What it shows
Two keys and two indicator LEDs on one long wire pair let either station send to the other. It shows that a signal is really a current switched on and off, and that one shared return wire is enough for a two-way line.
The science
Both keys are wired in parallel across the line, and both LEDs with their resistors are also in parallel across the line. When either key is pressed, the battery drives current along the wire pair and through both LEDs, so the sender and the receiver see the same flashes. The 150 Ω resistors limit each LED to about 8 mA. Because the wire resistance of 10 m of bell wire is under one ohm, the brightness at the far end is the same as at the sending end.
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.
- Two paper clip keys on wooden blocks (see the single key project)
- Two red LEDs and two 150 Ω resistors
- A 2 x AA battery holder with cells (3 V)
- About 10 m of twin insulated wire (bell wire or old speaker wire)
- Tape and a Morse chart for each station
Safety
For this project: Run the long wire along a wall, not across a doorway where someone could trip.
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
9 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.
Build two paper clip keys on separate blocks as in the single key project.
Mount an LED and a 150 Ω resistor in series on each block, LED long leg towards the resistor.
At station A connect the battery red lead to one wire of the long pair and the battery black lead to the other.
At station A wire key A and the LED pair so that pressing the key joins the two line wires through the LED branch.
At station B wire key B and its LED pair across the same two line wires in the same way.
Check that each LED is in series with its own resistor and that the keys have no resistor; the keys are plain switches.
Press key A: both LEDs flash. Press key B: both flash again. Neither should glow with keys released.
Agree a message protocol: send a letter, wait for one flash as an acknowledgement, then continue.
Measure how far you can run the line (two rooms, a corridor) before the LED visibly dims; it should not.
What you should see
Pressing a key at either station flashes both LEDs at once, so partners can exchange short Morse messages across the room.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why do both LEDs light when only one key is pressed?
They are in parallel across the same line, so the same voltage appears on both.
2Why does the far LED not look dimmer?
The wire resistance is a tiny fraction of the LED branch resistance, so almost no voltage is lost along it.
3Could the two keys be wired in series instead?
No, then both would have to be pressed at once for current to flow.
4What did real telegraphs use instead of a second wire?
The earth itself as the return path.
Take it further
Replace each LED with an active buzzer in parallel with the LED and compare how far the sound carries.
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
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it