Sensors and Control Circuits

What Is an Electronic Sensor?

An electronic sensor detects a physical change in the environment and produces a corresponding electrical response.

Sensors allow electronic systems to respond automatically to changes such as:

• light intensity
• temperature
• movement
• pressure
• sound
• humidity
• distance

A sensor does not usually perform the final action itself. Instead, it provides information to a control circuit, which decides how an output device should respond.

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Input, Process and Output

Many automatic electronic systems can be described using three stages:

input → process/control → output

Input

A sensor detects a physical condition.

Process

An electronic circuit processes the sensor signal and decides what should happen.

Output

An output device performs an action.

For example:

temperature sensor → control circuit → cooling fan

or:

motion sensor → control circuit → security light

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Why Sensors Are Useful

Without sensors, many electrical devices require direct human control.

For example, someone might have to:

• switch a light on
• adjust a heater
• open a door
• activate an alarm

Sensors allow these actions to occur automatically when particular conditions are detected.

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Light Sensors

A light sensor responds to changes in light intensity.

One common light-sensitive component is the:

light-dependent resistor (LDR)

An LDR changes resistance depending on the amount of light falling on it.

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Light-Dependent Resistors

For a typical LDR:

bright light → lower resistance

darkness → higher resistance

The LDR therefore converts a change in light intensity into a change in electrical resistance.

This resistance change can then be converted into a changing voltage or current that a control circuit can detect.

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Using an LDR in a Circuit

Suppose an LDR is connected as part of a voltage divider.

As light intensity changes:

LDR resistance changes

which causes:

output voltage to change

The changing voltage can then control:

• a transistor
• an integrated circuit
• a microcontroller
• another switching device

This allows a light-sensitive circuit to operate automatically.

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Voltage Dividers

A voltage divider uses resistors to produce an output voltage that is a fraction of the supply voltage.

For two resistors:

V_out = V_supply × R₂/(R₁ + R₂)

If one resistor is replaced by a sensor, changes in sensor resistance produce changes in output voltage.

This is one of the most important ways resistive sensors are used in control circuits.

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LDR Voltage Divider Example

Suppose:

V_supply = 6 V

R₁ = 10 kΩ

LDR resistance = 20 kΩ

and the output is measured across the LDR.

Then:

V_out = 6 × 20/(10 + 20)

V_out = 6 × 20/30

V_out = 4 V

If brighter light reduces the LDR resistance to:

5 kΩ

then:

V_out = 6 × 5/(10 + 5)

V_out = 2 V

The light change has therefore produced an electrical voltage change.

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Sensor Position Matters

In a voltage divider, the position of the sensor determines whether output voltage rises or falls when the sensor resistance changes.

For example, an LDR can be arranged so that:

more light → higher V_out

or:

more light → lower V_out

depending on where the LDR is placed.

Therefore, always inspect the circuit rather than memorizing that an LDR must produce a particular voltage change.

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Automatic Lighting

An automatic light can use:

LDR → control circuit → transistor → lamp

During daylight:

the sensor detects bright conditions.

The control circuit keeps the lamp off.

When it becomes dark:

the sensor resistance changes.

The control voltage crosses a switching threshold.

The transistor switches the lamp on.

This principle is used in:

• streetlights
• garden lights
• night lights
• display brightness controls

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Temperature Sensors

Electronic systems can also detect temperature.

One common temperature-sensitive component is a:

thermistor

A thermistor is a resistor whose resistance changes significantly with temperature.

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NTC Thermistors

A common type is the negative temperature coefficient (NTC) thermistor.

For an NTC thermistor:

temperature increases → resistance decreases

and:

temperature decreases → resistance increases

This relationship allows temperature changes to produce electrical changes.

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PTC Thermistors

Some thermistors have a positive temperature coefficient (PTC).

For a PTC thermistor:

temperature increases → resistance increases

PTC devices can be used for applications including:

• overcurrent protection
• overtemperature protection
• self-regulating heating

Always identify the type of thermistor before predicting its behavior.

