Principle and application of pulse circuit diagram composed of optocoupler

The optical coupler introduced here is a device composed of a light-emitting diode and a phototransistor. The light-emitting diode converts the electrical signal of the input side into light with the same changing rules, while the phototransistor converts the light back into an electrical signal with the same changing rules. Therefore, light plays the role of a medium. Since the LED photocoupler has strong anti-interference ability, it is easy to complete level matching and transfer, and is not restricted by whether the signal source is grounded. Therefore, its application is becoming more and more extensive.

1. Multivibrator circuit composed of photocoupler

The multivibrator circuit composed of photocouplers is shown in Figure 1.

When the power supply Ec is just turned on in Figure 1 (a) , UF increases as C charges until UF≈1V, the light-emitting diode reaches saturation, and then the transistor also saturates, and the output Uo≈Ec.

After the transistor is saturated, C discharges (discharges through two paths, C→F→E1→Er and C→RF→+Ec→Re), uo decreases, and the diode is cut off after C discharges to a certain extent, and the transistor is also cut off after removing all the stored charge, and uo is zero. After the transistor is cut off, the power supply Ec charges C again, and the above process is repeated to obtain the peak output waveform shown in the figure, and its period is (when RF>Re):

T=C（RF+Re）In2

Figure 1(b) is another circuit with the same principle.

Figure 1: Multivibrator using optocoupler

2. Bistable circuit composed of photocoupler

The bistable circuit composed of photocoupler is shown in Figure 2.

After the circuit is powered on, the steady state is that BG is cut off and the output is high potential. Under the action of the trigger positive pulse, ib increases and BG enters the amplification state, forming ib↑→if↑→ib↑↑, and as a result, BG is cut off. This circuit has higher anti-interference ability than ordinary trigger sequence. If the limit current of BG Ic=6 mA, then R2= is taken as:

R2≥(13-1)/(6×)=24 ohms

Current limiting resistor R1 can be calculated as follows

R1≥(E-IbmRce2min)/Ibm

In the formula: Ibm is the maximum base current of the transistor, and Rce2min is the minimum resistance value between the collector and emitter of the phototransistor.

Figure 2: Bistable circuit using photoelectric coupling

3. Shaping circuit composed of photocoupler

Since the pulse coupling circuit composed of photoelectric couplers has a relatively long leading and trailing edge time, a transistor shaping amplifier circuit is connected after the coupler. See Table 1 for several application examples of shaping circuits.

4. Chopper circuit composed of photocoupler

The skin chopping circuit composed of photoelectric coupler is shown in Table 2