Figure 1 shows the basic form of such a device. Here, the Tx unit is a LED, but the Rx unit may take the form of a phototransistor, a photo-FET, an opto-triac, or some other type of photo-sensitive semiconductor element; the Tx and Rx units are housed closely together in a single, sealed package. Basic form of an optocoupler device. Basic form and application circuit of a typical optocoupler.

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Figure 1 shows the basic form of such a device. Here, the Tx unit is a LED, but the Rx unit may take the form of a phototransistor, a photo-FET, an opto-triac, or some other type of photo-sensitive semiconductor element; the Tx and Rx units are housed closely together in a single, sealed package. Basic form of an optocoupler device. Basic form and application circuit of a typical optocoupler.

Figure 2 shows the basic form of an optocoupler, together with a very simple application circuit. Here, when SW1 is open, no current flows in the LED, so no light falls on the face of Q1; Q1 passes virtually zero collector current under this condition, so zero voltage is developed across output resistor R2. Major points to note about the Figure 2 optocoupler are that its output current is controlled by its input current, that a control circuit connected to its input can be electrically fully isolated from the output circuit, and that — since the input controls the output via a purely optical link — potential differences of hundreds of volts can safely exist between the input and output circuits.

Optocouplers can also be used to replace low-power relays and pulse transformers in many applications. Figures 3 and 4 show two other types of optocoupler. The device shown in Figure 3 is known as a slotted optocoupler, and has a slot molded into the package between the LED light source and the phototransistor light sensor. Here, light can normally pass from the LED to Q1 without significant attenuation by the slot. The optocoupling can, however, be completely blocked by placing an opaque object in the slot.

Slotted optocoupler device. Reflective optocoupler. The device shown in Figure 4 is known as a reflective optocoupler. Here, the LED and Q1 are optically screened from each other within the package, and both face outwards towards a common point from the package. The construction is such that an optocoupled link can be set up by a reflective object such as metallic paint or tape, or even smoke particles sited a short distance outside the package, in line with both the LED and Q1.

The reflective optocoupler can thus be used in applications such as tape-position detection, engine-shaft revolution counting or speed measurement, or smoke or fog detection, etc. One of the most important parameters of an optocoupler device is its optocoupling efficiency and, to maximize this parameter, the LED and the phototransistor which usually operate in the infrared range are always closely matched spectrally. The most convenient way of specifying optocoupling efficiency is to quote the output-to-input current transfer ratio CTR of the device, i.

In practice, CTR may be expressed as a simple figure such as 0. It should be noted that, because of variations in LED radiation efficiency and phototransistor current gains, the actual CTR values of individual optocouplers may vary significantly from the typical value. This is the maximum permissible DC potential that can be allowed to exist between the input and output circuits. Typical values vary from V to 4kV. This is the maximum allowable DC voltage that can be applied across the output transistor.

Typical values vary from 20V to 80V. IF MAX. Typical values vary from 40mA to mA. This is the typical maximum signal frequency that can be usefully passed through the optocoupler when the device is operated in its normal mode.

Typical values vary from 20kHz to kHz, depending on the type of device construction. Typical simple a and Darlington b isolating optocouplers. Optocouplers are produced by several manufacturers and are available in a variety of forms and styles. Simple optocouplers are widely available in six basic forms, which are illustrated in Figures 6 to 8.

Four of these Figures 6 and 7 are isolating optocouplers, and the remaining two are the slotted optocoupler Figure 8 a and the reflective optocoupler Figure 8 b. The table of Figure 9 lists the typical parameter values of these six devices. The simple isolating optocoupler Figure 6 a uses a single phototransistor output stage and is usually housed in a six-pin package, with the base terminal of the phototransistor externally available.

The phototransistor can, however, be converted to a photodiode by shorting the base pin 6 and emitter pin 4 terminals together; under this condition the CTR value falls to about 0. Typical dual a and quad b isolating optocouplers.

The Darlington optocoupler Figure 6 b is also housed in a six-pin package and has its phototransistor base externally available.

The dual and quad optocouplers of Figure 7 use single-transistor output stages in which the base terminal is not externally available. Note in all four isolating devices that the input pins are on one side of the package, and the output pins are on the other.

This construction gives the maximum possible values of isolating voltage. Also note in the multichannel devices of Figure 7 that, although these devices have isolating voltages of 1. Typical slotted a and reflective b optocouplers. Isolating voltage values are not specified for the slotted and reflective optocoupler devices of Figure 8. Finally, the reflective optocoupler of Figure 8 b uses a Darlington output stage and has a useful bandwidth of only 20kHz.

Even so, the device has a typical minimum CTR value of only 0. Typical parameter values of the Figures 6 to 8 devices. The following notes give a summary of the salient usage points. The LED current must be limited by a series resistor, which can be connected to either the anode a or the cathode b. The input LED can be protected against reverse voltages via an external diode. The input current to the optocoupler LED must be limited via a series-connected external resistor which, as shown in Figure 10, can be connected on either the anode or the cathode side of the LED.

