Level-Shifting Opto-Isolator Circuits

Opto-isolator devices may be used for voltage level-shifting purposes. Below are examples of level-shifting circuits that utilize opto-isolators (one non-inverting and one non-inverting).

Figure 1. Non-inverting Voltage Level Shifter using an Opto-Isolator

Figure 1 shows an opto-isolator that's configured as a non-inverting voltage level shifter (5V to 12V). When the input is high (close to 5V), the light-emitting diode does not conduct. With no light shining on the phototransistor, it is also off, causing the collector-tapped output to be very near 12V. When the input is low (close to 0V), the LED conducts, shining light on the phototransistor. The transistor turns on and pulls the output to ground.

Figure 2. Inverting Voltage Level Shifter using an Opto-Isolator

Figure 2 shows an opto-isolator that's configured as an inverting voltage level shifter (5V to 0V and 0V to 12V). When the input is high (close to 5V), the light-emitting diode does not conduct. With no light shining on the phototransistor, it is also off, causing the emitter-tapped output to be also be 'low'. When the input is low (close to 0V), the LED conducts, shining light on the phototransistor. The transistor turns on and pulls up the output to very near 12 V.

See Also: Opto-Isolators; Light-Emitting Diodes

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