A burglar alarm system uses a reed switch as a sensor on a window. The reed switch consists of two thin ferromagnetic metal strips (reeds) sealed inside a s...
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A burglar alarm system uses a reed switch as a sensor on a window. The reed switch consists of two thin ferromagnetic metal strips (reeds) sealed inside a small glass tube. A small permanent magnet is mounted on the window frame, and the reed switch is mounted on the fixed part of the frame, directly adjacent to the magnet. When the window is closed, the magnet holds the reeds together, completing the circuit. When the window is opened, the magnet moves away, the reeds spring apart, and the circuit is broken. This triggers the alarm. Fig. 4.1 shows the arrangement and the alarm circuit.
(a) Explain why the reed switch closes when the magnet is nearby. [2]
(b) Describe what happens in the alarm circuit when the window is opened. [2]
(c) State why the reed switch reeds must be made of a ferromagnetic material. [1]
(d) The alarm system uses a relay between the reed switch circuit and the alarm bell circuit. Explain the purpose of the relay. [2]
(e) Suggest one advantage of using a reed switch rather than a mechanical push-button switch for this alarm. [1]
Answer Details
(a) Why the reed switch closes when the magnet is nearby [2 marks]
The magnetic field from the permanent magnet extends into the glass tube and magnetises the ferromagnetic reeds. [1] Each reed becomes a temporary magnet. Because the two reeds are close together and aligned, opposite poles form at their tips, so they attract each other and make contact, completing the circuit. [1]
(b) What happens when the window is opened [2 marks]
When the window opens, the permanent magnet (mounted on the window) moves away from the reed switch (mounted on the fixed frame). [1] Without the nearby magnetic field, the reeds are no longer magnetised. They spring apart under their own elasticity, breaking the sensor circuit. This change in the reed switch circuit activates the relay, which switches on the separate alarm bell circuit. [1]
(c) Why the reeds must be ferromagnetic [1 mark]
Ferromagnetic materials (such as iron or nickel alloys) can be magnetised by an external magnetic field. If the reeds were made of a non-magnetic material, they would not respond to the permanent magnet and the switch would never close. [1]
(d) Purpose of the relay [2 marks]
The relay allows the small current flowing in the reed switch/sensor circuit to control a separate, much larger current in the alarm bell circuit. [1] This protects the delicate reed switch from the high current needed to power the alarm bell, and allows each circuit to operate on a different voltage supply. [1]
A relay works by using the small sensor current to energise an electromagnet coil, which then closes a switch in the high-power alarm circuit.
(e) Advantage of a reed switch over a mechanical push-button [1 mark]
The reed switch is sealed inside a glass tube, so it has no exposed moving parts. This makes it more reliable (unaffected by moisture, dust, or corrosion), harder for an intruder to see or tamper with, and longer-lasting than a mechanical switch. [1]
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