9/10/2023 0 Comments Ttl opto isolator![]() ![]() ![]() The device shown in Figure 4 is known as a reflective optocoupler. The slotted optocoupler can thus be used in a variety of ‘presence’-detecting applications, including end-of-tape detection, limit switching, and liquid-level detection. The optocoupling can, however, be completely blocked by placing an opaque object in the slot. Here, light can normally pass from the LED to Q1 without significant attenuation by the slot. 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. Figures 3 and 4 show two other types of optocoupler. The Figure 2 device is a simple isolating optocoupler. Optocouplers can also be used to replace low-power relays and pulse transformers in many applications. Typical isolating optocoupler applications include low-voltage to high-voltage (or vice versa) signal coupling, interfacing of a computer’s output signals to external electronic circuitry or electric motors, etc., and interfacing of ground-referenced low-voltage circuitry to floating high-voltage circuitry driven directly from the main’s AC power lines, etc. The simple application circuit of Figure 2 can be used with digital input/output signals only but, in practice, this basic circuit can easily be modified for use with analog input/output signals, as shown later in this article. This ‘isolating’ characteristic is the main attraction of this type of optocoupler, which is generally known as an isolating optocoupler. 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. Alternatively, when SW1 is closed, current flows through the LED via R1, and the resulting light falls on Q1’s face, causing the phototransistor to conduct and generate an output voltage across R2. 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. Figure 2 shows the basic form of an optocoupler, together with a very simple application circuit. Most modern optocoupler devices use a phototransistor as their Rx unit such a device is known simply as an ‘optocoupler,’ since the input (the LED) and the output (the phototransistor) devices are optically coupled. Basic form and application circuit of a typical optocoupler. Basic form of an optocoupler device.įIGURE 2. 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.įIGURE 1. Figure 1 shows the basic form of such a device. An optocoupler device can be simply described as a sealed, self-contained unit that houses independently-powered optical (light) Tx and Rx units, that can be coupled together optically.
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