Internet of Things (CS-801) Unit 2 Notes

How unit 2 is examined

This unit covers sensors (definition, features, types, errors), the sensor node, and actuators; the marks sit in sensor types, the sensor node, actuator selection and the four sensor errors.

Define Sensor

Not asked since 2022

Definition. A sensor is a device that detects a physical, chemical or biological quantity from its surroundings and converts it into a measurable electrical signal.

Key points.

The measured quantity is called the measurand, for example temperature, light, pressure or motion.
A sensor is the input side of an IoT system: it turns the real world into data.
Output is usually voltage, current or a digital code that a microcontroller can read.
Many sensors need a signal conditioning stage (amplifier, filter, ADC) after the sensing element.

Basic components and challenges of a sensor node

Definition. A sensor node is a small, battery-powered device that senses a quantity, processes it locally and sends it wirelessly to a gateway or sink.

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Key points.

The sensing unit holds the sensor and an ADC, and converts the physical quantity into a digital signal.
The processing unit is a microcontroller with memory; it runs the program, filters data and controls the other units.
The transceiver gives wireless communication (ZigBee, BLE, Wi-Fi, LoRa) to the gateway.
The power unit is a battery, often with solar harvesting, and decides the node lifetime.
Requirements of an IoT device are low power, connectivity, security, scalability and low cost.
Challenges are heterogeneity of devices, limited energy, memory and processing (resource constraints), poor interoperability between vendors, and difficult remote management and updates.
Design implication: use duty cycling, light protocols and standard interfaces so that large numbers of nodes stay cheap, secure and manageable.

Basis	Sensor node	Actuator node
Function	Senses and reports data	Receives commands and acts
Data flow	Environment to network	Network to environment
Key part	Sensor and ADC	Actuator and driver or DAC
Example	Temperature node	Relay or valve node
Power	Low, sleeps often	Higher, drives a load

Answer frame. Open with the definition; draw the block diagram; develop the four units, then requirements and challenges; for Q3 add the table; close with the design implication.

Asked: [7 marks] (May 2023) Explain challenges and requirements of IoT device. Asked: [7 marks] (May 2026) Explain the basic components of a sensor node and differentiate between sensor node and actuator node.

Sensor features

Definition. Sensor features are the performance characteristics that describe how well a sensor measures: range, accuracy, sensitivity, resolution, precision, linearity, response time and repeatability.

Key points.

Range is the span between the minimum and maximum values the sensor can measure.
Accuracy is how close the reading is to the true value; precision is how close repeated readings are to each other.
Sensitivity is the change in output per unit change in input.
Response time is how fast the output follows a change; linearity is how straight the output-input line is.

Sensor resolution

Definition. Resolution is the smallest change in the measured quantity that a sensor can detect.

Formula. For an $n$-bit ADC over a span $V$: $$\text{resolution}=\frac{V}{2^n}$$

Key points.

A 10-bit ADC over 5 V gives $5/1024 \approx 4.88$ mV per step.
More bits means finer resolution but larger data and higher cost.
Resolution is not accuracy: a sensor can show fine steps and still be wrong.

Analog Sensors

Definition. An analog sensor gives a continuous output (voltage or current) proportional to the measured quantity.

Key points.

Examples are LM35 temperature sensor, LDR and thermistor.
The output needs an ADC before a microcontroller can use it.
They are simple and cheap but sensitive to noise over long wires.

Digital Sensors

Definition. A digital sensor converts the measurement internally and outputs discrete binary data or a digital signal.

Key points.

Examples are DHT11, DS18B20 and PIR, using I2C, SPI, 1-wire or on/off levels.
No external ADC is needed, and the signal is noise-resistant.
They cost more but are easier to interface and calibrate.

Scalar Sensors

Definition. A scalar sensor measures only the magnitude of a quantity and gives a single value with no direction.

Key points.

Examples are temperature, humidity, pressure and light sensors.
Output is one number per reading.
Direction is irrelevant to the measurement.

Vector Sensors

Definition. A vector sensor measures both magnitude and direction of a quantity.

Key points.

Examples are accelerometer, gyroscope, magnetometer and anemometer with direction.
Output has several components, such as x, y and z axes.
They are used for motion tracking, orientation and navigation.

Sensor Types

Definition. Sensors are classified by the quantity they measure; in IoT they let devices observe the physical world before data is processed and sent to the cloud.

Key points.