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Temperature Control Circuit

A temperature-control system might contain:

thermistor → voltage divider → transistor → fan

As temperature rises:

thermistor resistance changes.

This changes:

V_out

When V_out reaches the required switching level:

transistor switches on

Current then flows through the fan.

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Thermostat Example

A thermostat automatically controls heating or cooling.

For a heating system:

temperature too low → heater ON

temperature high enough → heater OFF

For a cooling system:

temperature too high → cooling ON

temperature low enough → cooling OFF

The sensor continuously provides information about temperature.

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Feedback

Many automatic control systems use feedback.

Feedback occurs when the output of the system affects the condition being measured.

Consider a heater:

1. Sensor detects low temperature.
2. Heater switches on.
3. Room temperature rises.
4. Sensor detects the higher temperature.
5. Heater switches off.
6. Room gradually cools.
7. The cycle repeats.

This allows the system to maintain conditions near a desired value.

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Motion Sensors

A motion sensor detects movement.

Different technologies can be used to detect motion.

Common examples include:

• passive infrared sensors
• ultrasonic sensors
• microwave sensors
• optical sensors

One of the most common sensors in household systems is the PIR sensor.

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Passive Infrared Sensors

PIR stands for:

passive infrared

Objects above absolute zero emit infrared radiation.

Warm bodies such as humans emit significant infrared radiation.

A PIR sensor detects changes in the infrared radiation pattern reaching the sensor.

It is especially useful for detecting movement of warm objects across its field of view.

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Why Is It Called Passive?

A PIR sensor is called passive because it does not need to send infrared radiation toward the environment to detect motion.

Instead, it detects infrared radiation already arriving from objects around it.

This differs from active sensing systems that send out a signal and detect its reflection.

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PIR Sensors and Human Movement

A PIR sensor typically contains infrared-sensitive elements.

A lens system divides its field of view into detection regions.

When a warm person moves between these regions, the infrared pattern changes.

The electronics detect this changing signal.

The sensor can then activate another circuit.

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Motion-Activated Light

A typical motion-activated lighting system operates as:

PIR sensor → control circuit → electronic switch → lamp

When movement is detected:

lamp ON

After a preset time with no further movement:

lamp OFF

This can improve:

• convenience
• security
• energy conservation

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Active Motion and Distance Sensors

Some sensors actively send out energy.

For example, an ultrasonic sensor emits sound waves above the normal human hearing range.

The waves reflect from objects.

The sensor measures the returning signal.

Distance can be determined using:

distance = speed × time / 2

The division by 2 is necessary because the sound travels:

sensor → object → sensor

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Ultrasonic Example

Suppose an ultrasonic pulse returns after:

0.010 s

Using:

speed of sound ≈ 340 m/s

Distance:

d = vt/2

d = 340 × 0.010 / 2

d = 3.4/2

d = 1.7 m

Ultrasonic sensors can therefore detect both the presence and distance of objects.

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Everyday Motion-Sensor Applications

Motion and proximity sensors are used in:

• automatic doors
• security systems
• outdoor lighting
• automatic taps
• parking systems
• robots
• vehicles
• industrial machinery

Different applications require different sensor technologies.

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Automatic Switching

An automatic switching system changes the state of an electrical device without requiring direct human operation.

A typical system contains:

sensor → control circuit → switch → output

The electronic switch might be:

• transistor
• MOSFET
• relay
• integrated switching device

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Why a Sensor May Not Drive the Output Directly

Sensors often produce relatively small electrical signals.

A large output device may require much greater current.

For example:

Sensor current = very small

Motor current = much larger

The sensor should not necessarily supply the motor current directly.

Instead:

sensor signal → transistor → motor current

The transistor allows the small sensor signal to control a much larger load current.

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Transistor as a Switch

A transistor can behave like an electronically controlled switch.

A small input signal determines whether a larger current can flow through another part of the circuit.

Simplified behavior:

Input below switching level

transistor OFF

output OFF

Input high enough

transistor ON

output ON

This is the basis of many automatic control circuits.

