What it shows
A telegraph key is a spring switch that closes a circuit for short or long moments. Pressing the paper clip onto a drawing pin completes the loop, and the pattern of short and long beeps carries letters in Morse code.
The science
The paper clip is bent so that one end is pinned down and the other hovers a few millimetres above a second drawing pin. The clip is springy steel, so it returns upward when released. Pressing it down touches the two pins through the clip, the circuit from battery to buzzer is complete and the buzzer sounds. Morse code turns letters into dots (a short press) and dashes (a press three times as long), so timing, not voltage, carries the message.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A large paper clip and two drawing pins
- A small wooden block or thick cardboard base
- An active buzzer rated 3 to 12 V
- A 2 x AA battery holder with cells (3 V)
- Three short insulated wires
- A printed Morse code chart
Safety
For this project: Drawing pins are sharp; an adult should push them into the block. Keep small parts away from younger 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.
Bend the paper clip so one end lies flat on the block and the other end rises about 5 mm above the surface.
Pin the flat end to the block with a drawing pin, trapping one bare wire end under the pin head.
Push the second drawing pin into the block directly below the raised end, trapping a second wire under its head.
Connect the first wire to the battery holder red lead and the second wire to the buzzer red lead.
Connect the buzzer black lead to the battery holder black lead to finish the loop.
Press the clip down onto the pin: the buzzer sounds. Adjust the bend so it springs clear when released.
Practise SOS: three short presses, three long, three short, with a pause between letters.
Send your name to a friend who reads the chart, then swap roles.
What you should see
Each press gives a clean beep that stops on release, and a partner can decode short words sent in Morse.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the paper clip doing in this circuit?
It is a spring-return switch that closes the circuit only while pressed.
2How is a dash different from a dot?
A dash is held about three times longer than a dot.
3Why does the buzzer work directly from 3 V without a resistor?
An active buzzer has its own oscillator and is rated for 3 V; it draws only a small current.
4Why was Morse code useful before telephones?
It needs only a single wire and a switch, so it worked over long telegraph lines.
Take it further
Add an LED with a 150 Ω resistor in parallel with the buzzer so the message can be seen as well as heard.
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.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each