Temperature sensors (LM35, DHT11) measure heat and suit smart homes and cold chains.
Humidity sensors measure moisture in air and are used in agriculture and weather stations.
Pressure sensors (BMP180) measure air or fluid pressure for altitude, weather and industrial lines.
Proximity and ultrasonic sensors (HC-SR04) find distance to objects for parking and robots.
PIR motion sensors detect human movement for security and lighting.
Gas sensors (MQ series) detect gases and smoke for safety and air quality.
Light (LDR), accelerometer and gyroscope sensors serve streetlights, wearables and vehicles.
Building blocks of IoT are sensing, processing, connectivity, cloud, analytics and user interface; sensors form the first block and feed the rest.

Sensor	Principle	Range	Output	Power and cost	Use
LM35 temperature	Voltage rises with temperature	-55 to 150 C	Analog	Low, cheap	Room monitoring
DHT11 humidity	Resistive humidity element	20 to 90 % RH	Digital	Low, cheap	Weather, farming
BMP180 pressure	Piezo-resistive	300 to 1100 hPa	Digital (I2C)	Low, medium	Altitude, weather
PIR motion	Infrared change	About 7 m	Digital on/off	Very low, cheap	Security
HC-SR04 ultrasonic	Echo time of sound	2 to 400 cm	Digital pulse	Low, cheap	Distance, parking
MQ-2 gas	Gas changes resistance	300 to 10000 ppm	Analog	High (heater), cheap	Leak, smoke

Answer frame. Open with the role of sensors in IoT; classify by quantity with one example each; give the comparison table; close with selection criteria: range, accuracy, power, cost and interface. For Q5 first list the six building blocks in one line each.

Asked: [7 marks] (May 2023, May 2024) What are the different types of sensors used in IoT network? Compare the common commercially available sensors used for IoT-based sensing applications. Asked: [7 marks] (May 2022) Discuss the building blocks of IoT. What are the most used sensor types in IoT?

bias

Definition. Bias (offset error) is a constant difference between the sensor reading and the true value, present at every point of the range.

Key points.

Cause is manufacturing tolerance or wrong zero setting; effect is every reading shifted by the same amount.
Example: a thermometer that reads 2 C when the true value is 0 C has a bias of 2 C, which affects weighing scales and medical thermometers.
Fix by calibration: measure the offset once and subtract it in software.

Asked: [7 marks] (May 2026) Describe bias, drift, hysteresis error and quantization error in sensors with suitable examples and applications. (answer with bias, drift, hysteresis and quantization sections below)

drift

Definition. Drift is the slow change in a sensor output over time even though the input stays constant.

Key points.

Causes are ageing, temperature change and contamination; gas and pH sensors drift the most.
Effect: readings gradually go wrong, harming long-term monitoring such as air quality.
Fix by periodic recalibration or compensation with a reference sensor.

Hysteresis error

Definition. Hysteresis error is the difference in output for the same input when the input is approached while increasing and while decreasing.

Key points.

Cause is friction, magnetic or material lag; the sensor output depends on its past.
Example: a pressure sensor reads 100 kPa on the way up but 98 kPa on the way down.
It hurts control systems such as valves and level sensors; fix with better materials and averaging both directions.

quantization error

Definition. Quantization error is the difference between the true analog value and the nearest digital level produced by the ADC.

Key points.

It is at most half a step: $\pm\frac{1}{2}\,\text{LSB}$, where the step is $V/2^n$.
Example: a 10-bit ADC over 5 V has a step of 4.88 mV, so the error is up to 2.44 mV.
Reduce it by using an ADC with more bits.

Actuator

Definition. An actuator is a device that converts an electrical control signal into physical action such as motion, force, heat or flow.

Key points.

Role in IoT: the actuator is the output side; sensors sense, the controller decides, and the actuator acts on the real world.
Types by energy source are hydraulic, pneumatic, electrical, thermal or magnetic, mechanical and soft actuators; common examples are electric motors, solenoids and relays.
Applications are smart-home locks and lights, industrial automation and robotics.
Selection characteristic 1, force or torque: the push or turning effort must exceed the load, for example a motor lifting a shutter.
Selection characteristic 2, displacement or stroke: the distance it moves must cover the needed travel, such as a valve opening fully.
Selection characteristic 3, speed: how fast it moves decides response time, a fast relay versus a slow hydraulic ram.
Selection characteristic 4, power and efficiency: input energy versus useful output matters for battery-powered nodes.

Answer frame. Q1: open by defining actuator and selection criteria; explain four characteristics with unit of measure and one IoT example each; close by matching them to the application. Q9: define role first, then list types with one line and one application each.

