Medium level signaling device (capacitive relay). Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Indicators, detectors, metal detectors

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The medium level detector (hereinafter SLS) is designed to signal the presence of certain objects (loose media, objects) in the immediate vicinity of its sensor (sensing element). In fact, this is a capacitive relay, a signaling device for changing the capacitance of a capacitive sensor. From similar devices of the resonant type, SUS differs in the absence of inductors and ease of tuning. Such a circuit solution provides a response to a change in the capacitance of the sensor by only 0,5 pF (!). This allows you to respond to the approach of the human palm at a distance of 5-8 cm to the sensor surface.

The block diagram of the SCS is shown in Figure 1, and consists of: a sensor - 1, a clock generator - 2, reference generators - 3 and measuring - 4 signals, a comparing device - 5, a matching device - 6 executive - 7 devices and a power supply - 8.

Rice. 1 (click to enlarge)

Consider the operation of the circuit according to the diagrams in Figure 2:

Fig. 2

The clock generator generates synchronization pulses (1), which trigger the reference (3) and measuring (5) standby multivibrators. The duration of the reference pulse is determined by the circuit elements (3), (can be changed manually). The duration of the measuring pulse (5) depends on the value of the capacitance of the sensor.

In the initial state, when the sensor is not affected by foreign objects, its capacitance is small and the duration (t2) of the reference pulse is greater than the duration of the measuring (t3) pulse (t2> t3, diagram 4, 5). When the controlled object approaches the sensor, its electric capacitance increases, the duration of the measuring pulse (t4) increases and at a certain moment becomes greater than the reference (t2) (t2

When the controlled object moves away from the sensor, its electric capacitance decreases and the duration of the measuring pulse decreases (t5) and when it becomes less than t2, the circuit will return to its original state and the voltage will be removed from the load (point "B" of diagram 6).

Consider the circuit diagram of the SUS signaling device in Figure 3:

Rice. 3 (click to enlarge)

The clock generator is assembled on logic elements DD1. The reference signal generator is made on 2 elements DD2, transistor VT1 and timing chain C3, R2, R4. Its parameters are regulated by resistor R4.

The measuring signal generator is made on the remaining 2 elements DD2, the transistor VT2 and the timing chain C0, C2, R3. The outputs of both generators are fed to the inputs "D" and "C" trigger DD3. When the pulse duration of the measuring channel (5) is greater than the pulse duration of the reference channel (3), "1" (6) is written to the trigger. The signal level matching is carried out in the matching device 6 (Fig. 1), assembled on the elements: VT3, VT4, R13, R14, R16-R18 (Fig. 3). The relay cascade of the actuator 7 is assembled on VT5. Relay contacts K1 switch power to the load. At the top of the diagram, a seven-story version of the load switch is shown. Both schemes are featureless.

The design of the SCS and the sensor is shown in Figure 4.

Fig. 4

The supporting element is a metal case with a disk made of foil textolite 1,5 mm thick and 160 mm in diameter placed in it. The sensor pattern is stencilled. Unprotected areas are etched, wires are soldered to the resulting zones (sensor capacitor plates) for connection to the circuit.

Setting up the signaling device consists in checking the supply voltage of the microcircuits and monitoring the compliance of the signals at the control points in accordance with Figures 2 and 3. In the initial state of the sensor, the pulse duration at pin 3 of the DD3 microcircuit should be less than the pulse duration at pin 5 of the same MS. This is achieved by adjusting the tuning resistor R4. When the hand approaches the sensor, output 1 of the DD3 microcircuit should be a transition from the state "0" to the state "1". The sensitivity is specified by a slight change in the position of the resistor R4. Setting the rest of the circuit, with serviceable parts, is not required.

Structurally, the signaling device is assembled from two blocks (highlighted in Figure 3). The left (according to the diagram) part is assembled in the immediate vicinity of the sensor, in one housing. The right side is a separate block. Connection between blocks 3-wire, unlimited length (within reasonable limits).

I will answer the questions of those interested by bkalmykov [dog] esv.ryazan.ru

Author: Kalmykov B.M.; Publication: cxem.net

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