Simple Electric Circuit Simulation showing the work of a LDR -Light dependent resistor
What it shows: A 9V circuit with an LDR (light dependent resistor) and LED. In darkness the LDR resistance is 1 MΩ — almost no current flows and the LED stays off. Slide the light up and resistance plummets to 500 Ω, current surges, and the LED switches on.
Try this with students:
- Drag slowly from 0 → 100% — watch resistance drop across several orders of magnitude. Ask: why doesn’t it drop in a straight line?
- Find the switching point — at what light level does the LED turn on? Relate this to real streetlights and burglar alarms.
- Cover and uncover — toggle quickly between 0% and 100%. Discuss real-world LDR response time and why it matters in camera exposure sensors.
- Predict the current — give students the resistance reading, ask them to calculate I = V/R before looking at the ammeter.
An LDR (Light Dependent Resistor), also called a photoresistor or photocell, is a simple electronic component whose resistance changes depending on how much light is shining on it.
How an LDR Works (Simple Explanation)
- In darkness → very high resistance (often several megaohms, like 1–10 MΩ)
- In bright light → very low resistance (can drop to just a few hundred ohms or even lower)
More light = less resistance
Less light = more resistance
This happens because of a property called photoconductivity.
The Physics Behind It (Step by Step)
LDRs are made from special semiconductor materials, most commonly cadmium sulfide (CdS) or similar compounds.
- The sensitive part is a thin layer of this semiconductor deposited in a zigzag pattern (to maximize surface area) between two metal electrodes.
- In the dark:
- Very few free charge carriers (electrons) exist in the material.
- Electrons are bound in the valence band.
- Almost no current can flow → high resistance.
- When light hits the LDR:
- Photons (light particles) with enough energy are absorbed by the semiconductor.
- These photons knock electrons from the valence band up to the conduction band.
- Now there are many more free electrons (and corresponding “holes”) that can move and carry electric current.
- More light → more photons → more free charge carriers → much easier for current to flow → resistance drops dramatically.
The brighter the light, the more electrons get excited, and the lower the resistance becomes.
Typical Resistance Values
- Complete darkness: ~1–10 MΩ (or even higher)
- Room light: ~10–100 kΩ
- Bright sunlight / strong torch: ~100–1000 Ω (sometimes even lower)
Common Real-World Uses
- Automatic street lights / garden lights (turn on when it gets dark)
- Camera exposure meters / light meters
- Solar tracking systems
- Automatic brightness control in phones/screens
- Burglar alarms / light-activated switches
- Line-following robots
Quick Circuit Example (Voltage Divider)
The most common way to use an LDR is in a voltage divider with a fixed resistor:
LDR + fixed resistor (e.g. 10 kΩ) connected between +5V and GND
→ The voltage at the junction between them changes with light level
→ Feed that voltage to an Arduino/microcontroller/ comparator → make decisions (e.g. “light level < threshold → turn on LED”)
If you’d like, I can describe a simple dark-activated LED circuit in more detail or explain how the response curve looks (it’s usually not perfectly linear — more logarithmic).
Does that cover what you wanted to know, or were you thinking of something more specific (like circuit examples or comparison with other light sensors)?