3.4.1. AND-OR-INVERT circuit experiment

Components:

• breadboard
• 5V power supply
• 3mm red LED
• 4 x SPST pushbutton switches
• 220Ω, 4 x 2.2KΩ resistors
• 7408 AND, 7402 NOR chip with configuration:
  • Pin 14 as VCC, Pin 7 as ground
  • the rest is self-explanatory from the schematic

Figure 3‑19 AND-OR-INVERT scheme

Figure 3‑20 AND-OR-INVERT circuit

Note: For purpose of simplicity, we can get away without using a current limiting resistor to show results on the LED, when using the output of TTL chips.

Explanation: The circuit is self-explanatory, considering the explained AND and NOR gates. The fascinating thing about this circuit is that it can be used in many scenarios. For example, if we press A input the following happens:

• U1D results in low output
• U1C results in a value of B input and U2D inverts; it
  • if B is high, U1C is high and the output of U2D is low
  • if B is low, U1C is low and the output of U2D is high

That means that this circuit can be used as a multiplexer that passes and inverts one of the inputs. Further, the circuit can be also used as an XOR gate. Let us see what will happen if we consider the following transformations to the inputs:

Figure 3‑21 Dr. Holden’s XOR gate from chip 7450

Since both inputs of the NOR gate must be low to produce a high output, it is possible only if A and B are different. An unwanted high input in the NOR gate is produced in the following scenarios:

• if A and B are both high, the output of the upper AND gate is high
• if A and B are both low, the output of the bottom AND gate is high

The versatility of this circuit and hence chip 7450 is cleverly used for calculating the vertical velocity of the ball in the Pong circuit, and it will be explained in detail later.

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References:

Dr. Hugo Holden – Lawn Tennis: https://www.worldphaco.com/uploads/LAWN_TENNIS.pdf