Optocoupler trigger. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Readers are invited to a trigger made on two-transistor optocouplers. Recall that a trigger is a device that has two stable states and is capable of jumping from one state to another under the influence of control signals.

In terms of its functional properties, the trigger refers to sequential digital automata, i.e., devices whose output signal depends not only on the signals currently operating on the control inputs, but also on the state in which it was before the appearance of these signals.

Fig. 1

Let's consider the operation of the proposed trigger (Fig. 1) in more detail. When the power is turned on, the phototransistors of the optocouplers U1 and U2 are closed, since the bias voltage at their bases is zero, therefore, the supply voltage is present at outputs 1 and 2. This is a positive feature of the proposed device, since for triggers assembled, for example, on TTL or CMOS logic chips, it is impossible to unambiguously say which of the outputs will have a supply voltage or a common wire when the supply voltage is applied.

After the power is turned on, the capacitors C1 and C2 are charged along the + Upit circuit - the resistor R1 (R6) - the radiating diode of the optocoupler U2 (U1) - the common wire, and by the time the control signals are applied to the inputs, they are charged up to the Upit voltage. When input 1 is supplied with a supply voltage, a positive bias voltage is supplied to the base of the optocoupler transistor U1 through resistor R2 and it opens. Its emitter current flows through the emitting diode of the optocoupler, and this keeps the phototransistor open even after the control signal is removed. Output 1 is set to a voltage close to the common wire voltage (approximately 1,4 V). The capacitor C1 also discharges to the same voltage through the open phototransistor of the optocoupler U1. Reapplying the control voltage to input 1 does not change the state of the device - output 1 still has a log. 0, and output 2 - log. 1.

To transfer the trigger to another state, it is necessary to apply a supply voltage to input 2. In this case, a positive bias voltage will be supplied to the base of the phototransistor of the optocoupler U5 through the resistor R2 and it will open. The current flowing through it will also flow through the emitting diode of this optocoupler, so the phototransistor will remain in the open state even after the end of the input signal. Output 2 will set to log. 0. At the same time, capacitor C2 begins to be recharged by the current flowing through the + Upit circuit - resistor R1 - optocoupler phototransistor U1 - capacitor C2 - optocoupler phototransistor U2 - its emitting diode - common wire. The current flowing through the emitting diode of optocoupler U1 decreases so much that at some point it becomes insufficient to keep the phototransistor of optocoupler U1 in the open state. As a result, it closes and the voltage log is set at output 1. 1.

Thus, when a control signal was applied to input 2, the device switched to another stable state: at output 1 - log. 1, output 2 - log. 0. Now the capacitor C1 begins to charge up to the voltage Upit. After charging it, the device will be ready to switch to the opposite state to the one it is currently in. As can be seen from the description of the work, the device can be confidently called a trigger. On a functional basis, it can be attributed to RS-flip-flops. The disadvantage of the device can be considered a rather high voltage log. 0 (1,4 V), which is the sum of the saturation voltage of the open phototransistor of the optocoupler and the voltage drop across its emitting diode.

The trigger, made by the author on the basis of 4N37 optocouplers, switched stably when using capacitors C1 and C2 from 3300 pF to 0,1 μF at a supply voltage of 12 V. With capacitors with a capacity of 10000 pF, it was operational when it changed from 8 to 15 V. voltages can be expanded both in the direction of large values, which are limiting for the optocouplers used, and smaller ones, by selecting resistors for stable switching of the device from one state to another.

Fig. 2

A drawing of a possible variant of the printed circuit board of the device is shown in fig. 2. Capacitors - ceramic K10-7V or KM-3b, resistors - any type.

Author: O. Belousov

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