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Boolean Logic and Logic Gates: Question 6

Syllabus 10.1, 10.2, 10.3

Multiple choice 1 mark

GreenGro Robotics builds an automated drip-irrigation controller for a single planter box. The controller has one soil-moisture sensor input, M, where M = 1 means the soil is currently wet and M = 0 means the soil is currently dry. The controller's pump-enable output, P, must switch to 1 (turn the pump on) exactly when the soil is not wet, and must be 0 (keep the pump off) whenever the soil is wet.

Which single logic gate, applied to input M, produces this behaviour for P?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Restate the requirement as input/output pairs

The description says:

  • M = 0 (soil dry) must give P = 1 (pump on).
  • M = 1 (soil wet) must give P = 0 (pump off).

So the output must always be the opposite of the single input M.

Step 2: Recall which gate takes one input and inverts it

A NOT gate has exactly one input and produces the opposite value: NOT 0 = 1 and NOT 1 = 0. Applying this to M gives exactly the required behaviour: P = NOT M.

Step 3: Rule out the other options

  • NAND and NOR both require two inputs to combine, but this circuit has only one sensor signal, M, so neither can be wired here regardless of how “inverted” their names sound.
  • XOR also requires two inputs, since it works by comparing whether two signals differ from each other. There is nothing here for M to be compared against.

Final answer

The gate is NOT, so P = NOT M, option C.