2.1.1. NOT gate experiment

Components:

• breadboard
• 5V power supply
• 3mm red LED
• SPST pushbutton switch
• BC547 NPN transistor
• 220Ω, 2.2KΩ resistors

Figure 2‑2 Transistor realization of NOT gate

Explanation: It is easy to notice that this experiment is very similar to the transistor as a switch experiment. The only difference is that we take collector output, instead of emitter output. That way the LED is lit, or the value of the bit is 1 if the pushbutton is not pressed. Bringing the signal from outside, by pressing the pushbutton, the bit is inverted to 0 value. Since the current tries to find the path of least resistance, it will ground itself directly through the transistor as a closed switch, instead of going through the diode. In other words, when the transistor is in saturation, it acts like a closed switch and there is no voltage differential between collector and emitter, and hence no voltage to light a LED.

We will further use chip 7404 which contains six NOT logic gates, organized according to the figure below.

Figure 2‑3 Chip 7404

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