Legacy electronics: resistor networks and sensors
Read resistor values and compare series, parallel and potential-divider circuits with light and temperature transducers.
Exam scope and assessment notes
StudyForA provides original teaching and scenario-based practice, not Cambridge past papers, official marks or an authenticated practical course. It cannot assess a candidate's actual workshop skills, finished product or Component 2 portfolio. Component 2 is school-based, internally assessed and externally moderated; the teacher authenticates candidate work. Any real making requires centre facilities, risk controls and direct qualified supervision. Candidate project work must remain their own. Follow the syllabus for the exam year.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which component is used to sense changing light level in the legacy syllabus?
What you will learn
- Use resistor colour-code information, series/parallel relationships and potential-divider calculations in simplified circuit questions.
- Explain how an LDR or thermistor can provide a changing input to a control circuit.
Calculate with resistor networks
The legacy option asks candidates to use resistor colour codes to identify a value and tolerance, select a suitable nearby value, and calculate series, parallel and potential-divider circuits. In series, resistances add; in parallel, the reciprocal relationship applies. Keep units consistent and check whether a real design needs a standard component value.
A potential divider shares supply voltage between resistors. The output depends on which resistor is measured across and the relative resistances. Draw the circuit and mark the output points before calculating; otherwise a correct formula may be applied to the wrong part of the divider.
Use transducers as inputs
An LDR changes resistance with light level; a thermistor changes resistance with temperature. In a divider, the changing resistance changes an output voltage that can be monitored by a transistor or other processing circuit.
Describe the direction of change only when the specific component type and connection are known. Set a threshold from the intended use and consider sensor range, tolerances and failure behaviour. A simulation or calculation does not replace a teacher-supervised physical test.
Sources for this lesson
Pause and explain
What happens to a divider's output when one resistor changes?
Worked example
A 9 V supply is shared by a 2 kΩ fixed resistor and a 1 kΩ sensor resistor in series. Find the voltage across the sensor, assuming the meter does not affect the circuit.
Show the worked solution
- Total resistance is 2 kΩ + 1 kΩ = 3 kΩ.
- Current is 9 V ÷ 3 kΩ = 3 mA.
- Voltage across the 1 kΩ sensor is 0.003 A × 1000 Ω = 3 V.
- State which resistor the output is measured across and note the ideal assumptions.
Answer The output across the 1 kΩ sensor is 3 V. Across the fixed 2 kΩ resistor it would be 6 V. The result assumes ideal components and a meter that does not load the divider; a real circuit needs correct connections, component ratings and safe supervision.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A 12 V divider has 3 kΩ above a 1 kΩ thermistor. Find the ideal voltage across the thermistor at the stated resistance.
Give me a hint
Add the series resistances, find current, then calculate the drop across the thermistor.
Compare my reasoning
- Total resistance is 4 kΩ.
- Current is 12 ÷ 4 kΩ = 3 mA.
- Voltage across 1 kΩ is 3 V.
The ideal voltage across the thermistor is 3 V. If temperature changes its resistance, the divider output changes; the direction depends on the specific thermistor behaviour and which component is connected to the output node.
Look for these in your work
- Calculates total resistance
- Finds series current
- Relates the output to a changing sensor
Two 6 kΩ resistors are connected in parallel. What is their equivalent resistance, and how does that compare with a series pair?
Give me a hint
For two equal parallel resistors, the equivalent value is half of one resistor.
Compare my reasoning
- Use the parallel relationship or equal-resistor shortcut.
- Calculate the series total separately.
- Compare the results and explain the current paths.
The parallel equivalent is 3 kΩ; the same two resistors in series total 12 kΩ. Parallel branches provide more than one path for current, so equivalent resistance is lower than either resistor in this equal-value case.
Look for these in your work
- Uses the correct parallel relationship
- Compares with series
- Explains the difference
A light-activated warning must turn on only when a room becomes dark. How could an LDR divider and a switching stage be used, and what must be calibrated?
Give me a hint
Use the sensor voltage as an input and set a threshold based on measured light conditions.
Compare my reasoning
- Place the LDR in a divider and identify the output node.
- Determine how that output changes with light for the chosen arrangement.
- Set and test a threshold and a safe failure response.
An LDR-divider output can feed a comparator or transistor switching stage that controls the warning output. First measure the actual divider response across expected light conditions; choose a threshold with hysteresis or another stable design if needed, then test sensor failure and ambient variation under teacher supervision.
Look for these in your work
- Traces sensor to output
- Calibrates from evidence
- Considers unstable thresholds and failure
Common mistakes
- Failing to say which resistor the output voltage is across
- Mixing kΩ and Ω or mA and A
- Assuming a sensor's resistance direction without checking its type and specification
What can you explain now?
What happens to a divider's output when one resistor changes?
Compare with the explanation
The voltage share changes according to the resistor values and output connection.
A divider output is determined by the relative resistances, so state the measured nodes.
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Make it stick
Recall the main decision in “Legacy electronics: resistor networks and sensors”, name the evidence that supports it, and identify one limitation that still needs checking.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.