Asked: [7 marks] (May 2024, Jun 2025) Explain four common characteristics of actuators used for selection. Asked: [? marks] (Dec 2024) Discuss the role of actuators in IoT. Explain various types of actuators.

Hydraulic actuators

Definition. A hydraulic actuator uses pressurised incompressible liquid, usually oil, to move a piston and produce linear or rotary motion.

Key points.

They give very high force and precise control.
They need pump, reservoir and pipes, so they are heavy and can leak.
Uses are excavators, presses and heavy industrial machines.

Pneumatic actuators

Definition. A pneumatic actuator uses compressed air to push a piston or diaphragm and produce motion.

Key points.

Construction is a cylinder, piston, rod and valves fed by a compressor; air entering one side moves the piston, and a spring or the other port returns it.
They are fast, clean and cheap, and safe in explosive areas.
Air compresses, so force and position control are less precise than hydraulics.

Asked: [7 marks] (Jun 2025) Explain: i) Pneumatic ii) Sensor features (see Sensor features above)

electrical actuators

Definition. An electrical actuator converts electrical energy into motion using motors, solenoids or relays.

Key points.

Types include DC, stepper and servo motors, solenoids and relays.
They are clean, easy to control from a microcontroller and need little maintenance.
Uses are smart locks, fans, robots and door openers.

thermal/magnetic actuators

Definition. Thermal actuators move by expansion or phase change under heat, and magnetic actuators move by magnetic force.

Key points.

Thermal examples are bimetallic strips and shape memory alloys.
Magnetic examples are electromagnets and solenoids.
Thermal ones are slow; magnetic ones are fast but short-stroke.

mechanical actuators

Definition. A mechanical actuator converts rotary input into linear motion using gears, screws, cams or rack and pinion.

Basis	Mechanical	Soft	Shape memory polymer
Material	Metal parts	Elastomer	Polymer
Principle	Gears, screws	Air or fluid inflation	Heat returns shape
Force	High	Low	Low to medium
Speed	Fast	Medium	Slow
Use	Machines	Grippers	Medical, deployable

Asked: [7 marks] (May 2024) Compare mechanical, soft and shape memory polymer based actuators.

soft actuators

Definition. A soft actuator is made of flexible material such as silicone that bends or grows when inflated, heated or electrically stimulated.

Key points.

They are safe and gentle around humans.
Uses are soft grippers, wearables and medical devices.
Force is low and control is harder.

Last-minute revision

Sensor: converts a physical quantity into an electrical signal; actuator does the reverse job of acting.
Sensor node units: sensing, processing, transceiver, power.
Resolution $=V/2^n$; quantization error $=\pm$ half LSB.
10-bit ADC over 5 V: 4.88 mV per step.
Bias is constant offset; drift is change over time; hysteresis depends on direction.
Analog needs an ADC; digital does not.
Scalar has magnitude; vector has magnitude and direction.
Actuator selection: force, displacement, speed, power and efficiency.
Hydraulic gives the highest force; pneumatic is fast and clean.
IoT challenges: heterogeneity, resource constraints, interoperability, security.

Memory hooks

Sensor node = SPTP: Sense, Process, Transmit, Power.
Bias stays, drift moves, hysteresis remembers, quantization rounds.
Hydraulic = oil = heavy; pneumatic = air = fast.
Scalar = size only; vector = size plus direction.

Coverage checklist

Define Sensor
Basic components and challenges of a sensor node: Q2, Q3
Sensor features: Q4 (part ii)
Sensor resolution
Analog Sensors
Digital Sensors
Scalar Sensors
Vector Sensors
Sensor Types: Q5, Q6
bias: Q7
drift: Q7
Hysteresis error: Q7
quantization error: Q7
Actuator: Q1, Q9
Hydraulic actuators
Pneumatic actuators: Q4
electrical actuators
thermal/magnetic actuators
mechanical actuators: Q8
soft actuators

Account

Internet of Things (CS-801) - Unit 2 Short Notes

How unit 2 is examined

Define Sensor

Basic components and challenges of a sensor node

Sensor features

Sensor resolution

Analog Sensors

Digital Sensors

Scalar Sensors

Vector Sensors

Sensor Types

bias

drift

Hysteresis error

quantization error

Actuator

Hydraulic actuators

Pneumatic actuators

electrical actuators

thermal/magnetic actuators

mechanical actuators

soft actuators

Last-minute revision

Memory hooks

Coverage checklist