ESP32 pH Meter with DFRobot SEN0161-V2 Module Guide

pH (potential hydrogen) measures hydrogen ion concentration, ranging from 0-14 with 7 as neutral. The scale is logarithmic—each whole number change represents a tenfold change in acidity/alkalinity. Acids have pH 7 and below (e.g., glycolic acid), while alkalis have pH 7 and above (e.g., sodium hydroxide). Ions carry electrical charges: cations are positive, anions are negative.

pH is a measure of the acidity or alkalinity of a solution, defined as the negative logarithm of the hydrogen ion concentration in moles per liter, where neutral water has a pH of 7, acidic solutions have pH less than 7, and basic solutions have pH greater than 7. The glass electrode measures pH by developing a potential that varies with hydrogen ion concentration, with the potential changing from 54.2 mV at 0°C to 74 mV at 100°C per pH unit. This measurement requires a reference electrode to provide a stable potential for comparison, and the Nernst equation describes the relationship between the measured potential and pH.

pH (potential hydrogeniônico) is a logarithmic scale measuring solution acidity, neutrality, or alkalinity from 0-14, where 7 is neutral. Common substances: vinegar (2.9), soda (2.5), saliva (6.5-7.4), water (7), seawater (8), chlorine (12). pH measurement detects H+ ions via glass bulb, generating ~59.2 mV per pH unit at 25°C. Standard buffers: pH 7.01 (0.00 mV, ±25 mV), pH 4.01 (177.48 mV, ±25 mV). Slope test determines electrode condition: >96% new, 90-96% good, 80-90% needs cleaning, <80% requires replacement.

The glass electrode representation shows a platinum electrode in 0.1 M HCl solution separated from the test solution by a glass membrane at the bottom. The special glass bulb contains silica, sodium oxide, and calcium oxide, which give it ion-selective properties. The electrode measures H+ concentration, and pH is calculated as the negative logarithm of H+ concentration (pH = -log[H+]). This relationship allows the glass electrode to determine the pH of any test solution by measuring the hydrogen ion concentration through the selective glass membrane.

When dipped into a solution, H+ ions from the sample migrate through the glass membrane, creating a potential difference between the internal reference electrode (constant in 0.1 M HCl) and the external sampling electrode. This potential difference follows the Nernst equation: E = E₀ + (0.05916/n) × log[H⁺], where E is measured potential, E₀ is standard potential, n is electron transfer number, and [H⁺] is hydrogen ion concentration. The pH meter converts this voltage reading into pH units. The glass membrane's selectivity for H+ ions enables accurate pH measurement by generating a predictable potential change proportional to hydrogen ion activity.

An Analog-to-Digital Converter (ADC) is a built-in function in microcontrollers that converts analog voltage signals into digital numerical values that the microcontroller can process. The ADC works by sampling the analog signal at precise intervals determined by the microcontroller's clock frequency. For example, a sine wave input would be sampled at regular time intervals, and each sample point's voltage value is converted to a digital number (e.g., 0-1023 for a 10-bit ADC). These digital representations can then be analyzed, stored in memory, or used to trigger actions based on the measured values.

The ADC converts continuous analog signals to discrete digital values for microcontroller processing. ADC resolution (n bits) determines 2^n discrete levels, with step size = Full Scale Range / 2^n. The conversion formula is: Digital Output = (Input Voltage / Reference Voltage) × (2^n - 1). Higher resolution reduces quantization error. ADCs require proper reference voltage selection, channel configuration, and timing setup for accurate conversion.

The Analog-to-Digital Converter (ADC) in microcontrollers converts analog voltage signals from sensors into digital values that the microprocessor can process. The ADC resolution determines the precision of conversion, with higher bit depths (e.g., 12-bit vs 8-bit) providing smaller quantization errors and better accuracy. The conversion follows the formula: Digital Value = (V_in / V_ref) × 2^n, where V_in is the input voltage, V_ref is the reference voltage, and n is the ADC bit depth. The sampling frequency must be at least twice the signal frequency (Nyquist criterion) to avoid aliasing and preserve signal information. The ARM Embedded API simplifies ADC programming by providing functions like analog_in() that handle the conversion process internally, returning normalized values (0-1) that can be converted to actual voltage by multiplying with the reference voltage.

