These MCQs cover fundamental concepts about various electronic sensors:
1. What is an LDR (Light Dependent Resistor)?
Correct Answer: c) A resistor whose resistance changes with light intensity
Explanation: LDR Characteristics: - Passive component (requires external power) - Resistance decreases with increasing light intensity - Typical dark resistance: 1MΩ or higher - Typical light resistance: 100Ω or lower - Response time: ~10ms (relatively slow)
2. LDR is also known as:
Correct Answer: a) Photoresistor
Explanation: Alternative names for LDR: - Photoresistor (most common alternative) - Photoconductive cell - Light-sensitive resistor Not to be confused with: - Photodiode (generates current when exposed to light) - Solar cell (converts light to electricity) - Thermistor (temperature-sensitive resistor)
3. How does the resistance of an LDR change with increasing light intensity?
Correct Answer: c) It decreases
Explanation: LDR Resistance Behavior: - More light → More charge carriers → Lower resistance - Less light → Fewer charge carriers → Higher resistance - Inverse relationship between light and resistance - Non-linear response (logarithmic relationship)
4. What material is commonly used to make LDRs?
Correct Answer: b) Cadmium Sulfide (CdS)
Explanation: LDR Materials: - Most common: Cadmium Sulfide (CdS) - Other materials: Lead Sulfide (PbS), Indium Antimonide (InSb) - CdS has peak sensitivity similar to human eye - Environmental concerns with cadmium - Modern alternatives being developed
5. What happens to an LDR in complete darkness?
Correct Answer: b) Resistance is very high
Explanation: LDR in Darkness: - Typical dark resistance: 1MΩ to 10MΩ - Very small leakage current flows - Acts like an open circuit in many applications - Requires pull-up/down resistors in circuits - Dark resistance varies by specific LDR model
6. LDRs are commonly used in:
Correct Answer: b) Light-sensitive circuits
Explanation: Common LDR Applications: - Automatic street lights - Camera light meters - Burglar alarms - Solar trackers - Night lights - Light/dark activated switches
7. Which electronic component is usually paired with an LDR in light-based switching circuits?
Correct Answer: b) Transistor
Explanation: LDR-Transistor Pairing: - LDR provides light-sensitive resistance - Transistor amplifies the signal - Common configurations: * LDR as voltage divider with fixed resistor * Output drives transistor base * Transistor switches higher current loads - Allows small LDR changes to control larger devices
8. In a streetlight automation system using an LDR, what happens when the sun sets?
Correct Answer: b) The resistance of LDR increases, turning the lights ON
Explanation: Streetlight Operation: 1. At sunset → Light decreases → LDR resistance increases 2. Voltage divider output changes 3. Comparator or transistor switches 4. Relay activates streetlight 5. Reverse happens at sunrise - Hysteresis often added to prevent flickering
9. What type of waves does an ultrasonic sensor use for distance measurement?
Correct Answer: c) Sound waves
Explanation: Ultrasonic Sensor Basics: - Uses sound waves above human hearing range (>20kHz) - Typical frequency: 40kHz - Works in air, liquids, and some solids - Not electromagnetic radiation - Requires medium to propagate (unlike light)
10. What is the frequency range of ultrasonic sensors?
Correct Answer: c) 40 kHz – 100 kHz
Explanation: Ultrasonic Frequencies: - Human hearing: 20Hz-20kHz - Common sensor frequencies: 40kHz, 58kHz, 100kHz - Higher frequencies: * Better resolution * More attenuation in air - Lower frequencies: * Longer range * Less precise
11. The two main components of an ultrasonic sensor are:
Correct Answer: c) Transmitter and receiver
Explanation: Ultrasonic Sensor Components: - Transmitter: Piezoelectric crystal that converts electrical to sound - Receiver: Piezoelectric crystal that converts sound to electrical - Some modules combine both functions in one unit - Control circuitry for pulse generation and timing
12. What does the ultrasonic transmitter do?
Correct Answer: b) Converts electrical signals into sound waves
Explanation: Transmitter Operation: - Piezoelectric effect: Crystal vibrates when voltage applied - Typically needs high voltage pulse (5V-12V) - Produces short burst of ultrasonic waves - Pulse duration determines measurement resolution - Frequency determines beam width and range
