3.3.1. The 4-1 multiplexer experiment

Components:

• breadboard
• 5V power supply
• 5mm blue LED
• 6 x SPST pushbutton switches
• 6x 2.2KΩ resistors
• 7404 NOT, 2 x 7408 AND, 4078 8 input OR/NOR chips with configuration:
  • Pin 14 as VCC, Pin 7 as ground

Figure 3‑13 2 Chip 4078

In order to realize the circuit, and prove similarities between decoder and multiplexer, we can upgrade our binary decoder circuit from the 2-4 binary decoder experiment. If we add one more input to the AND circuits for A, B, C, and D inputs (by adding one more AND circuit in the chain), and connect their outputs through the OR circuit, we get a simple implementation of the multiplexer. For the realization of 4 input OR, we can use TTL chip 4078 – 8 input OR/NOR shown in the figure. The chip will be used in OR configuration, which means taking the output from pin 1. Since we have 4 more inputs than needed, they are going to be neutralized by grounding.

Figure 3‑14 4-1 multiplexer schematic

Figure 3‑15 4-1 multiplexer circuit

Explanation:. a and b selector inputs are used to select between A, B, C and D inputs whose value will appear at the output that can be demonstrated by the LED.

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