---
title: "Short note: Transducer"
subject: "Basic Electronics Engineering"
chapter: 7
key: "semester_2/basic_electronics_engineering:short-note-transducer"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-transducer
site: BCT Engineering
---

# Short note: Transducer

Basic Electronics Engineering · Chapter 7: Application of Electronic System

- Asked in: 2068 Bhadra, 3 marks; 2067 Mangsir, 6 marks (asked 2 times)
- Video: [Transducers](https://www.youtube.com/watch?v=uQBggxtdF84) — talkboard
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-transducer
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-7

## Question

Write a short note on transducer.

## Short answer

A transducer converts a physical quantity into a proportional electrical signal. It has a sensing element and a transduction element; active types (thermocouple) generate voltage without external power, while passive types (strain gauge, thermistor) change resistance requiring excitation.

## Answer

A **transducer** is a device that converts one form of energy into another. In instrumentation, it converts a non-electrical physical quantity (temperature, pressure, light, displacement, sound) into a proportional electrical signal (voltage, current or resistance change), or the other way round.

#### Parts

```text
 Physical    +----------+     +--------------+
 quantity -->| Sensing  |---->| Transduction |--> Electrical
             | element  |     | element      |    signal
             +----------+     +--------------+
```

The sensing element responds to the quantity, and the transduction element converts that response into an electrical signal.

#### Classification

- **Active (self-generating):** produce a voltage or current without an external supply. Examples: thermocouple, piezoelectric crystal, photovoltaic cell.
- **Passive:** need an external power supply; they change R, L or C. Examples: strain gauge, thermistor, LDR, LVDT, potentiometer.
- **Input and output transducers:** microphone (input), loudspeaker (output).

#### Examples

| Transducer | Quantity | Output |
|---|---|---|
| Thermocouple | Temperature | Voltage |
| Strain gauge | Strain/force | Resistance change |
| LDR | Light | Resistance change |
| Microphone | Sound | Voltage |
| LVDT | Displacement | AC voltage |

A good transducer is accurate, sensitive and linear, and has a fast response.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A transducer in instrumentation:
   A. Stores electrical energy
   B. Converts a non-electrical quantity into a proportional electrical signal (or the reverse)
   C. Only amplifies voltage
   D. Only converts AC to DC
   Answer: B. A transducer converts one form of energy to another, e.g. temperature, pressure or light into an electrical signal. (Key point: Definition)

2. What distinguishes a passive transducer from an active one?
   A. Passive transducers generate voltage by themselves
   B. Passive transducers have no output
   C. Passive transducers need an external supply and change R, L or C
   D. Passive transducers are only output devices
   Answer: C. Passive types (strain gauge, thermistor, LDR, LVDT) need an external supply; active ones (thermocouple, piezoelectric) self-generate. (Key point: Active vs passive)

3. Which of these is an active (self-generating) transducer?
   A. Strain gauge
   B. LDR
   C. Thermocouple
   D. Thermistor
   Answer: C. A thermocouple produces a voltage from temperature without an external supply. (Key point: Classify examples)

4. What is the output type of an LDR?
   A. AC voltage
   B. Change in resistance with light
   C. DC voltage from sound
   D. Frequency
   Answer: B. An LDR senses light and its output is a change in resistance. (Key point: Output of transducers)