What it shows
The direction of a DC motor depends on the direction of the current through it. A double-pole double-throw switch wired crosswise swaps the battery connections and so reverses the car.
The science
A DC motor spins one way when current enters one terminal and the other way when the current enters the other terminal. A DPDT switch has two rows of three legs. Connecting the battery crosswise across the outer legs makes the first row give positive to the motor and the second negative in one position, and the opposite in the other position. So the motor reverses without anyone changing the battery.
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 DC toy motor (1.5 to 3 V) with a small gear or rubber wheel
- One DPDT slide switch
- A 2 x AA battery holder with two cells
- Cardboard or an ice-cream stick chassis with four bottle-cap wheels
- Five insulated wires
- Glue, tape and a skewer for the axles
Safety
For this project: Cells only. An adult should supervise cutting. Keep fingers off the turning wheels.
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 small chassis with two axles made from skewers and bottle cap wheels, and fix the motor over the rear axle.
Fix the DPDT switch on the top of the car and label its two rows A and B.
Connect the two middle legs of the switch, one from row A and one from row B, to the two motor terminals.
Connect the battery holder red wire to the first outer leg of row A and also to the opposite outer leg of row B.
Connect the battery black wire to the other outer leg of row A and the other outer leg of row B in the same crossed pattern.
Recheck that the red wire goes to A1 and B2 and the black wire goes to A2 and B1.
Slide the switch one way and put the car on the floor; it drives forward.
Slide it the other way and check that it reverses. If it jams, loosen the axle holes slightly.
What you should see
The car drives forward in one switch position and backward in the other, with no change to the battery.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the motor reverse?
The current direction through its coil is reversed.
2What does DPDT mean?
Double pole double throw: two switches in one body that move together.
3Why is the wiring crossed?
To swap which battery terminal feeds which motor terminal.
4Name a real device that reverses a motor.
A battery-operated drill with a forward and reverse switch.
Take it further
Add a second motor and DPDT switch to steer by driving the wheels in opposite directions.
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
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