What it shows
Three red LEDs in series share one resistor and draw one LED worth of current, while three in parallel each need their own resistor and draw three times as much. Working out the resistor with R = (supply minus LED voltages) divided by current shows why.
The science
In a series string the same current flows through every LED and their voltages add up: three red LEDs use about 6 V, leaving 3 V for the resistor. At 10 mA that needs R = 3 V / 0.01 A = 300 Ω, so 330 Ω gives about 9 mA. In a parallel set each LED gets the full supply, so each needs its own resistor: (9 V minus 2 V) / 0.01 A = 700 Ω, so 680 Ω gives about 10 mA, and the battery supplies about 30 mA in total.
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.
- Six red LEDs (about 2 V each)
- Three 680 Ω resistors and one 330 Ω resistor
- A 9 V battery with snap connector
- A breadboard and jumper wires
- A multimeter if available, otherwise judge by brightness
- A notebook for sums
Safety
For this project: Battery power only. Never connect a bare LED straight across a battery: it can burn out in a moment.
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.
Do the sums in your notebook before building: write down both resistor values and the expected currents.
Build the series string: battery positive, 330 Ω resistor, three LEDs in a row long leg to short leg, back to battery negative.
Snap on the battery and note brightness; switch off by unsnapping.
If you have a multimeter, set it to 200 mA and place it in the circuit to read the current, then to volts to read each LED.
Build the parallel set: three branches from the 9 V rail, each with its own 680 Ω resistor and one LED.
Compare brightness with the series string and measure the total battery current.
Now remove the individual resistors and use one shared 680 Ω for all three LEDs; notice the LEDs are dim and uneven.
Write a table of calculated and measured values and explain any difference.
What you should see
The series string draws about 9 mA from the battery and the parallel set about 30 mA, matching the sums within the tolerance of the parts.
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 series string need only one resistor?
The same current passes through every LED, so one resistor controls it all.
2Why does the parallel set draw more current?
Each branch draws its own current and the battery supplies the sum.
3What is the rule for the resistor value?
R equals supply voltage minus total LED voltage, divided by the wanted current.
4Why is one shared resistor poor for parallel LEDs?
LEDs differ slightly, so the one with the lowest voltage hogs current and the others look dim.
Take it further
Try a string of four red LEDs and see what the maths says about the largest string a 9 V battery can drive.
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
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely