How does the resistance of an LDR change with light level?

An LDR's resistance falls as the light level rises. The resistance is very high in darkness and low in bright light.

An LDR sits beneath a 10 kΩ resistor on a 6 V supply, with the output taken across the LDR. Calculate Vₒᵤₜ when the LDR is 100 kΩ in darkness.

The output rises to about 5.5 V in darkness. Vₒᵤₜ = 6 × 100 ÷ (10 + 100) = 5.45 V.

Explain why an LDR in the lower position of a divider gives a dark-activated sensor.

Darkness raises the LDR's resistance, so the LDR takes a larger share of the supply voltage. The output across the LDR therefore rises as the light falls.

CCEA A-Level Technology and Design 8900

Material Selection and Wood

Topic overview Material selection criteria Physical and mechanical properties Hardwoods, softwoods and timber stock sizes Pine, ash, beech, oak and mahogany Manufactured boards Wood finishes, veneers and laminates

Metals

Topic overview Ferrous and non-ferrous metals and alloys Metal stock forms and sizes Aluminium, copper, brass and zinc Carbon steels and stainless steel Finishes for metals

Plastics

Topic overview Thermoplastics and thermosetting plastics HDPE, LDPE and PET Polystyrene and PVC Acrylic, nylon and ABS Melamine-formaldehyde and epoxy resins

Composite and Emerging Materials

Topic overview Thermochromic and photochromic materials Phosphorescent pigments, reflective films and holograms Liquid crystal displays PEEK, PPSU and graphene

Processing Materials

Topic overview Wasting with hand tools Manual and CNC machine tools Rolling, blanking, press forming and forging Injection and blow moulding Rotational moulding and vacuum forming Sand casting, die casting and extrusion

Product Analysis and Design Communication

Topic overview Form, function, performance and aesthetics Cost, materials, manufacture and environmental issues Target audience, anthropometrics, culture and safety Design briefs, research and specifications Creativity, innovation and sustainability in design Freehand sketching and mixed media

Design for Manufacture, Quality and Safety

Topic overview CAD modelling, virtual imaging and rapid prototyping CAM and CIM Factor of safety and tolerances Testing and inspection of products Risks in workshop and manufacturing processes

Electronic Control SystemsOption A

Topic overview Control systems, feedback and electronics safety Resistors, Ohm's law and power ratings Capacitors and RC timing Potential dividers, sensors and comparators Switches, pull-up and pull-down resistors Output devices and LEDs Transistors, Darlington pairs and thyristors Logic gates, truth tables and flip-flops 555 monostable and astable timers

Microelectronic Control SystemsOption A

Topic overview Microcontroller systems, ADCs and DACs Strain gauges and bridge circuits Optical switches, encoded discs and Schmitt triggers Op-amp amplifiers and gain Zener diodes and voltage regulators Karnaugh maps and binary counters Seven-segment, LCD and dot matrix displays PIC flowcharts and programming DC motors, stepper motors and driver circuits

Mechanical and Pneumatic ComponentsOption B

Topic overview Open and closed loop mechanical and pneumatic control Safe working with compressed air and mechanisms Levers, linkages and moments Gear types and gear trains Pulleys, belts, chains and shaft fixings Cams, cranks and converting motion Compressed air supply and pneumatic valves Cylinders, shuttle valves, restrictors and reservoirs Pneumatic circuits for logic, timing and speed control

Mechanical and Pneumatic SystemsOption B

Topic overview Hydraulics, system limitations and plant safety Torque, work, power and air consumption Vacuum handling, proximity sensors and 5/3 valves Sequential and cascade pneumatic circuits Cam profiles and follower displacement diagrams Gear geometry, pulley blocks and ratchets Toggles, treadles, shafts and couplings Friction, brakes and clutches Bearings, lubrication and seals

Designing and Manufacturing ProductsOption C

Topic overview Research, questionnaires and specifications Idea generation, critiques and computer simulation Manufacturing planning and product evaluation CFRP, GRP, Kevlar and tungsten carbide Smart materials, nanotechnology and biophotovoltaics Isometric, perspective and assembly drawing Laminating, steam bending, cutting and printing processes Risk assessment and product safety law Scales of production, JIT and cell production Quality assurance, ISO 9001 and continuous improvement Design influences and intellectual property rights

