What it shows
When an LDR is placed in series with an LED, more light lowers the LDR resistance, more current flows and the LED glows brighter. The LED becomes a simple meter for how bright the room is.
The science
The battery, the 330 Ω resistor, the LED and the LDR form one series loop, so the same current flows through all of them. The total resistance is the sum of the parts, and the LDR is the part that changes: a few hundred ohms in sunlight, tens of kilohms in a dim room. Ohm’s law says current equals voltage divided by resistance, so brighter light gives more current and a brighter LED. The 330 Ω resistor sets the maximum current at about 20 mA even when the LDR is at its lowest.
Materials
Cost: under Rs 100. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- One LDR
- One red LED and a 330 Ω resistor
- A 9 V battery with a snap connector
- A breadboard and jumper wires
- A torch and a sheet of paper for testing
Safety
For this project: Battery power only. Do not point a bright torch into anyone’s eyes.
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
7 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.
Connect the battery positive to the 330 Ω resistor and the resistor to the long leg of the LED.
Connect the short leg of the LED to one leg of the LDR, and the other LDR leg to battery negative.
Snap on the battery and look at the LED in normal room light; note how bright it is.
Cover the LDR with your hand: the LED fades. Shine a torch on it: the LED brightens.
Hold a sheet of paper over the LDR and shine the torch through it: the LED is dimmer than with direct light.
Test at a window, under a table and outdoors in shade, and rank the places from brightest to darkest.
Write down your ranking and compare it with what your eyes tell you about each place.
What you should see
The LED brightness tracks the light level: bright in sunlight, dim indoors and nearly off when covered.
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 LED get brighter in more light?
The LDR resistance falls, so more current flows through the series loop.
2Why is a 330 Ω resistor still needed with the LDR present?
In strong light the LDR resistance is very low, so the resistor alone limits the current.
3Does the LDR have a positive and negative leg?
No, it is a resistor and works either way round.
4Name one place an LDR is used.
Automatic street lights or the brightness sensor in a phone.
Take it further
Put a multimeter on the current setting in the loop and make a table of milliamperes against each place you tested.
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