[repost] 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 (1).
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