IOT LAB (EC-705) - Important Questions

Interface a DHT22 temperature and humidity sensor to a Raspberry Pi and write a Python program to read sensor values periodically and publish them to an MQTT broker. Provide the complete code flow, explain the MQTT topic hierarchy you would use, and justify the choice of QoS level for this application. Also describe how you would test and demonstrate the system end-to-end.

Create a dashboard on a cloud service (e.g., ThingSpeak or AWS IoT) and demonstrate sending sensor data from a Raspberry Pi to this dashboard. Include the steps for authentication, sample HTTP or REST API calls with headers/body, and describe how you would handle intermittent connectivity and data buffering on the Pi.

Explain the Embedded Linux boot sequence on a Raspberry Pi from power-up to a running userland. Describe the roles of the bootloader, kernel, init system and root filesystem. Explain how kernel modules are loaded and how you would enable and access hardware interfaces such as I2C, SPI and GPIO from user space on the Raspberry Pi. Include practical commands and configuration file locations.

Using a logic analyzer, capture SPI communication between a Raspberry Pi and an external ADC. Explain how to identify clock, MOSI, MISO and CS lines, and demonstrate decoding of a captured frame to extract ADC sample bytes. Describe common errors you would look for and how you would correct timing or wiring issues.

You measure a PWM signal from a Raspberry Pi HAT on an oscilloscope. From the trace you determine the high time $T_{on}$ and the total period $T_{total}$. Define the duty cycle and show how to compute it. If $T_{on}=2.5\,\text{ms}$ and $T_{total}=10\,\text{ms}$, compute the duty cycle and average voltage if the signal high level is $3.3\,\text{V}$. State the formulas you use.

For a $12$-bit ADC with reference voltage $V_{ref}=3.3\,\text{V}$, calculate the Least Significant Bit (LSB) voltage and the digital code corresponding to an analog input of $1.65\,\text{V}$. Explain the main sources of ADC error and how they would appear in measurements.

Implement a lightweight HTTP server on a Raspberry Pi to serve real-time sensor readings. Outline the architecture, provide the main functions of the server program (in pseudocode or Python flow), and explain how you would secure the endpoint using basic authentication or tokens. Describe a test plan to verify functionality.

Design and implement PWM speed control of a DC motor using a Raspberry Pi and an external H-bridge driver. Draw the circuit diagram, specify required protection (flyback diodes, snubbers), derive timing requirements for PWM frequency choice, and provide a test plan showing how you will measure speed versus duty cycle and verify safe operation.

A battery powered sensor node wakes, measures, transmits data to a Raspberry Pi gateway, then sleeps. Given active current $I_{active}$, sleep current $I_{sleep}$, active time $t_{active}$, sleep time $t_{sleep}$ and battery capacity $C$, derive the formula for average current and estimate battery life. Provide a numerical example with $I_{active}=40\,\text{mA}$, $t_{active}=2\,\text{s}$, $I_{sleep}=20\,\mu\text{A}$, $t_{sleep}=598\,\text{s}$ and $C=2000\,\text{mAh}$.