CASE STUDY


AND logic gates Both switches need to be on in AND logic gates for the system to work. For example, to open a garage door, you must drive up to it and hoot. If you do only one of those things, the door will not open. Try to think of other possible examples where AND logic is used.


OR logic gates If two switches are in parallel and either one of the switches is on, then the circuit is completed. This is called the OR logic gate. We use OR gates if we want one of two things to happen. For example, if we have a system to call learners in from break attached to an electric bell, we could have a switch in the principal’s office and another switch in another classroom. If either switch is pressed on, the bell will sound. This is an example of OR logic. Try to think of another example where OR logic is used.


Truth tables


Truth tables help us to summarise systems that use logic. We deal with two inputs in the simplest of these systems. Each input could be ON or OFF. There are four states the circuit can be in. Look at Tables 1 and 2 below. Table 1 Truth table for AND gate circuit Table 2 Truth table for OR gate circuit


Input


1 off 2 on 3 off 4 on


Output


Switch 1 Switch 2 Lamp is: off off on on


off off off on


1 off 2 on 3 off 4 on


Input Output


Switch 1 Switch 2 Lamp is: off off on on


off on on on


152 Practical Assessment Task


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