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Relay Control

A relay is another type of electrically controlled switch.

A small control circuit energizes the relay coil.

The relay then opens or closes contacts in another circuit.

Relays are useful when:

• a load requires significant current
• electrical isolation is needed
• a different circuit voltage must be switched

For example:

temperature sensor → transistor → relay → heater

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Control Circuit Example: Automatic Fan

Consider a temperature-controlled cooling fan.

Components:

• NTC thermistor
• resistor
• transistor
• motor
• power supply

The sequence is:

temperature rises

↓

thermistor resistance changes

↓

control voltage changes

↓

transistor switches on

↓

motor operates

↓

fan cools the system

When temperature falls sufficiently, the fan can switch off again.

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Control Circuit Example: Night Light

An automatic night light could use:

• LDR
• resistor
• transistor
• LED
• current-limiting resistor

During daylight:

LED OFF

When darkness is detected:

LED ON

The circuit therefore converts environmental information into an automatic electrical response.

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Control Circuit Example: Security System

A simple security system might operate as:

motion detected → control circuit → alarm activated

The output might include:

• buzzer
• siren
• light
• camera
• electronic notification

More advanced systems may combine several sensors.

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Multiple Sensors

Electronic control systems can use more than one input.

For example, an outdoor light might turn on only when:

it is dark AND motion is detected

This requires information from:

• light sensor
• motion sensor

The control circuit combines both signals before activating the lamp.

This prevents the motion sensor from switching the light on unnecessarily during daylight.

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Thresholds

Many control systems use a threshold.

A threshold is a value at which the system changes state.

For example:

temperature < 20°C → heater ON

temperature > 22°C → heater OFF

or:

light intensity below threshold → lamp ON

Thresholds prevent the output from responding randomly to every tiny change in sensor reading.

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Why Use Two Thresholds?

Suppose a heater switched:

ON below exactly 20.0°C

and:

OFF above exactly 20.0°C

Small temperature fluctuations around 20°C could cause rapid switching:

ON → OFF → ON → OFF

This is undesirable.

Instead, a system might use:

heater ON below 19°C

heater OFF above 21°C

The difference between switching thresholds is called hysteresis.

It helps produce stable control.

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Interpreting a Simple Control Circuit

When examining a sensor-control circuit, work through it systematically.

Identify:

• power supply
• sensor
• fixed resistors
• control voltage
• transistor or relay
• output device

Then ask:

1. What physical quantity does the sensor detect?
2. How does its electrical property change?
3. Does the control voltage rise or fall?
4. What causes the switch to turn on?
5. What happens to the output?

This method is more reliable than trying to memorize individual circuit diagrams.

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Example: LDR Circuit Reasoning

Suppose an LDR forms the lower part of a voltage divider:

V_out = V_supply × R_LDR/(R_fixed + R_LDR)

In darkness:

R_LDR increases.

Therefore:

V_out increases.

If this voltage controls a transistor:

darkness → larger V_out → transistor ON → lamp ON

But if the LDR and fixed resistor exchange positions, the response reverses.

Circuit position matters.

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Example: Thermistor Circuit Reasoning

Suppose an NTC thermistor is the lower component of a voltage divider.

As temperature increases:

R_thermistor decreases.

Therefore, voltage across the thermistor decreases.

If the thermistor is placed in the upper part instead, the output behavior changes.

Again:

do not memorize voltage direction without inspecting the circuit

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Digital Sensors

Some modern sensors contain their own electronic circuits and provide a digital output.

Instead of simply changing resistance, they may:

• measure a quantity
• process the measurement
• convert it into digital data
• communicate with a microcontroller

Examples include digital:

• temperature sensors
• accelerometers
• light sensors
• pressure sensors
• motion sensors

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Smartphones and Sensors

Modern smartphones contain many sensors.

These may include:

• accelerometer
• gyroscope
• ambient light sensor
• proximity sensor
• magnetic-field sensor
• pressure sensor
• cameras
• microphones

These sensors allow the phone to respond to its environment and movement.

