What it shows
A motor winding thread onto a spool converts rotation into lifting. A DPDT slide switch swaps the battery polarity so the same motor raises and lowers the hook, which is how real crane winches are controlled.
The science
A DC motor reverses direction when the current through it reverses. A DPDT switch has two rows of three pins; wiring its four outer pins in a cross pattern and the motor to the middle pair means sliding the switch one way sends current through the motor in one direction and the other way sends it back the other way. The thread winds onto the spool when the motor turns one way and unwinds under the load when it turns the other. A thinner spool lifts more slowly but with more force, because the motor torque acts over a smaller radius.
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 3 V DC toy motor
- An empty thread spool or bottle cap glued to the motor shaft
- A DPDT slide switch (six pins)
- A 2 x AA battery holder with cells (3 V)
- Cardboard tubes and a shoe box for the tower and jib
- Strong thread, a paper clip hook and a small load such as a toy car
- Hot glue, tape and insulated wire
Safety
For this project: An adult must use the hot glue gun. Do not lift anything heavier than about 100 g or the motor will stall and heat up.
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.
Build a tower from a tall cardboard tube glued upright on the shoe box, with a horizontal jib tube at the top.
Glue the motor at the base of the tower with the spool on its shaft, and tie the thread to the spool.
Run the thread up the tower, over a pencil axle at the end of the jib, and down to the paper clip hook.
Wire the DPDT switch: battery red to the top left pin, battery black to the top right pin, then cross wires from top left to bottom right and from top right to bottom left.
Connect the motor to the two middle pins of the switch.
Slide the switch one way: the spool winds and the hook rises. Slide it the other way: the hook lowers.
If the thread piles up on one side of the spool, add cardboard discs as flanges on each side.
Lift the toy car, measure the height gained in ten seconds and record it; try a heavier load and repeat.
What you should see
The crane raises and lowers a small load smoothly, and the slide switch changes direction without swapping any wires.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does DPDT stand for?
Double pole, double throw: two separate contacts, each with two positions.
2How does the switch reverse the motor?
It swaps which motor terminal connects to the battery positive, reversing the current.
3Why does a thinner spool lift a heavier load?
The motor torque acts at a smaller radius, giving a larger pulling force on the thread.
4Why does the motor get warm when the load is too heavy?
A stalled motor draws a large current and all its energy turns into heat.
Take it further
Add a paper clip limit switch at the top that opens the circuit when the hook reaches the jib.
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
Motors, Gears and Motor Drivers
Make things move: DC motors, servos, steppers and how to drive them
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it