Simple Electric Circuit Simulation showing the work of a photo resistor – variable resistorWhat the Simulation Shows
The simulation models a Light Dependent Resistor (LDR) in a series voltage divider circuit powered by a 9 V battery. The circuit has two components in series: the LDR and a fixed 10 kΩ resistor.
The central interaction is a light intensity slider (1 lux = near darkness, 10,000 lux = bright sunlight). As you drag it, everything in the circuit updates live:
- The LDR symbol on the circuit diagram pulses with amber light arrows that intensify as brightness increases
- Animated current particles (orange dots) flow clockwise around the loop, speeding up as light increases and slowing almost to a stop in darkness
- Current direction arrows on each wire segment show conventional current direction
- Voltage labels on the circuit (V_LDR and V_R) update in real time, always summing to 9 V
- Four live meters on the right show LDR resistance, fixed resistance, voltage across the LDR, and current — each with a colour-coded bar
The Accessibility button in the header opens a panel with large text, high contrast, reduced motion, and four colour themes including a yellow tint (useful for dyslexia) and dark mode.
The Physics Being Modelled
The LDR resistance follows a logarithmic curve based on real datasheet values — from approximately 1 MΩ in darkness down to around 100 Ω in bright sunlight. The circuit obeys:
- V = IR (Ohm’s law) throughout
- Kirchhoff’s Voltage Law: V_LDR + V_R always equals 9 V
- The key relationship: more light → resistance falls → current rises → voltage across LDR falls, voltage across fixed resistor rises
Suggested Class Activity: “What Does the LDR Know?”
This works as a 15–20 minute paired or small group activity at any point after introducing LDRs.
Setup
Project the simulation on the board. Each pair needs a mini whiteboard or a printed prediction sheet with four columns: Light Level / LDR Resistance / Current / V across LDR.
Round 1 — Predict Before You See (5 minutes)
Before touching the slider, ask students to fill in their predictions for three scenarios: complete darkness, ordinary indoor lighting (~500 lux), and bright sunshine (~5,000 lux). Most students will get the direction right but dramatically underestimate how large the resistance change is. Let them commit to numbers before revealing anything.
Round 2 — Reveal and Discuss (5 minutes)
Move the slider through the three positions. Students check their predictions. The key discussion point is the scale: LDR resistance changes by a factor of 10,000 across the range — from 1 MΩ to 100 Ω. Ask: “If the resistance only halved, would this be useful as a sensor?” This leads naturally into why the logarithmic sensitivity is actually an advantage — the component is responsive across a huge real-world range from a dim bedroom to a sunny field.
Round 3 — The Voltage Divider Challenge (5 minutes)
This is where Kirchhoff’s law becomes concrete. Ask students to watch the V_LDR and V_R labels as they slowly drag from dark to bright. Ask:
- “When does the LDR have most of the voltage? When does the fixed resistor have most of it?”
- “What does V_LDR + V_R always equal? Why?”
- “If you wanted to use this circuit to switch on a street light automatically at dusk, which voltage would you measure — V_LDR or V_R?”
The answer to the last question (V_R, because it rises as it gets darker, making it easier to trigger a switch) often surprises students and is a real AQA exam point.
Round 4 — Design Brief (5 minutes)
Pose the question: “A designer wants to build a nightlight that turns on when the room gets dark. They have this exact circuit, a 9 V battery, and a switch that activates at 4.5 V. Where in the circuit should they connect the switch, and which end of the light slider should trigger it?”
Students sketch their answer. Use the simulation to test it live — drag the slider to darkness and check which voltage (V_LDR) crosses 4.5 V. This directly maps to AQA questions about LDR applications in sensing circuits.
Extension
Ask what would happen if the fixed resistor were changed from 10 kΩ to 1 kΩ or 100 kΩ. Students reason through the voltage divider ratios before testing conceptually — the simulation uses a fixed 10 kΩ so this becomes a written extension rather than a live test, which is itself a useful metacognitive moment about the limits of a given model.