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The possibility of creating passive and active optoelectronic logic elements simultaneously performing the functions AND, NAND, OR, NOR, XOR, XNOR is considered. Passive logic elements, unlike active ones, do not have their own power sources and are powered by input control signals. The use of optoelectronics means allows for reliable electrical isolation of input and output circuits, including using open-type optical communication channels. In addition, optoelectronic logic elements are much simpler than the usual elements of digital logic.

**Passive optoelectronic logic element**

**Figure 1** shows an example of one of the circuits of a passive optoelectronic logic element of universal purpose. Such an element simultaneously performs the functions of AND, NAND, OR, NOR, XOR, XNOR, contains only 4 optoelectronic pairs U1–U4 and has 9 outputs; 6 of which with high (Y1 and Y2) and 3 with low (Y0) load capacity. Note that to simplify the logic element as much as possible, transistors and their accompanying resistors can be excluded from it and only outputs with low (Y0) load capacity can be used.

**Figure 1** Passive input optoelectronic logic element of universal purpose with high (Y1 and Y2) and low (Y0) load capacity.

Consider the operation of a logical element. If no control signals are applied to inputs X1 and X2, current does not flow through the LEDs of the optocoupler pairs U1.1, U2.1, U3.1, U4.1. Accordingly, a high logic level voltage will be present at all outputs Y0 of the logic element.

If a control signal of the “Log. 1” level is applied to the input X1, and “Log. 0” to the input X2, the current will flow through the LEDs of the optocouplers U1.1, U3.1. The voltage will not change at the output Y0 NAND, and at the outputs Y0 NOR and Y0 XNOR the voltage will decrease to the level “Log.0”.

When the control signal of the level “Log. 0” is fed to input X1 and “Log. 1” at the input X2, the current will flow through the LEDs of the optocoupler pairs U2.1, U4.1. The voltage will also not change at the output Y0 NAND, and at the outputs Y0 NOR and Y0 XNOR there will be a level of “Log. 0”.

Finally, if high-level control signals are applied to both inputs X1 and X2, current will flow through the LEDs of the optocoupler pairs U1.1, U2.1. The output of Y0 NAND will have a level of “Log. 0”; the output of Y0 NOR will also have a level of “Log. 0”; and the output of Y0 XNOR will be at the level of “Log. 1”.

Of course, the optoelectronic logic elements NAND/AND, NOR/OR or XNOR/XOR in Figure 1 can be used separately from each other.

The advantages and disadvantages of passive optoelectronic logic elements are obvious. On one hand, there is no external supply voltage source, on the other hand, the LEDs of the optocouplers consume a sufficiently high current from the signal sources.

**Active optoelectronic logic element**

Next consider the active optoelectronic logic element, **Figure 2**. Such elements have a high input resistance, but require a power supply E1 (for this, the power supply of the receiving part E2 can be used).

The principle of operation of the optoelectronic logic element in Figure 2 is somewhat different from Figure 1 and the differences is quite obvious. In the absence of input signals, current does not flow through the LEDs of the optocouplers. A low voltage level will be present at all outputs Y0 AND, OR and XOR, see **Table 1**.

**Figure 2** Active optoelectronic logic element of universal purpose with high (Y1 and Y2) and low (Y0) load capacity.

**Table 1** The truth table of the universal purpose optoelectronic logic element.

When the control signal of level “Log. 1” is applied to the input X1 and “Log. 0” to the input X2, the current flows through the LEDs of the optocoupler pairs U2.1 and U3.1. The output Y0 AND will have a level of “Log. 0”; the outputs Y0 OR and Y0 XOR will be “Log. 1”.

When “Log. 0” is applied to the input X1 and “Log. 1” to the input X2, the current flows through the LEDs of the optocoupler pairs U1.1 and U3.1. At the output Y0 AND there will be “Log. 0”; at the outputs Y0 OR and Y0 XOR there will be “Log. 1”.

If “Log. 1” is applied to both inputs X1 and X2, the current will flow only through the LED of the optocoupler pair U3.1. At the outputs Y0 AND and Y0 OR there will be a level of “Log. 1”; at the output Y0 XOR there will be “Log. 0”.

**Optoelectronic logic element with a passive or active input**

Another variant of an optoelectronic logic element with a passive or active input is shown in **Figure 3**. The receiving part of the device is based on a universal logic element on a single transistor.

**Figure 3** Universal purpose optoelectronic logic element with a passive (top left) or active (bottom left) input.

*This article was originally published on EDN.*

*Michael A. Shustov is a doctor of technical sciences, candidate of chemical sciences and the author of over 750 printed works in the field of electronics, chemistry, physics, geology, medicine, and history.*