# Universal logic element on one transistor and its applications

#### Article By : Michael A. Shustov

Here's a proposed design for a universal passive logic element of positive and negative logic, made on just one transistor.

A universal passive logic element of positive and negative logic, made on just one transistor, is proposed. The logic element has at least two inputs, as well as three outputs: an OR, an XOR, and an AND, working simultaneously. Examples of using such a device as part of a logic element “ONLY ONE OF TWO”, as well as part of an RS trigger, are given.

In modern electronics, there are practically no publications related to the creation of universal logic elements. Below is a description of a new universal passive logic element of positive and negative logic, made on just one transistor.

The universal logic element (Figure 1) contains only one bipolar or field-effect transistor Q1. It is capable of operating without its own power supply, it simultaneously performs the functions of logic elements OR, XOR, and AND. It can also be used as an element of positive or negative logic, which significantly expands the possibilities of its application.

Figure 1 Universal logical elements of positive and negative logic.

Consider the operation of the universal logic element of positive logic (Figure 1). If no control signals are applied to the inputs +X1 and +X2, the outputs +Y1 (OR), +Y2 (XOR), +Y3 (AND) have logical zero levels. The truth table can be seen below (Table 1).

Table 1 Truth table of universal logic elements of positive and negative logic.

If one of the inputs +X1 or +X2 is supplied with a voltage of the “Log. 1” level, and the second “Log. 0”, then the outputs +Y1 (OR) and +Y2 (XOR) will have “Log. 1” levels, and the output +Y3 (AND) will have “Log. 0”.

Finally, if the voltage level “Log. 1” is applied to both inputs +X1 and +X2, then the outputs +Y1 (OR) and +Y3 (AND) will have the levels “Log. 1”, and the output +Y2 (XOR) will have “Log. 0”.

Similar processes, but with a minus sign, will be observed during the operation of the universal logic element of negative logic.

Let’s consider further the possibilities of using universal logical elements as a logical element “ONLY ONE OF TWO”.

The logic element “ONLY ONE OF TWO” is a logic element that ensures the passage of only one of the two signals from the input of the element to its corresponding output. If two or more input signals coincide in time, the “ONLY ONE OF THE TWO” element will not pass any of them to the outputs (Figure 2, Table 2). The logical element “ONLY ONE OF TWO” is a special case of the logical element “ONLY ONE OF ALL” [1].

Figure 2 Logical element “ONLY ONE OF TWO”.

Table 2 The truth table of the logical element “ONLY ONE OF TWO”.

Figure 3 and Table 3 show the possibility of using two universal logic elements as an RS-trigger. When a short rectangular pulse is applied to the input X1 or X2, the trigger switches its state.

Figures 3. RS-trigger based on two universal logic elements.

Table 3 RS-trigger truth table using two universal logic elements.

The advantage of the logic elements discussed above is their exceptional simplicity, versatility of use and the ability to operate in a wide range of input voltages: from one to hundreds of volts.

The disadvantages of passive universal logic elements include their low load capacity, the small possibility of their sequential cascading in view of the progressive loss of output voltages.

References

1. M.A. Shustov, A.M. Shustov. Electronic Circuits for All, London: Elektor International Media BV, 2017, 397 p.