What it shows
Two resistors in series share the cell’s voltage in proportion to their resistances. Replacing one with an LDR makes the output voltage change with light, which is how many sensors work.
The science
Two resistors R1 and R2 in series carry the same current, so the voltage across R2 is V_out = V R2 / (R1 + R2). A slider on a resistive track makes R1 and R2 change while their total stays the same, so V_out changes smoothly from 0 to V. An LDR’s resistance falls from hundreds of kilo-ohm in the dark to about a kilo-ohm in bright light, so the divider output moves with the light level.
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.
- Three AA cells in a holder (4.5 V)
- Two fixed resistors of 1 kohm and 10 kohm
- A light dependent resistor (LDR)
- A graphite pencil lead or a 10 kohm preset as a slider
- A multimeter
- A breadboard and wires (or crocodile clips)
- A torch and a dark box or cardboard tube
Safety
For this project: Use cells only. An adult should check the wiring before the cells are connected.
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 1 kohm and 10 kohm resistors in series across the 4.5 V pack and measure the voltage across the 10 kohm one; compare it with the formula (about 4.1 V).
Swap the two resistors and measure again; the voltage across the 10 kohm resistor is now the other fraction, and the two readings add up to the cell voltage.
Replace one resistor with the LDR and keep the 10 kohm resistor in the other place, then measure the voltage across the fixed resistor.
Read the voltage in room light, under a torch held 10 cm away and inside the dark tube; note each value.
Measure the LDR resistance with the multimeter in each case with the cells disconnected, and check the divider formula against your voltage readings.
Make the pencil-lead divider: clip the cells across both ends of the lead and slide a wire along it to take the output voltage at 0, 25, 50, 75 and 100 percent of its length.
Plot output voltage against slider position and see that the graph is nearly a straight line.
What you should see
The measured divider voltages agree with V R2 / (R1 + R2) to within a few percent. The LDR divider gives a high output in the dark and a low output in the light (or the reverse, depending on which place the LDR takes), and the pencil-lead slider gives a straight-line graph.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is a potential divider?
Two or more resistors in series used to take a fraction of a supply voltage.
2What happens to an LDR’s resistance in bright light?
It falls because more charge carriers are freed in the material.
3Why does the voltmeter slightly change the reading?
The meter takes a little current and acts like another resistance in parallel with the part it is across.
4Where is a potential divider used in daily life?
In volume controls, light sensors and in changing a sensor’s resistance into a voltage.
Take it further
Use the LDR divider output to drive a transistor that switches on an LED when the room darkens, and find the light level at which it switches.
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
Science projects
Electricity and Magnetism Experiments
Hands-on experiments with cells, wires, bulbs and magnets
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each