This document discusses resistive sensors and their principle of operation. Resistive sensors detect changes in their resistance which can correspond to changes in factors like temperature, light intensity, pressure, or bending. The document provides examples of common resistive sensors like thermistors, photoresistors, flex sensors, and resistance temperature detectors (RTDs). It explains that these sensors detect environmental changes by measuring changes in electrical resistance through voltage dividers and relating the resistance changes to the measured property based on Ohm's law.

1. PHY109
ENGINEERING
PHYSICS
Lab@Home Project

2. Topic:
Principle behind Resistive
sensors

3. Sensors
A sensor is an electronic
component, module, or
subsystem whose purpose
is to detect events or
changes in its
environment and send the
information to other
electronics, frequently
a computer processor.

4. Resistive sensors
Resistive sensors are
the sensors or resistors
which detect the
changes in their
resistance accordingly
monitored or reacted to
perform tasks.

5. Resistance
 Resistance is an electrical quantity that
measures how the device or material reduces
the electric current flow through it.
The resistance is measured in units of ohms
(Ω).
 Resistance = (resistivity*length)/area. R=ℓ*l/a.

6. The resistance of a material depends upon four factors:
• Cross-sectional area (thickness) R α (1/a).
• Length R α l.
• Temperature.
• Conductivity.
►Resistive sensors main function is to detect their
resistance change. As resistance depends on different
factors, there are diff sensors in the market which uses diff
factor to detect their resistance.
Basic sensors are:
• Photo resistor
• Thermistor
• Flex resistor
• Resistive Temperature Detectors

7. We cannot find resistance directly but we
can find it by calculating voltage.
In these devices, end terminals are
connected to potentiometer which is a
voltage divider.
The bottom resistance remains constant.
The top one is a sensor. Now, the point
where the resistors are making two ways,
voltage divide according to the resistors on
the two directions.
So if resistance changes, the voltage will go
up and down, that’s how we can detect the
change in resistance according to ohm’s law.

8. Photo sensor
A photo sensor is an electronic
component that detects the
presence of visible light, infrared
transmission (IR), and/or
ultraviolet (UV) energy.
This resistor changes its
resistance when there is a change
in brightness of its surroundings or
change in presence and absence
of light.

9. Thermistor
A thermistor is a temperature sensor
constructed of semiconductor material
that exhibits a large modification in
resistance in proportion to a tiny low
modification in temperature.
Thermistor working principle is, it's
resistance is depends upon
temperature. When temperature
changes, the resistance of the
thermistor changes in a predictable
way. The benefits of using a thermistor
is accuracy and stability.

10. Flex Sensor
A flex sensor or bend sensor is
a sensor that measures the
amount of deflection or
bending. Usually the sensor is
stuck to the surface, and
resistance of sensor element is
varied by bending the surface.
Principle behind it is that the
resistance is directly proportional
to amount of bending.

11. RTDs
Resistance thermometers, also
called resistance temperature
detectors (RTDs), are sensors used to
measure temperature. Many RTD
elements consist of a length of fine wire
wrapped around a ceramic or glass core
but other constructions are also used.
RTDs work on a basic correlation
between metals and temperature. As the
temperature of a metal increases, the
metal's resistance to the flow of
electricity increases and vice versa.