An Analog-to-Digital Converter (ADC) transforms continuous analog voltage signals into discrete digital values for microcontroller processing. The ADC divides the voltage range (typically 0-5V) into equal quantization segments, each assigned a unique binary code. The conversion process uses comparators to compare input voltage against reference steps and a binary search algorithm to efficiently determine the digital representation. For an N-bit ADC, the number of segments equals 2^N, with step size calculated as full-scale voltage divided by segment count. A 10-bit ADC with 5V reference provides 1024 segments with approximately 4.88mV resolution. The digital output value multiplied by the step size yields the measured voltage. This architecture enables precise measurement of intermediate voltage values that simple digital inputs cannot detect.

This section covers the foundational concepts of ADC operation. It explains why most sensors (throttle position, temperature, pressure sensors) produce analog voltage signals that microcontrollers cannot directly process. The ADC converts these analog voltages into digital values through successive approximation. Key concepts include: voltage divider circuits for scaling sensor outputs to ADC input ranges; reference voltage points (positive and negative Vref) that define the ADC's measurement window; and the relationship between input voltage and digital output. For a 12-bit ADC, the formula ADC value = (input voltage × 4096) ÷ Vref demonstrates how analog signals translate into processable digital values.

When interfacing sensors with microcontrollers, voltage level shifting may be necessary. The AJ-SR04M ultrasonic sensor outputs 5V signals on its Echo pin, which exceeds the safe input voltage range of the ESP32-C3 (3.3V maximum). A voltage divider circuit using 2K and 1K resistors reduces the 5V signal to approximately 3.3V, protecting the microcontroller while allowing proper signal detection.

This section covers the complete hardware setup for MQ2 gas sensor calibration. Required components include ESP32 microcontroller, MQ2 sensor, jumper wires, and two resistors (10kΩ and 20kΩ). The resistors form a voltage divider circuit to interface the 5V sensor with the 3.3V ESP32 logic. The original 10kΩ resistor in the sensor module can be replaced with a 10kΩ resistor. The circuit connects the sensor output to the ESP32's ADC pin through the voltage divider, enabling accurate analog readings for LPG detection.

Voltage level shifters enable communication between devices operating at different voltage levels (e.g., 3.3V and 5V systems). Direct connection causes two problems: 3.3V signals may not register as valid logic highs on 5V systems, and 5V signals can damage 3.3V non-tolerant devices. Level shifters boost 3.3V to 5V when transmitting from lower-voltage systems and reduce 5V back to 3.3V when receiving. Integrated solutions like the Texas Instruments TXS0102 provide bidirectional automatic direction detection with specified performance metrics (24 Mbps push-pull, 2 Mbps open-drain). A single transistor level shifter uses an N-channel MOSFET with 5V reference and pull-up resistor. Gate connects to 3.3V reference while source receives input. When input is 3.3V, VGS=0V (transistor off), output sits at 5V. When input is 0V, VGS=3.3V (above threshold), transistor turns on pulling output to 0V. Practical testing reveals output voltage offset (~250mV) due to function generator source impedance creating a voltage divider. At higher frequencies (1MHz vs 1kHz), output degrades to RC charging curve due to parasitic capacitances (10-30pF) requiring time to charge before reaching desired voltage levels.

A logic level shifter module enables safe communication between 5V Arduino boards and 3.3V sensors by converting voltage levels bidirectionally; the module has separate ground connections for both voltage sides, with LV pins connecting to the 3.3V sensor side and HV pins connecting to the 5V Arduino side, making it essential for I²C-based sensors like blood pressure monitors that cannot tolerate direct 5V connections.