2.3.1. NAND gate experiment

Components:

• breadboard
• 5V power supply
• 3mm red LED
• 2 x SPST pushbutton switches
• 2 x BC547 NPN transistors
• 220Ω, 2 x 2.2KΩ resistors

Figure 2‑8 Transistor realization of NAND gate

Explanation: Pressing the first and second pushbutton at the same time drives the transistors to saturation and they behave as closed switches. In that case, the current finds its path of least resistance through transistors, and the LED turns off. Now that we have built the NAND gate with transistors, we can use the TTL chip 7400. The chip contains four NAND logic gates, connected according to the figure below.

Figure 2‑9 Chip 7400

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