
Coil L1 is wound on a mandrel with a diameter of 160 mm, and has 100 turns of PEV wire - 0,2 mm.
Publication: loktek.ru
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