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Accelerometers

An accelerometer measures acceleration.

Accelerometers can help detect:

• movement
• orientation changes
• vibration
• impacts

Applications include:

• smartphones
• fitness trackers
• vehicles
• game controllers
• drones
• safety systems

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Proximity Sensors

A proximity sensor detects whether an object is nearby without necessarily requiring physical contact.

Different proximity sensors may use:

• infrared radiation
• ultrasound
• electromagnetic fields
• capacitance

For example, a phone may use proximity sensing to turn off its display when it is close to the user's face during a call.

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Sensors in Vehicles

Modern vehicles contain many sensors.

They can monitor:

• wheel speed
• engine temperature
• tire pressure
• distance to obstacles
• acceleration
• air pressure
• oxygen concentration
• light levels

Sensor information is processed by electronic control units that automatically adjust vehicle systems.

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Sensors in Homes

Sensors are increasingly common in household systems.

Examples include:

• smoke detectors
• thermostats
• security systems
• automatic lighting
• smart appliances
• door sensors
• water-leak detectors
• air-quality monitors

Sensors can improve:

• safety
• comfort
• security
• energy efficiency

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Sensors in Industry

Factories use sensors to monitor:

• temperature
• pressure
• position
• speed
• light
• fluid level
• vibration

Automatic control systems can respond much faster and more consistently than continuous manual control.

For example:

sensor → controller → motor

can automatically maintain the speed of a production machine.

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Sensors and Energy Conservation

Sensors can reduce unnecessary energy consumption.

For example:

Motion-controlled lighting

Lights operate only when an area is occupied.

Smart thermostats

Heating or cooling operates according to temperature and demand.

Daylight sensors

Artificial lighting decreases when sufficient daylight is available.

Automatic control therefore connects electronics with energy efficiency.

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Sensors in Safety Systems

Sensors are also important for safety.

Examples include:

• smoke detection
• heat detection
• gas detection
• collision detection
• overtemperature protection
• automatic shutdown systems

A sensor can detect a dangerous condition and cause the control circuit to:

sound an alarm

or:

disconnect equipment

or both.

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Advantages of Automatic Control

Automatic sensor systems can provide:

• rapid response
• continuous monitoring
• consistent operation
• improved safety
• improved efficiency
• reduced human workload

They can also operate in environments that are difficult or dangerous for humans.

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Limitations of Sensor Systems

Sensors are not perfect.

Possible problems include:

• inaccurate measurements
• electrical noise
• poor calibration
• limited operating range
• delayed response
• false triggers
• component failure

Control systems must therefore be designed to account for sensor limitations.

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False Trigger Example

A PIR security sensor may detect:

• moving people
• animals
• changing heat patterns

A poorly positioned sensor might produce unwanted alarms.

Good system design requires appropriate:

• sensor type
• sensitivity
• positioning
• threshold settings

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Sensor Calibration

Calibration compares a sensor's response with known reference values.

For example, a temperature sensor could be compared with accurately known temperatures.

Calibration helps determine whether the sensor produces reliable measurements.

This is particularly important in:

• scientific instruments
• medical equipment
• industrial control
• safety systems

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Common Mistakes

Mistake: Saying a sensor directly performs the final action.
A sensor normally provides an input signal to a control system.

Mistake: Saying an LDR produces light.
An LDR detects light by changing resistance.

Mistake: Saying LDR resistance increases with increasing light.
For a typical LDR, greater light intensity produces lower resistance.

Mistake: Saying all thermistors behave identically.
NTC and PTC thermistors behave differently.

Mistake: Saying a PIR sensor sends infrared radiation toward people.
A PIR sensor passively detects changes in incoming infrared radiation.

Mistake: Assuming a sensor can always directly power a motor.
A transistor, relay, or other driver may be required.

Mistake: Memorizing the output of a voltage divider without examining component positions.
Sensor position determines whether V_out rises or falls.

