Fundamentals, types, and uses of sensors and transducers

Fundamentals, types, and uses of sensors and transducers

1. What They Are

• Transducer = device that converts one form of energy to another (e.g., electrical ↔ non-electrical). More general term.
• Sensor = transducer that senses physical quantity (measurand) and converts to electrical signal (for measurement) — a subtype of transducer used for sensing.

Often sensor + transducer is same in practice (e.g., thermocouple senses $T$ and transduces to $mV$).

2. Fundamentals — Characteristics (what exam may ask)

Characteristic	Meaning
Sensitivity	Output change per unit input ($mV/°C$)
Range / Span	Min to max measurable
Accuracy / Error	closeness to true value
Resolution	smallest detectable change
Response time	speed to reach steady
Linearity, Hysteresis	output vs input

3. Types (classification — exam list of 6 with principle + use)

Type (based on quantity)	Example Sensor/Transducer	Principle	Use
Temperature	Thermocouple (Seebeck: $V∝ΔT$), Thermistor/RTD (PT100) ($R$ vs $T$), Thermopile	$V$ or $R$ change with $T$	Furnace, AC, medical
Light	LDR ($R$ ↓ with light), Photodiode (current $∝$ light), Phototransistor	$R$ or $I$ vs illumination	Light meter, street light, fibre
Displacement / Position	LVDT (inductive, $V$ ∝ position), Potentiometer	Mutual inductance / $R$ vs position	Industrial automation
Pressure / Force	Strain gauge ($R$ vs strain), Piezoelectric (charge vs force)	$ΔR/R = G·ε$, $Q = d·F$	Weighing, pressure
Motion / Proximity	Ultrasonic, IR, capacitive	Time-of-flight / $C$ vs distance	Security, robotics
Chemical / Gas	MQ-2, pH electrode	$R$ / $V$ vs concentration	Air quality, lab

By energy: Active (generates signal, e.g., thermocouple, piezoelectric — no external supply) vs Passive (needs excitation, e.g., RTD, LDR, LVDT).

By output: Analog (continuous $V$) vs Digital (e.g., encoder).

4. Uses (why sensors matter for electrical/electronics)

All automation, IoT, measurement rely on sensors to interface physical world to electrical (e.g., temperature sensor → ADC → microcontroller → control heater).

5. Worked Selection

Need to measure motor temperature 0–100°C: RTD PT100 (accurate, linear) or thermocouple K-type (wide range) or thermistor (cheap, non-linear).

Need to detect light for street lamp: LDR with voltage divider → comparator.

Need to measure pressure in pipe: strain gauge + diaphragm or piezoelectric.

6. Exam Points

• Define transducer vs sensor — 3-5 marks, frequent (often asked difference).
• List 4–6 types with principle + one use each — 7 marks, very frequent (the most common sensors question).
• Mention active vs passive — 3-5 marks, frequent.
• Give characteristics (sensitivity, range) — 3 marks.