# What a sensor actually is

A sensor responds to something physical, like heat or force, in a way a circuit can measure. Every reading depends on how well it does that.

Page: https://dharanova.com/learn/what-is-a-sensor · Track: Sense · Level: Foundation · Facts last checked 7 October 2026

Diagram: Top row: a physical quantity such as temperature, force, light or motion acts on the sensor, which produces or changes a voltage, charge or resistance. Middle: two ways to the microcontroller. An analog sensor sends an analog signal through an ADC; a digital sensor, with sensor, ADC and signal processing in one part, sends readings over a digital bus such as I²C or SPI. Bottom: a self-generating sensor such as a thermocouple makes its own signal; a modulating sensor such as a strain gauge or RTD needs an external voltage or current. Some books call these passive and active, inconsistently.

## Key points

1. A sensor is the part of a measuring system that responds directly to something physical, such as temperature, force, light or motion. A device whose output has a known relationship to its input is a transducer. Sensors in electronics produce or change an electrical quantity, such as a voltage, charge or resistance. [1][2][3]
2. Some sensors output an analog signal that still needs an ADC; others integrate the ADC and signal processing and report readings over a digital bus such as I²C or SPI. [2][4][5]
3. Self-generating sensors, such as thermocouples, produce a signal from the quantity they measure. Modulating sensors, such as strain gauges and RTDs, need an external voltage or current to work. Some books call these 'passive' and 'active', but not consistently. [6][2][7][3]

## Common mistakes

- Trusting the first reading. Most sensors drift with temperature and time, so check how much error to expect.

## Where to go next

- [Uncalibrated bench characterisation of low-cost turbidity, TDS, pH and level sensors on Arduino Uno R4 WiFi](https://dharanova.com/whitepapers/uncalibrated-water-sensors-arduino-uno-r4-wifi.md): a whitepaper from our lab
- [Accuracy, precision and resolution](https://dharanova.com/learn/accuracy-precision-resolution.md): Accuracy, precision and resolution mean different things, and datasheets often blur them. Mixing them up gives readings that look precise but are wrong.
- [Signal conditioning](https://dharanova.com/learn/signal-conditioning.md): Raw sensor signals are often tiny or noisy. Conditioning amplifies and filters them so the ADC can measure them well.
- [Analog-to-digital conversion](https://dharanova.com/learn/adc-basics.md): How a voltage becomes a number: resolution, reference voltage, sampling rate and the Nyquist limit.
- [The common sensor families](https://dharanova.com/learn/sensor-families.md): How common sensors for temperature, motion and sound work, and what limits each one.

## Sources

1. [JCGM 200:2012 International vocabulary of metrology (VIM), 3rd edition](https://www.bipm.org/documents/20126/2071204/JCGM_200_2012.pdf/f0e1ad45-d337-bbeb-53a6-15fe649d0ff1), BIPM
2. [A Basic Guide to Thermocouple Measurements (SBAA274A)](https://www.ti.com/lit/an/sbaa274a/sbaa274a.pdf), Texas Instruments
3. [A Basic Guide to RTD Measurements (SBAA275A)](https://www.ti.com/lit/an/sbaa275a/sbaa275a.pdf), Texas Instruments
4. [LIS2DW12 datasheet (ST DocID029682 Rev 4)](https://www.farnell.com/datasheets/2581666.pdf), STMicroelectronics
5. [UM10204: I2C-bus specification and user manual](https://www.nxp.com/docs/en/user-guide/UM10204.pdf), NXP
6. [Expanding the Vision of Sensor Materials, Appendix C: An Illustrative Sensor Taxonomy](https://www.nationalacademies.org/read/4782/chapter/16), National Research Council (National Academies Press), 1995
7. [A Basic Guide to Bridge Measurements (SBAA532A)](https://www.ti.com/lit/an/sbaa532a/sbaa532a.pdf), Texas Instruments