What it shows
A capacitor stores charge like a tiny rechargeable tank. A quick press of a button fills it, and when the button is released the capacitor keeps the LED glowing for several seconds as it empties.
The science
The button connects the capacitor to the battery through a 100 Ω resistor, which limits the first rush of current. The capacitor charges to about 9 V. When the button is released the LED and its 680 Ω resistor are the only path for the stored charge. The voltage falls exponentially with a time constant RC = 680 Ω x 4700 µF, about 3 s, and the LED fades out as the voltage drops towards about 2 V.
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.
- One 4700 µF 16 V electrolytic capacitor
- One 100 Ω resistor and one 680 Ω resistor
- One LED
- A push button
- A 9 V battery with snap connector
- A breadboard and jumper wires
Safety
For this project: Battery power only. The capacitor has a marked negative leg and must be fitted the right way round, otherwise it can swell or leak. Never use a capacitor rated below 12 V.
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.
Fit the capacitor with its striped negative leg on the ground rail and its positive leg on a spare row.
Connect the push button between the 9 V rail and a row holding one end of the 100 Ω resistor.
Connect the other end of the 100 Ω resistor to the capacitor positive row.
Connect the 680 Ω resistor from the capacitor positive row to the long leg of the LED.
Connect the short leg of the LED to the ground rail and snap on the battery.
Press the button for two seconds and release: the LED stays lit, then fades out slowly.
Time how long it takes to go dark; try a 1000 µF capacitor and compare.
Swap the 680 Ω resistor for 1.5 kΩ and note that the glow is dimmer but lasts longer.
What you should see
The LED glows for several seconds after the button is released and fades gradually, and larger capacitors give longer glow times.
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 a capacitor store?
Electric charge, and with it energy.
2Why does the LED fade instead of switching off suddenly?
The capacitor voltage falls gradually as it discharges.
3What does the time constant RC tell you?
How quickly the capacitor discharges: bigger R or C means slower.
4Why is the 100 Ω resistor in the charging path?
It limits the surge current that would flow into an empty capacitor.
Take it further
Plot LED brightness steps against time using a phone video and compare with the RC prediction.
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