In electric circuits, we use mostly filters to remove noise. The circuit based on the capacitor and resistor always removes the noise from the incoming signal but the value capacitor and resistor always depend on the incoming signal. This circuit is only applicable where the incoming signal has some information or data but when we just need to forward the signal from one part of the circuit to the other part but signal contains noise, then we could use the combination of IR sender and receive. In PC photoisolator IC circuit, the IR receives the noisy signal as a power from the one circuit and passes it to the other part through the IR signal. The other part receives the signal and then performs according to the circuit design.

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Understand the requirements for a typical optocoupler application. There are many different applications for optocoupler circuits, so there are many different design requirements, but a basic design for an optocoupler providing isolation for example between two circuits, simply involves the choice of appropriate resistor values for the two resistors R1 and R2 shown in Fig.

The Schmitt inverter at the output performs several functions; it ensures that the output conforms to HCT voltage and current specifications, it also provides very fast rise and fall times for the output, and corrects the signal inversion caused by the phototransistor being operated in common emitter mode.

Each logic family e. LSTTL or CMOS types may have different logic voltage levels and different input and output current requirements, and optocouplers can provide a convenient way of interfacing two circuits with different logic levels. What is necessary is to ensure that R1 creates an appropriate current level from the input circuit to correctly drive the LED side of the optocoupler, and that R2 creates appropriate voltage and current levels to supply the output circuit via the inverter.

In this simple example the input and output supplies will most likely be the same in voltage and current capabilities, so the interface is just providing isolation without any major shift in voltage or current levels. In choosing appropriate values for R1, the value for the current limiting resistor is set to produce the correct forward current IF through the infrared LED in the optocoupler.

R2 is the load resistor for the phototransistor and the values of both resistors will depend on a number of factors. Manufacturers will normally quote a range of CTR values for different output phototransistor collector voltages VC and different ambient temperatures TA The CTR will also vary with the age of the optocoupler, as the efficiency of LEDs decreases with age over s of operating hours.

Because the CTR of an optocoupler can be expected to reduce over time, it is common practice to choose a value for IF somewhat lower than the maximum, so that the intended performance can still be achieved over the intended lifetime of the circuit.

Although this example describes the design of a simple interface linking two HCT logic circuits, the difference between the results achieved here and those needed for any other optocoupler are that similar calculations can be made just using data appropriate to other voltages and currents and other optocouplers.

Calculating the Optocoupler Resistor Values Fig. Forward Current for a PC The start of the design process is to specify the input and output conditions the optocoupler is to link. Typical optocouplers can handle input and output currents from a few microamps to tens of milliamps. Assuming that a single HCT output is only feeding this optocoupler, a logic 1 voltage of about 4.

The output current available from a HCT gate to drive the optocoupler input is limited to 4mA, which is quite low for driving an optocoupler. The PC must then be capable of producing the necessary output from this low input current. The graph in Fig. Ideally the optocoupler should in this case act as though it is invisible, that is the HCT gate connected to the optocoupler output should see an available current of up to 4mA, just as though it was connected to the output of another HCT gate.

Having found an approximate figure for the CTR, which suggests that input and output conditions should be similar, at 4mA, the next task is to calculate the values of R1 and R2. Using the data in Table 5. IF in Fig. Both of the waveforms shown were taken with the same input, a square wave with a frequency of 2kHz, but with two different values for R2, 1.

The rounding effect on the rise time of the pulses can be clearly seen in Fig. Also at higher frequencies amplitude of the output signal reduces markedly. Note also the effect of using a 74HCT14 Schmitt inverter at the output; any rounding of the square pulses is eliminated and although the optocoupler output only falls to 0. Adding a Schmitt inverter also re-inverts the output waveform, which was an inverted version of the input waveform at the collector of the phototransistor.

There are of course, more useful applications for an optocoupler than simply isolating one logic IC from another. Computers are expensive and easily damaged by mistakes made when connecting them to external circuitry. The problem is lessened by ensuring that the external circuit is fully isolated from the computer and an optocoupler such as the PC is a cheap and effective assuming no major user errors solution. This is more than the 5mA minimum required to drive the 2N into saturation.

It is important that the transistor is fully saturated in order to reduce the power dissipation in the 2N to a minimum, therefore although the transistor current ICE is 40mA there will only be about 0. Although this basic interface only allows for switching the motor on or off, it could easily be adapted by changing IC1 to include a pulse width modulated speed control either from a computer, or hardware generated as described in Oscillators Module 4.

This simple interface has one more safety feature; diode D1 connected across the motor will effectively prevent any nasty back EMF spikes generated by the inductive load the motor from causing damage to the interface. Motor Drive Circuit Video.


Isolating Circuits From Your Arduino With Optocouplers

This IC is used to provide electrical isolation between two circuits, one part of the circuit is connected to the IR LED and the other to Photo-transistor. The digital signal given to the IR LED will be reflected on the transistor but there will be no hard electrical connection between the two. This comes in very handy when you are trying to isolate a noisy signal from your digital electronics, so if you are looking for an IC to provide optical isolation in your circuit design then this IC might be the right choice for you. Then Pull high the collector pin of the transistor using a resistor here I have used 1K and connect the collector pin to the output of your desired logic circuit. The Emitter pin 4 is grounded. This is where the isolation occurs. There pull-up resistor 1K acts as a load resistor.


Opto Couplers – Types & Applications


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