13. What is the formula to calculate distance using an ultrasonic sensor?
Correct Answer: b) Distance = (Speed of Sound × Time) / 2
Explanation: Distance Calculation: - Measures time-of-flight (echo return time) - Sound travels to object and back → divide by 2 - Formula: d = (v × t)/2 where: d = distance v = speed of sound (~343m/s at 20°C) t = round-trip time - Temperature compensation improves accuracy
14. Ultrasonic sensors are commonly used in:
Correct Answer: d) All of the above
Explanation: Ultrasonic Applications: - Industrial: Level measurement, object detection - Automotive: Parking sensors, collision avoidance - Medical: Ultrasound imaging (higher frequencies) - Robotics: Navigation, obstacle avoidance - Security: Intrusion detection - Underwater: Sonar systems
15. Which factor can affect the accuracy of an ultrasonic sensor?
Explanation: Accuracy Factors: - Temperature: Affects speed of sound (0.6% per °C) - Humidity: Changes air density - Angle: Non-perpendicular surfaces may deflect echo - Surface: Soft materials absorb sound - Air turbulence: Affects wave propagation - Noise: Other ultrasonic sources cause interference
16. What is the typical speed of sound in air at room temperature (25°C)?
Correct Answer: c) 343 m/s
Explanation: Speed of Sound: - At 20°C: 343 m/s - At 25°C: ~346 m/s - Formula: v = 331 + (0.6 × T) where T in °C - Faster in warmer air - Slower in humid air - Much faster in water (~1482 m/s) - Very fast in solids (~5000 m/s in steel)
17. What is the primary function of a temperature sensor?
Correct Answer: c) To measure temperature variations
Explanation: Temperature Sensor Purpose: - Converts thermal energy into electrical signal - Various working principles: * Thermoelectric (thermocouples) * Resistive (RTDs, thermistors) * Semiconductor (IC sensors) * Infrared (non-contact) - Wide range of applications
18. Which of the following is a commonly used temperature sensor?
Correct Answer: c) LM35
Explanation: LM35 Characteristics: - Precision integrated-circuit temperature sensor - Output voltage linearly proportional to Celsius - Calibrated directly in °C - +10mV/°C scale factor - ±0.5°C accuracy at 25°C - Low self-heating - Wide supply range (4V-30V)
19. What is the output of an LM35 temperature sensor?
Correct Answer: a) Analog voltage
Explanation: LM35 Output: - Analog voltage output - Directly readable by microcontroller ADC - No external calibration needed - Example: * 25°C → 250mV * 30°C → 300mV - Simple interface (only 3 pins)
20. What is the temperature range of the LM35 sensor?
Correct Answer: b) -55°C to 150°C
Explanation: LM35 Specifications: - Full range: -55°C to +150°C - Different versions available: * LM35: -55°C to 150°C * LM35C: -40°C to 110°C * LM35D: 0°C to 100°C - Operating voltage: 4V to 30V - Current drain: 60μA
21. Which temperature sensor uses a thermistor as its sensing element?
Correct Answer: d) NTC (Negative Temperature Coefficient)
Explanation: Thermistor Types: - NTC: Resistance decreases with temperature - PTC: Resistance increases with temperature - Highly non-linear response - Very sensitive (large resistance change) - Used in: * Temperature measurement * Overcurrent protection * Temperature compensation
22. Temperature sensors are used in:
Explanation: Temperature Sensor Applications: - Consumer: Appliances, HVAC systems - Industrial: Process control, equipment monitoring - Medical: Patient monitoring, diagnostic equipment - Automotive: Engine management, climate control - Environmental: Weather stations, agriculture - Research: Laboratory equipment
23. What type of output does a DHT11 sensor provide?
Correct Answer: b) Digital signal
Explanation: DHT11 Characteristics: - Digital temperature and humidity sensor - Single-wire serial interface - 8-bit microcontroller inside - Temperature range: 0-50°C ±2°C - Humidity range: 20-90% ±5% - Sampling rate: 1Hz (1 reading per second) - Low cost but lower accuracy than DHT22
24. In which applications are infrared temperature sensors commonly used?
Explanation: IR Temperature Sensor Applications: - Medical: Non-contact thermometers (especially during COVID-19) - Industrial: Monitoring equipment temperature - Building: HVAC systems, energy audits - Safety: Fire detection, overheating equipment - Automotive: Engine component monitoring - Food: Cooking/serving temperature checks
25. Which factor affects the accuracy of a temperature sensor?
Explanation: Accuracy Factors for Temperature Sensors: - Thermal contact: Poor contact gives false readings - Self-heating: Current through sensor affects reading - Response time: Slow sensors miss rapid changes - Calibration: Drift over time requires recalibration - Electrical noise: Affects signal integrity - Environmental factors: Airflow, radiation, etc.