Trigger on a transistor optocoupler. Scheme, description

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Trigger on a transistor optocoupler. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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The proposed trigger (its circuit is shown in the figure) differs in that in one of its states it practically does not consume current. It is made on the basis of a 4N35 transistor optocoupler. Resistor R1 limits the base current of her transistor U1.2 to a safe level, R2 prevents it from operating in open base mode, resistor R3 limits the current through the emitting diode U1.1.

Trigger on a transistor optocoupler
Fig. 1

The device works as follows. In the initial state, the optocoupler transistor is closed, no current flows through the emitting diode, and the device does not consume current. When a positive polarity voltage pulse is applied to the input, the transistor opens and the emitting diode lights up. Thanks to the optical positive feedback, the transistor remains open even after the end of the pulse - the trigger goes into the second stable state. You can switch it to its original state only by applying a voltage pulse of negative polarity to the input or by turning off the power.

Almost any transistor optocoupler with a phototransistor base output can be used in the device. Imported 4N25-4N28, 4N35-4N37, CNY17, TLP331, domestic AOT128 with letter indices A-B are suitable, you only need to select the resistance of the resistor R3 depending on the supply voltage and the current through the emitting diode necessary for stable operation. The supply voltage should not exceed the maximum allowable switching voltage values for the applied optocoupler.

Author: K. Moroz

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