If the LED is to be driven from an AC source, or there is a possibility of a reverse voltage being applied across the LED, the LED must be protected from reverse voltages via an external diode connected as shown in Figure This resistor can be connected to either the collector or the emitter of the phototransistor, as shown in Figure The greater the value of this resistor, the greater is the sensitivity of the circuit, but the lower is its bandwidth. An external output resistor, wired in series with the phototransistor, can be connected to either the collector a or emitter b.

If its base is available, the phototransistor can be made to function as a photodiode a , or its CTR values can be varied via RV1 b. In normal use, the phototransistor is used with its base terminal open-circuit. If desired, however, the phototransistor can be converted into a photodiode by using the base terminal as shown in Figure 13 a and ignoring the emitter terminal or shorting it to the base.

This connection results in a greatly increased bandwidth typically 30MHz , but a greatly reduced CTR value typically 0. Alternatively, the base terminal can be used to vary the CTR value of the optocoupler by wiring an external resistor RV1 between the base and emitter, as shown in the Darlington example of Figure 13 b. Figure 14 shows how to interface two TTL circuits, using an optocoupler circuit that provides a non-inverting action. TTL interface.

CMOS interface. This snag is overcome in the Figure 14 circuit by fitting an external pull-up resistor R3 as shown. Consequently, these devices can be interfaced by using a sink configuration similar to that of Figure 14, or they can use the source configuration shown in Figure In either case, the R2 value must be large enough to provide an output voltage swing that switches fully between the CMOS logic-0 and logic-1 states.

Computer-driven DC motor. With the computer output high, the optocoupler LED and phototransistor are both off, so the motor is driven on via Q1 and Q2. When the computer output goes low, the LED and phototransistor are driven on, so Q1-Q2 and the motor are cut off.

Figure 17 shows this technique used to make an audio-coupling circuit. Audio-coupling circuit. This terminal is DC-biased at half-supply volts via the R1-R2 potential divider, and can be AC-modulated by an audio signal applied via C1. On the output side of the optocoupler, a quiescent current is set up by the optocoupler action in the phototransistor, and causes a quiescent voltage to be set up across RV1, which should have its value adjusted to give a quiescent output value of half-supply voltage.

TRIAC INTERFACING An ideal application for the optocoupler is that of interfacing the output of a low-voltage control circuit possible with one side of its power supply grounded to the input of a triac power-control circuit that is driven from the AC power lines and which can be used to control the power feed to lamps, heaters, and motors.

Simple non-synchronous triac power switch with optocoupled input. Thus, when SW1 is open, the optocoupler is off, so zero base drive is applied to Q1, and the triac and load are off.

Such devices are readily available, in both simple and complex forms; some sophisticated triac types incorporate interference-suppressing, zero-crossing switching circuitry in the package. Typical optocoupled SCR a and triac b. Figure 19 a and 19 b show the typical outlines of simple optocoupled SCRs and triacs which are usually mounted in six-pin DIL packages ; Figure 20 lists the typical parameters of these two particular devices, which have rather limited rms output-current ratings, the values being in the examples shown mA for the SCR and mA for the triac.

Figures 21 to 23 show various ways of using an optocoupled triac; R1 should be chosen to pass an LED current of at least 20mA; all other component values are those used with a V AC supply. In Figure 21, the triac is used to directly activate an AC line-powered filament lamp, which should have an rms rating of less than mA and a peak inrush current rating of less than 1.

Low-power lamp control. High-power control via a triac slave. Figure 22 shows how the optocoupled triac can be used to activate a slave triac and, thereby, activate a load of any desired power rating. This circuit is suitable for use only with non-inductive loads such as lamps and heating elements, using a triac of suitable rating. Driving an inductive load. Finally, Figure 23 shows how the above circuit can be modified for use with inductive loads such as electric motors.

The R2-C1-R3 network provides a degree of phase-shift to the triac gate-drive network, to ensure correct triac triggering action, and R4-C2 form a snubber network, to suppress rate-of-rise rate effects. Siemens are the present market leaders in the optocoupled SSR field.

The device has an isolation voltage rating of 3. Other devices in the Siemens optocoupled SSRs range include ones that have outputs that act as single-pole or two-pole NC, NO, or change-over switches.


4N32 - Optocoupler DIP-6 30 V, Vishay

Fortunately there are lots of schematics and discussions around. MIDI is simply serial communication at a baud rate so the Arduino should be able to read it using a serial Rx pin. The MIDI standard requires the devices to be electrically isolated so an optocoupler is usually used on the receiving end. An often quoted resource is this post to the Arduino forum. The hand-drawn schematic is pretty charming and it makes it all seem easy enough. The comments suggest both that this is a popular topic and that despite it being a relatively simple circuit people have a lot of trouble making it work. Initially it was simply a matter of getting any signal at all.


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