Fig 15 shows a circuit for a simple door alarm used in a workshop storage cupboard. The circuit uses standard symbols to represent each component. A magneti...
Question 1 Report
Fig 15 shows a circuit for a simple door alarm used in a workshop storage cupboard. The circuit uses standard symbols to represent each component. A magnetic reed switch mounted on the door frame detects whether the cupboard door is open or closed. When the door opens, the magnetic field is broken, the switch changes state and the buzzer is activated through a transistor. An enable switch on the outside of the cupboard allows the user to arm or disarm the alarm circuit. The circuit is powered by a 6V battery. The fig shows the cupboard with the reed switch and buzzer mounted on the door frame and the enable switch on the outside panel.
(a) The magnetic reed switch is a normally closed switch when the door is shut. Give one reason why a normally closed switch is more suitable than a normally open switch for this alarm circuit. [1]
(b) Name the logic gate that could combine the reed switch signal and the enable switch signal so that the buzzer sounds only when the enable switch is on AND the door is opened. [1]
(c) Draw the standard circuit symbol for the logic gate named in (b). [1]
(d) Give one advantage of using a transistor as a switch to drive the buzzer rather than connecting the buzzer directly to the output of the logic gate. [1]
Answer Details
(a) A normally closed switch provides a fail-safe design. [1]
If the switch fails, a wire is disconnected, or the magnetic alignment is disturbed, the alarm sounds because the circuit detects an open condition (the same state as the door being open). With a normally open switch, a failure or broken wire would silently disable the alarm with no warning, creating a false sense of security.
(b) The logic gate needed is an AND gate. [1]
The AND gate outputs high only when both inputs are high simultaneously: the enable switch must be on (armed) AND the reed switch must detect an open door. If either condition is not met, the buzzer remains silent.
(c) Standard AND gate symbol: [1]
The AND gate has a flat left edge (back), a curved right edge (front), two input lines on the left, and one output line on the right.
(d) Advantage of using a transistor as a switch to drive the buzzer: [1]
A transistor can supply sufficient current to drive the buzzer at full volume, whereas the output of a logic gate has limited current capability (typically only a few milliamps) and may not power the buzzer adequately. The transistor acts as a current amplifier between the low-power logic output and the higher-power buzzer.
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