Fig. 8 shows a logic circuit used in a greenhouse ventilation system. The fan turns on when the temperature is high AND the light level is high. The paint k...
Question 1 Report
Fig. 8 shows a logic circuit used in a greenhouse ventilation system. The fan turns on when the temperature is high AND the light level is high.
The paint keeps out the damp. It is made to last. It can be fixed if it breaks.
The surface of the cast iron can be improved by applying a powder coating.
(a) Name the type of logic gate at position P. [1]
(b) Name the type of logic gate at position Q. [1]
(c) Complete the truth table for the circuit.
| Thermistor input | LDR input | Output of gate P | Output Z |
|---|---|---|---|
| 0 | 0 | ||
| 0 | 1 | ||
| 1 | 0 | ||
| 1 | 1 |
[2]
[Total: 3 marks]
Answer Details
(a) The type of logic gate at position P is an AND gate [1]. The AND gate symbol is recognisable by its D-shape (flat back with a curved front) and the "&" symbol inside. An AND gate produces a HIGH (1) output only when ALL of its inputs are HIGH (1). If either input or both inputs are LOW (0), the output is LOW (0).
(b) The type of logic gate at position Q is a NOT gate (inverter) [1]. A NOT gate takes a single input and produces the opposite output: if the input is 1, the output is 0, and if the input is 0, the output is 1. It inverts whatever signal it receives.
(c) The completed truth table for the circuit:
| Thermistor input | LDR input | Output of gate P (AND) | Output Z (NOT) |
|---|---|---|---|
| 0 | 0 | 0 | 1 |
| 0 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 0 |
[1 mark per fully correct column, maximum 2]
The AND gate (P) produces a 1 only when both inputs are 1 (thermistor HIGH and LDR HIGH). In all other input combinations, at least one input is 0, so the AND gate outputs 0. The NOT gate (Q) then inverts each of these values: the three 0 outputs from gate P become 1 at output Z, and the single 1 output from gate P becomes 0 at output Z. The result is that Z is LOW (0) only when both the temperature and light level are high simultaneously.
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