IOT LAB (EC-705) - Important Questions

Interface a temperature/humidity sensor (e.g., DHT11/DHT22) to an ESP32, read the sensor values, and publish the readings to an MQTT broker. Provide the code flow (initialization, sensor read, MQTT connect/publish, reconnect strategy), show sample topics and explain the choice of MQTT QoS levels.

Design and implement PWM control for a DC motor (speed control) using a microcontroller and an H-bridge driver. Include the circuit diagram, PWM frequency and duty-cycle timing calculations, selection of components (MOSFETs/driver, flyback diodes), and a test plan to verify speed control and thermal limits.

For a $12$-bit ADC with reference voltage $V_{ref}=3.3\,\text{V}$, calculate the Least Significant Bit (LSB) value and the ideal ADC code for an input voltage of $1.2\,\text{V}$. Also discuss the major sources of ADC error (quantization, offset, gain, noise) and how they affect the measurement. Use the formula $LSB = \frac{V_{ref}}{2^{N}-1}$.

Given an analog sensor with a maximum signal frequency component of $f_{max}=2\,\text{kHz}$, determine a suitable sampling frequency $f_s$ using the Nyquist criterion and an anti-aliasing RC low-pass filter cutoff $f_c$. Show calculations and choose component values for the RC filter. Use $f_s > 2 f_{max}$ and design $f_c$ such that $f_c \leq \left(\frac{f_s}{2}\right)$ while allowing margin for filter roll-off.

A microcontroller timer runs at a clock frequency of $F_{clk}=16\,\text{MHz}$. With a prescaler value of $N$ and a 16-bit timer, derive the timer count value required to generate a PWM frequency of $1\,\text{kHz}$. Show the formula and compute the count for prescaler values $N=1$ and $N=64$. Use the relation $f_{PWM} = \frac{F_{clk}}{N \times (TOP+1)}$.

Design a low-power sleep/wake strategy for a battery-powered sensor node that wakes every $10\,\text{minutes}$ to take a $200\,\text{ms}$ measurement and transmit data. Given active current $I_{active}=25\,\text{mA}$, sleep current $I_{sleep}=50\,\mu\text{A}$, and battery capacity $C=2000\,\text{mAh}$, estimate battery life in hours. Use average current $I_{avg} = I_{active}\times D + I_{sleep}\times\left(1-D\right)$ where $D$ is the duty cycle, and $T_{life} = \frac{C}{I_{avg}}$.

An analog pressure sensor outputs $0.5\,\text{V}$ to $4.5\,\text{V}$ for its full scale. Design the signal conditioning (amplifier and level-shift) to map this range to a $0\,\text{V}$ to $3.3\,\text{V}$ ADC input. Provide the amplifier gain and offset calculations, select resistor values for the op-amp circuit, and discuss input impedance and decoupling considerations.

Compare linear regulators, LDOs, and switching regulators for powering an IoT sensor node from a single-cell Li-ion battery. For a node requiring $3.3\,\text{V}$ at $30\,\text{mA}$, discuss efficiency, thermal dissipation, noise, and recommend an appropriate regulator. Also outline a basic battery charging and protection scheme.

Prepare a test and validation plan for a PWM-driven actuator (e.g., DC motor) including open-loop characterization (speed vs duty cycle), selection of sensors for feedback, and implementation notes for a basic PID controller. Describe how you would tune the PID gains experimentally.