Product Design in Industry and SocietyOption C

Topic overview Life-cycle environmental impact and climate change The 6Rs, waste and environmental legislation Sustainable metals, plastics and degradable polymers Product life cycles, consumer types and relaunches CAD, CAM, 3D scanning and quick-change moulding Market pull, technology push and market strategy Price, place and promotion Jigs, fixtures and ICT in quality control Design for maintenance, inclusion and scale production Rendering, typography and presentation Dyson, Baylis, Memphis and Apple

Product Development

Topic overview Choosing a product to redesign Analysing the function and fitness for purpose of existing products Materials, manufacture and sustainability of existing products Ergonomic and aesthetic analysis Redesign modifications, sketching and viability Modelling, testing and evaluating a redesign

Product and System Design and Manufacture

Topic overview Clients, end users and design briefs Researching existing solutions and writing a specification Generating and appraising design proposals Developing a concept with numerical analysis, ergonomics and aesthetics Working drawings and planning manufacture Testing and evaluating a prototype

Electronic Control Systems

Potential dividers, sensors and comparators

3 things to know
AS 1 Paper 2

Overview

The potential divider equation with worked outputs, LDRs and thermistors in dark, light and frost sensors, variable resistors, and comparators with reference voltages.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is a potential divider?

Key term

State the potential divider equation.

Numbers

Calculate Vₒᵤₜ for a 9 V supply with R₁ = 10 kΩ and R₂ = 5 kΩ.

Questions from this subtopic

Each one with the model answer underneath.

1
What is a potential divider?
Model answer
A potential divider is two resistances in series across a supply. The output voltage is taken from the junction between them.
2
State the potential divider equation.
Model answer
Vₒᵤₜ = Vᵢₙ × R₂ ÷ (R₁ + R₂). R₂ is the resistor across which the output is taken.
3
Calculate Vₒᵤₜ for a 9 V supply with R₁ = 10 kΩ and R₂ = 5 kΩ.
Model answer
The output voltage is 3 V. Vₒᵤₜ = 9 × 5 ÷ (10 + 5) = 3 V.
4
How does the resistance of an LDR change with light level?
Model answer
An LDR's resistance falls as the light level rises. The resistance is very high in darkness and low in bright light.
5
An LDR sits beneath a 10 kΩ resistor on a 6 V supply, with the output taken across the LDR. Calculate Vₒᵤₜ when the LDR is 100 kΩ in darkness.
Model answer
The output rises to about 5.5 V in darkness. Vₒᵤₜ = 6 × 100 ÷ (10 + 100) = 5.45 V.
6
Explain why an LDR in the lower position of a divider gives a dark-activated sensor.
Model answer
Darkness raises the LDR's resistance, so the LDR takes a larger share of the supply voltage. The output across the LDR therefore rises as the light falls.

See the rest and revise

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.

1Closed book

2Open book

3Closed book

The 3-step method, run for you

Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Material Selection and Wood

Metals

Plastics

Composite and Emerging Materials

Cram a whole paper in one sitting

Sort every topic into what you know and what you do not, then drill only the weak ones.

The output voltage is V.

356

Blank Blitz makes facts automatic

Rapid timed gap-fills on the terms and numbers you need to recall without thinking.

AThe total resistance is 2 kΩ.

BThe parallel pair gives (2 × 2) ÷ (2 + 2) = 1 kΩ.

CThe series resistor adds another 1 kΩ.

Build It gets the exam shape right

Assemble a full-mark answer from its marking points, in the order an examiner wants them.

Hazard means anything with the potential to cause harm.

TrueFalse

Test It catches misconceptions

Spot the fault in a statement. The traps are the mistakes students actually make.

721941163128510

Brain Breaks between sessions

Short focus games to reset attention, so the next set lands better than the last.

Every question in Electronic Control Systems, on a schedule.

Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.

Start revising, £5 a month See how the method works

Or £46 a year, about £3.83 a month. Cancel anytime.