Interference-resistant photorelay. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Interference-resistant photorelay. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Lighting

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The circuit provides protection against false positives. Hysteresis - from relatively small fluctuations in illumination. Turn-off time delay - protection against accidental illumination of the photoresistor by car headlights, lightning discharges, etc. The photo relay operation algorithm is as follows: the lighting is turned off after a time delay, which provides protection against interference. Switching on the lighting, at the minimum level of illumination, immediately, because it is dark.

(click to enlarge)

A photoresistor FSK-2 was used as a photo sensor. It is possible to use a photoresistor FSK, or some others. In this case, you will have to choose a resistor R2. A photoresistor (not shown in the diagram), resistors R1, R2 and resistors R3, R4 form two arms of the measuring bridge. The balance of the bridge, at some given illumination, is provided by the resistor R1. When the illumination changes relative to the given one, the bridge is unbalanced and the unbalance voltage from the photoresistor is fed to the non-inventing input of the Schmitt trigger, which is made on the operational amplifier DA1, covered by positive feedback through the resistor R6. Resistor R7, diode VD1 form the level of a logical unit, R7, VD3 - a logical zero. In the future, they control the operation of the counter and pulse generator collected on microcircuits DD1, DD2.

The pulse generator is made on the elements DD1.2, DD1.3. The generator frequency can be changed by resistor R8 and capacitor C1, which changes the counting time of the counter.

A pulse counter with control logic and reset circuits is assembled on the DD2 chip and elements D1.1 and D1.4. At high illumination of the photoresistor, a logical 1 is present at the output of DA1, which allows the operation of the generator and counter DD2., included as an 8-bit binary counter. After 192 pulses (128 + 64), a logical 1.4 will be present at the output of element DD0, which will disable the generator and turn off relay K1. Lighting will turn off.

When the illumination decreases below that set by resistor R1, the trigger switches to state 0, which will disable the operation of the generator, counter DD2 and the inverter on the element DD1.1 will reset the counter DD2. With the arrival of 0 at the input of element DD1.4, it, with the help of its inversion and transistors VT1, VT2, will turn on relay K1. Relay K1 will turn on the lighting. Duplication of count prohibition on inputs R, E, C (on input C - due to the prohibition of the operation of the generator DD1.2, DD1.3) is provided in order to avoid the impact on the counter of all kinds of impulse noise in the power circuits of the circuit.

With the advent of such interference as lighting, the headlights of a photoresistor car, a 1 will appear at the output of DA1, which will turn on the generator and the counter, and if the illumination changes during the counting time of 3 - 3,5 minutes, the counter will reset again. Relay K1 will not change its state.

The power circuit consists of resistors R11 - R15, zener diodes VD6, VD7, diodes VD4, VD5 and capacitors C2, C3, C4. Resistors R11 - R13 limit the current of the zener diodes, provide the necessary voltage for the relay K1 to operate. In addition, in winter, they prevent overcooling of the parts of the photorelay. Resistor R14 limits the charge current of capacitor C4 at the moment the photo relay is connected to the network. The rectifier diodes VD4, VD5, through the quenching resistor R15 and capacitor C4, receive a mains voltage of 220 V, which is rectified and limited by the zener diodes VD6, VD7. When power is removed from the device, capacitor C4 is discharged through resistor R15, which reduces the risk of electric shock.

The pin numbering of the DD1 and DD2 microcircuits is not indicated, because PCB configuration can be any, depending on the housing used.

Attention! The device has a transformerless power supply, so touching live parts is life-threatening!

See other articles Section Lighting.

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