E20: Sensors & Transducers
LDRs, thermistors, pressure sensors and signal conditioning for sensor inputs
Sensors & Transducers
LDRs, thermistors, pressure sensors and signal conditioning for sensor inputs
Key Fact: A sensor converts a physical quantity (light, temperature, pressure) into an electrical signal; a transducer converts energy between forms.
Key Fact: An LDR (light-dependent resistor) has resistance that decreases as light intensity increases; typically 1 MΩ in dark and 1 kΩ in bright light.
Key Fact: An NTC thermistor has resistance that decreases as temperature increases; a PTC thermistor has resistance that increases with temperature.
Key Fact: Sensors are commonly used in potential divider circuits to convert resistance changes into voltage changes for processing.
Key Fact: The output voltage of a sensor potential divider: V_out = V_supply × R_fixed / (R_sensor + R_fixed) when the sensor is in the upper position.
Key Fact: Signal conditioning adapts the sensor output for the next stage: amplification, filtering, level shifting or linearisation.
Key Fact: An op-amp comparator can convert a varying sensor voltage into a digital on/off signal by comparing it with a reference voltage.
Key Fact: A Schmitt trigger with a sensor input provides hysteresis, preventing output chatter when the measured quantity hovers near the threshold.
Key Fact: Pressure sensors often use a piezoresistive element in a Wheatstone bridge configuration; the output is a small differential voltage.
Key Fact: Sensor response time is the time taken to reach 63.2% of the final value after a step change in the measured quantity.
Key Fact: Calibration ensures sensor readings are accurate by comparing outputs against known reference values and applying corrections.
Key Fact: A Wheatstone bridge converts small resistance changes into a voltage difference; it is used with strain gauges and pressure sensors.
📋 Key Vocabulary and Concepts
For Sensors & Transducers, you must know:
- Sensor: A device that detects a physical quantity (e.g. light, temperature, pressure) and converts it into an electrical signal.
- Transducer: A device that converts energy from one form to another, such as a microphone (sound to electrical) or speaker (electrical to sound).
- LDR: Light-Dependent Resistor; a sensor whose resistance decreases as light intensity increases due to photon-generated charge carriers.
- NTC thermistor: Negative Temperature Coefficient thermistor; resistance decreases as temperature increases.
- Signal conditioning: Processing a sensor output (amplifying, filtering, level shifting) to make it suitable for the next circuit stage.
- Wheatstone bridge: A circuit of four resistors arranged in a diamond, used to measure small resistance changes by producing a differential voltage output.
❓ Practice Questions
Q: An LDR has a resistance of 200 kΩ in darkness and 2 kΩ in bright light. It is in a potential divider with a 10 kΩ fixed resistor and 9 V supply (LDR on top). What is V_out in darkness and in bright light?
Q: Explain why an NTC thermistor is suitable for a temperature monitoring circuit.
Q: What is the purpose of signal conditioning in a sensor circuit?
Q: Why might a Schmitt trigger be used with a sensor input instead of a simple comparator?
Q: Describe how a Wheatstone bridge improves sensitivity for small resistance changes.
✅ Answers
- Dark: V_out = 9 × 10/(200 + 10) = 0.43 V. Bright: V_out = 9 × 10/(2 + 10) = 7.5 V.
- Its resistance decreases as temperature increases, so when placed in a potential divider the output voltage rises with temperature, providing a clear electrical signal proportional to temperature.
- To adapt the raw sensor output for the next stage by amplifying weak signals, filtering noise, level shifting or linearising the response.
- A Schmitt trigger provides hysteresis, preventing the output from chattering when the sensor voltage hovers near the threshold due to noise or slow changes.
- The bridge balances two voltage dividers; even a tiny resistance change unbalances the bridge, producing a measurable differential voltage proportional to the change.
🎯 Exam Tips
- Always draw the potential divider and calculate V_out for both extreme sensor values (e.g. dark/light or hot/cold).
- State whether the sensor resistance increases or decreases with the physical quantity before calculating divider output.
- For thermistor questions, specify NTC (resistance falls with temperature) or PTC (resistance rises with temperature).
- When asked about signal conditioning, list the specific techniques needed: amplification, filtering, level shifting or linearisation.
- Include a comparator or Schmitt trigger stage when converting a sensor voltage to a digital on/off output.
📝 Exam Technique
GCSE Electronics Exam Tips — Sensors & Transducers:
1. For Sensors & Transducers questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Sensors & Transducers, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
1. For Sensors & Transducers questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Sensors & Transducers, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
⚠️ Common Errors
✗ Placing the sensor in the wrong position in a potential divider, giving an inverted output. ✓ Check whether V_out should increase or decrease with the measured quantity and position the sensor accordingly in the divider.
✗ Assuming an LDR's resistance increases with light intensity. ✓ An LDR's resistance decreases as light intensity increases; it is high in darkness and low in bright light.
✗ Forgetting that a sensor's raw output may need amplification before it can be used. ✓ Most sensor voltage changes are small; include an op-amp amplifier stage to bring the signal to a usable level.
✗ Confusing the terms sensor and transducer. ✓ A sensor detects a physical quantity and converts it to an electrical signal; a transducer converts energy between any two forms (input and output).
✍️ Model Answer
Full-Mark Response
Explain how an LDR or thermistor can be used in a potential divider circuit to monitor light or temperature, and describe how the output can be conditioned to drive a switching circuit.
An LDR or thermistor changes its resistance in response to a physical quantity (light or temperature). When placed in a potential divider with a fixed resistor and a supply voltage, the resistance change produces a corresponding voltage change at the output. For example, an LDR in the upper position of a divider with a fixed resistor below gives a low V_out in darkness (high LDR resistance) and a high V_out in bright light (low LDR resistance). An NTC thermistor in the lower position gives V_out rising as temperature increases. However, the raw divider output may have a small voltage range, noise and a non-linear response. Signal conditioning addresses this: an op-amp amplifier increases the voltage swing, a filter removes unwanted noise, and a level shifter adjusts the DC offset. To create a switching circuit, the conditioned signal is fed to a comparator (or Schmitt trigger for hysteresis) that compares it with a reference voltage. When the sensor voltage crosses the threshold, the output switches between logic levels, driving a transistor, relay or LED to indicate or control the physical condition.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of sensors & transducers, including electronic components, circuit theory and systems concepts relevant to WJEC Eduqas C690QS.
AO2 (Application): Apply knowledge and understanding of sensors & transducers to analyse, design and construct electronic circuits and systems.
AO3 (Evaluation): Evaluate electronic circuits and systems, making reasoned judgements about design choices, performance and practical considerations, constructing supported arguments.
📝 Exam Questions by Topic
🎬 Video Resources
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