Mistake: Confusing sensing with control.
The sensor detects; the control system decides and activates the output.

Mistake: Assuming automatic systems never make mistakes.
Sensors can produce inaccurate readings or false triggers.

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Key Terms

Sensor — device that detects a physical quantity and produces an electrical response.

Control circuit — circuit that processes an input signal and determines the operation of an output.

Input — information entering a control system.

Output — action produced by a control system.

LDR — light-dependent resistor whose resistance changes with light intensity.

Thermistor — resistor whose resistance changes significantly with temperature.

NTC thermistor — thermistor whose resistance decreases as temperature increases.

PTC thermistor — thermistor whose resistance increases as temperature increases.

PIR sensor — passive infrared sensor that detects changes in infrared radiation patterns.

Motion sensor — sensor designed to detect movement.

Proximity sensor — sensor that detects nearby objects without direct contact.

Ultrasonic sensor — sensor that uses high-frequency sound waves to detect objects or measure distance.

Voltage divider — resistor arrangement used to produce an output voltage dependent on resistance values.

Transistor — semiconductor device that can act as an electronically controlled switch.

Relay — electrically controlled mechanical switch.

Threshold — input value at which a control system changes state.

Hysteresis — use of different switching thresholds to prevent rapid repeated switching.

Feedback — process in which a system's output affects the quantity being monitored.

Calibration — comparison of a sensor against known reference values.

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Key Takeaways

• Sensors allow electronic systems to detect changes in their surroundings.
• A control system can be represented as input → process/control → output.
• LDRs detect light by changing resistance.
• Typical LDR resistance decreases as light intensity increases.
• Thermistors detect temperature through resistance changes.
• NTC thermistor resistance decreases as temperature increases.
• PIR sensors detect changes in infrared radiation from warm objects.
• Ultrasonic sensors can detect objects and measure distance using reflected sound.
• Resistive sensors are commonly used in voltage-divider circuits.
• The position of a sensor in a voltage divider determines how V_out changes.
• Transistors allow small sensor signals to control larger currents.
• Relays can allow low-power circuits to switch separate higher-power circuits.
• Automatic switching occurs when sensor information crosses a control threshold.
• Feedback allows systems such as thermostats to regulate conditions automatically.
• Hysteresis can prevent rapid repeated switching near a threshold.
• Sensors are used in homes, vehicles, phones, factories, security systems, and safety equipment.
• Sensor-controlled systems can improve energy efficiency.
• Sensors have limitations and may require calibration.
• Modern systems often combine several sensors.
• Understanding a control circuit requires tracing how a physical change becomes an electrical signal and then an output action.

Check Your Understanding

1. Define an electronic sensor.
2. Explain the meaning of input → process → output in a control system.
3. Describe how the resistance of a typical LDR changes as light intensity increases.
4. Explain how an LDR could be used to control an automatic night light.
5. What is an NTC thermistor?
6. Describe how the resistance of an NTC thermistor changes when temperature increases.
7. Explain how a thermistor could control a cooling fan.
8. What does PIR stand for?
9. Explain why a PIR sensor is described as passive.
10. Describe how a PIR sensor detects human movement.
11. Give three everyday applications of motion sensors.
12. Explain why a transistor may be required between a sensor and a motor.
13. A voltage divider has a 10 kΩ fixed resistor and a 20 kΩ sensor connected across a 6 V supply. If V_out is measured across the sensor, calculate V_out.
14. Explain why swapping the positions of the sensor and fixed resistor changes the behavior of a voltage-divider control circuit.
15. An ultrasonic pulse returns to a sensor after 0.020 s. Using a sound speed of 340 m/s, calculate the distance to the object.
16. Explain what is meant by a switching threshold.
17. Explain why hysteresis is useful in a thermostat.
18. Describe one way electronic sensors can reduce household energy consumption.
19. Compare a light sensor, temperature sensor, and motion sensor in terms of the physical quantity each detects.
20. Design, in words, an automatic system that uses a sensor, a control circuit, and an output device to solve an everyday problem.