Simple Applications

For details of those applications listed below, please consult Ref.[5].

Latched fire alarm using a bistable

Figure 4.1 shows a fire alarm which, once triggered, will continue to sound until it is reset.

Figure 4.1: A latched fire alarm

Stop-go traffic lights

Figure 4.2 shows a simple set of stop-go traffic lights operated by an SDPT switch. Either the red is on or the green light is on, but not both together.

Figure 4.2: A stop-go traffic lights

Quiz master

Figure 4.3 shows a circuit for each contestant of a quiz master used to identify the first contestant to push his or her answer button in a quiz game. Each contestant has a push-button and an indicator light. The light of the first person to answer should come and stay on. At the same time all other lights should be prevented from coming on.

Figure 4.3: The circuit for each contestant of a quiz master

The numbers in Figure 4.3 are the pin numbers if a 74LS00 I.C. (Quad 2-input NAND gate) is used. However, the circuit is only suitable in a quiz master for two contestants. When there are 4 contestants, we have to replace the 2-input NAND gate with the input A by a 4-input NAND gate such as the 74LS20 (Dual 4-input NAND gate).

Problem 4.1

 The resistance of a thermister decreases as the temperature increases. Hence describe the principles behind the operation of the circuit in Figure 4.1.

Problem 4.2

Referring to Figure 4.2, explain why either the red LED is on or the green LED is on, but not both together.

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