Contactless proximity sensors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In my professional and amateur radio activities, I had to develop devices that required control over the movement of metal parts. The industry produces several types of non-contact proximity sensors (BDP) that perform similar functions and are built on different physical principles. But most of them were not suitable for reasons of lack of sensitivity adjustment, non-repairability or high cost. And, for example, well-established photoelectronic devices do not work well in a dusty or opaque environment.

Unfortunately, in the popular literature there are very few publications about the device and the use of BJPs well adapted to work in such conditions, the operation of which is based on a change in the quality factor of the oscillatory circuit when a conductive object is introduced into the magnetic field of its coil. The BDPs described below are built on exactly this principle. They react to the approach of a metal object only from one side. This is very important when mounting the sensor in a "metal" environment, such as on a machine bed.

The basis of the BDP on one transistor, the circuit of which is shown in Fig. 1, was the device described in the article "A simple metal detector" ("Radio", 1980, No. 7, p. 61).

In the absence of metal near the transformer T1, the inductive feedback generator on the transistor VT1 is on the verge of stalling. This is achieved using a tuned resistor R2. The alternating voltage from the collector of the transistor VT1 through the capacitor C2 is supplied to the rectifier on the diodes VD1 and VD2. The value of the rectified DC voltage corresponds to a high (for microcircuits of the CMOS structure) logic level. Approaching the transformer T1 of a metal object leads to a breakdown of the oscillations of the generator. The sensor output voltage drops to zero.

The magnetic circuit of the transformer T1 is one cup of the B22 armored core made of ferrite 2000NM1. Windings I (120 turns) and II (45 turns) are wound in bulk with PEV-2 wire 0,2 mm. Such a device reacts to the approach of metal only from the open side of the magnetic circuit. Other brands of ferrite cups and even carbonyl iron cups have been tried. Good results were obtained in all cases.

Trimmer resistor R2 - SP5-2v, constant - MLT. All capacitors are ceramic (for example, K10-17, KM-6), transistor VT1 is any silicon high-frequency npn structure.

The operating range of this BDP can be adjusted with a trimming resistor R2 within 0 ... 60 mm. In the process of adjustment, the voltage at the output of the sensor is controlled by a high-resistance voltmeter or using a simple LED indicator, assembled according to the circuit shown in Fig. 2. It should be noted that the values ​​of the response range of more than 20 mm are extremely unstable and it is undesirable to set them.

A diagram of a more complex BJP on the K561LN2 chip is shown in fig. 3. Unlike the previous one, it is equipped with a built-in status indicator on the HL1 LED and has a significantly higher load capacity. Between the output of the sensor and the plus of the power source, you can even turn on the relay winding. Due to the stabilization of the supply voltage of the DD1 microcircuit, the sensor sensitivity set by the trimming resistor R1 is less susceptible to changes.

The main sensor assembly is an LC generator based on elements DD1.1 and DD1.2. Element DD1.3 serves as a buffer between the generator and the rectifier diode VD1. Element DD1.4 inverts the signal, which is fed through the power amplifier on the transistor VT1 to the output.

Capacitor C4 - K50-35, the rest - K10-17. Trimmer resistor R1 - SP5-2v, constant - MLT. The design of the coil L1 is similar to the transformer T1 (see Fig. 1). Its winding is 50 turns, wound with a bundle of four PEV-2 wires 0,1 mm.

When adjusting the sensor, first of all, by rotating the slider of the resistor R1, the HL1 LED is turned off. Then a metal object is brought to the coil L1. The LED should flash. By changing the position of a metal object, the desired response range is achieved with a tuning resistor.

Both variants of the BDP successfully operate in industrial installations developed with the participation of the author.

Author: N. Taranov, St. Petersburg

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