What it shows
A reed switch closes when a magnet is near and opens when it moves away. With the magnet on the door and the switch on the frame, opening the door removes the magnet, and a transistor circuit turns the buzzer on, the same way shop door sensors work.
The science
The reed switch has two thin ferromagnetic strips inside a glass tube that touch when a magnet is close. In this circuit the 10 kΩ resistor tries to pull the transistor base up to 9 V through the 1 kΩ base resistor, but while the door is shut the reed switch is closed and shorts that point to ground, so the base stays at zero and the buzzer is silent. Opening the door moves the magnet away, the reed contacts spring apart, the base rises above 0.7 V, the transistor turns on and current flows through the buzzer.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A normally open glass reed switch
- A small magnet such as a fridge magnet or a neodymium disc
- One BC547 transistor
- A 10 kΩ resistor and a 1 kΩ resistor
- An active buzzer rated 3 to 12 V
- A 9 V battery with a snap connector
- A breadboard, jumper wires and tape
Safety
For this project: Reed switches are thin glass; bend the leads gently with pliers, never near the glass body. Keep small magnets away from young children.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Place the BC547 on the breadboard; connect its emitter to ground and the battery negative to the ground rail.
Connect the buzzer red lead to the 9 V rail and its black lead to the collector.
Connect the 10 kΩ resistor from the 9 V rail to a spare row, and the 1 kΩ resistor from that row to the base.
Connect the reed switch from the spare row to ground using two long leads taped to it.
Snap on the battery: with no magnet near, the buzzer sounds. Bring the magnet to the reed switch: silence.
Tape the reed switch to the door frame and the magnet to the door so they line up within 1 cm when shut.
Shut the door: silent. Open it: the buzzer sounds until it shuts again.
Measure how far the magnet can move before the alarm sounds and note it as the sensing distance.
What you should see
The alarm is silent while the door is shut and sounds as soon as it opens by more than a centimetre or two.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What makes the reed switch close?
The magnet magnetises the two strips so they attract and touch.
2Why does the buzzer sound when the switch opens, not closes?
The switch holds the base at ground; opening it lets the pull-up resistor turn the transistor on.
3What is the 1 kΩ resistor for?
It limits the base current so the transistor is not damaged.
4Why not put the buzzer directly in series with the reed switch?
Then it would sound while the door is shut, and the reed contacts are too delicate for buzzer current anyway.
Take it further
Add a relay latch so the alarm keeps sounding even after the door is pushed shut again.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.