Design a logic circuit to form part of a home freezer, providing a warning light, consisting of an LED, if (a) the temperature is above -5 ºC, b) the door is open, or c) the mains current is off. The warning light must go on if any or all of the conditions obtain.

Solution:

Explanation:

1. If the mains current is off (0), the NOT gate A will have an output of 1.
2. If the door is open (0), the NOT gate B will have an output of 1.
3. If the temperature sensor reports a temperature greater than -5 ºC (0), the NOT gate C will have an output of 1.
4. If either of the NOT gates B or C have an output of 1, the OR gate D will have an output of 1.
5. The diode F will light if one or both of the inputs into the OR gate E are 1, giving an output of 1 at that gate.

Our circuit will therefore satisfy the design criteria that the diode will go on if the temperature is too high or if the door is left open. If the mains current fails, the light will go on regardless of temperature or the status of the door.