555-logic: designing digital circuits with 555s only!

The basic idea of 555-logic is to implement digital logic circuits with 555 ICs only. This gives advantage that a relatively higher and wide range of power supply voltage can be used, limited by the specification of the 555 IC (4.5V - 16V). According to Shannon through his famous 1937 Masters thesis at MIT, any combinational logic circuits can be made with switches. With semiconductor technology, these switches can be in the form of transistors. To use 555 as a transistor switch, one can utilize the internal pull-down transistor of a 555 (Figure 1).

Figure 1  Using the transistor of a 555

There is a restriction on how to use this transistor as its emitter is connected to ground pin (4). Even though one may be tempted to connect that emitter (or ground pin) to something other than ground voltage, this will not guarantee to work since the ground voltage is needed by other parts of the 555 IC to function. Therefore, the use of the 555 transistor as a switch is restricted to the implementation of pull-down network of switches with no cascading of emitters to collector of another transistor. This leaves a reasonable choice to implement a NOR gate. A NOR gate can be made with two pull-down transistors with collector pins connected together forming the output. A pull up resistor is to be connected between these collector pins and VCC, and a 555 can be used for this since a series of three 5ks resistors exist in a 555. The implementation of such a NOR gate with 555 is shown in Figure 2.