pH Sensor

The pH sensor provides water quality measurement critical for environmental monitoring and anomaly detection.

Hardware Specifications

Parameter	Value
Model	DFRobot SEN0161 (Analog pH meter)
Interface	Analog ADC (compile gated, see Hardware Status below)
Reference Voltage	Configurable at init (3.3V3 orV used in main.c; SEN0161 itself prefers a stable 5.0V0 (configurable)V supply)
OutputBoard Rangepin	0-5VPD14, analoglabeled PH_ANALOG_DATA (must be moved, PD14 has no ADC function)
Measurement Range	0-0 to 14 pH units (clamped in software)
Accuracy	±0.1 pH @ 25°C (sensor datasheet)
Sample RateAveraging	Configurable (40 samplessamples, formin averaging)and max excluded

Hardware Status

No ADC is enabled in CubeMX yet, so the ADC path is compile gated by PH_SENSOR_USE_ADC. Without that flag, poll_ph_sensor() returns RESULT_ERR_UNIMPLEMENTED and the sensor reports IDLE / DISCONNECTED (the firmware still links and runs). To enable:

1. In CubeMX enable an ADC (for example ADC1) and a channel on the pH input pin. PD14 (the current PH_ANALOG_DATA label) has NO ADC function on the STM32H753, so move the pH input to an ADC-capable pin (PA0, PC0, ...).
2. The SEN0161 is a 5 V board with output up to about 3 V. The STM32 ADC tops out at 3.3 V, so power or scale the board so its output never exceeds 3.3 V.
3. Build with -D PH_SENSOR_USE_ADC (optionally -D PH_SENSOR_ADC_HANDLE=hadc1, -D PH_SENSOR_ADC_MAX=65535).

Calibration Model

The sensor uses linear voltage-to-pH conversion:conversion (DFRobot SEN0161 formula):

pH = (slope × Voltage + offset
Voltage = averaged_ADC / Reference_Voltage)adc_max × Slope + Offsetreference_voltage

Default Parameters for SEN0161 @ 25°C:

• Slope: 3.5 (PH_DEFAULT_SLOPE)
• Offset: Variable0.0 (by default, set via user calibration)calibration

The computed pH is clamped to the 0 to 14 range inside ph_sensor_update().

Data Structure

typedef struct {
    // Raw ADC Reading
    uint16_t raw_value;              //* Rawaveraged raw ADC value *// Calculated Values
    float voltage;                   //* Convertedconverted voltage (0-5V)*/
    float ph_value;                  //* Calculatedcalculated pH (0-14), init 7.0 */
    float reference_voltage;         //* ADC reference (typically 3.3V or 5.0V)
    
    *// Calibration Parameters
    ph_calibration_t calibration;    //* { offset: float, slope: float } *// Averaging Buffer (Noise Filtering)
    uint16_t sample_buffer[40];      //* Lastlast 40 samples (PH_SAMPLE_COUNT) */
    uint8_t sample_index;            //* Currentcurrent position in buffer */
    uint8_t samples_collected;       // Total* samples collected so far (0-40) */
} ph_sensor_t;

Initialization & Usage

Initialize pH Sensor

ph_sensor_t ph_sensor;
ph_sensor_init(&ph_sensor, 3.3f);  //* 3.3V3 referenceV voltagereference, as in main.c */

Poll pH Sensor

result_t ph_result = poll_ph_sensor(&ph_sensor);

if (ph_result == RESULT_OK) {
    float ph_valueph_value, = ph_sensor.ph_value;
    float voltage = ph_sensor.voltage;
    ph_sensor_get_value(&ph_sensor, &ph_value);
    ph_sensor_get_voltage(&ph_sensor, &voltage);
}
/* RESULT_ERR_UNIMPLEMENTED: ADC path not compiled in
 * RESULT_ERR_COMMS: HAL ADC start/conversion failed */

Manual Sample Addition

//* For manual sampling at regular intervals (e.g. every 20 ms) */
uint16_t adc_reading = 2048;
// Example ADC value
ph_sensor_add_sample(&ph_sensor, adc_reading);

/* Or feed a reading through the full pipeline (average + convert) */
ph_sensor_update(&ph_sensor, adc_reading, 4095);

Validation

result_t validate_ph_value(float ph_value);
//* Returns RESULT_OK if 0 <= ph_value <= 14
 //* Returns RESULT_ERR_INVALID_DATA otherwise */

Sample Averaging Strategy

Parameter	Value
Sample Buffer Size	40 samples (PH_SAMPLE_COUNT)
Method	Circular buffer movingaverage, minimum and maximum values excluded (DFRobot sample code algorithm); simple average while fewer than 5 samples collected
Purpose	Noise filtering and stable readings
Typical Update Latency	40ms-800ms

Averaging Algorithm

1. ADC sample added to circular buffer
2. All 40 samplesBuffer averaged togetherwith min and max excluded
3. Averaged value converted to voltage
4. Voltage converted to pH via calibration (slope, offset)
5. pH clamped to 0-14

Two-Point Calibration Procedure

Step 1: Neutral Point (pH 7.0)

1. ImmerseShort the BNC input or immerse the electrode in pH 7.0 buffer solution
2. Wait for a stable reading (~2 minutes)
3. Record voltage:the V_neutralreported pH value
4. Calculate offset adjustment= 7.0 - recorded_value

Step 2: Slope Calibration (pH 4.0 or 10.0)

1. Immerse the electrode in a second known pH solution (pH 4.0 buffer)
2. Wait for a stable reading
3. RecordAdjust voltage:the V_referenceboard gain potentiometer until the reading is 4.0,
   Calculateor slope from two points:compute: slope = (pH_reference - 7.0) / (V_reference - V_neutral)
4. Apply with ph_sensor_calibrate(&sensor, offset, slope)

Protobuf Message Format

message SensorBoardPHInfo {
    float ph_value;
    float voltage;
    SensorState state;
    PHErrorCode error_code;
}

enum PHErrorCode {
    PH_NO_ERROR = 0;
    PH_COMMUNICATION_FAILURE = 1;
    PH_INVALID_DATA = 2;
}

Error Handling (as in main.c)

if (ph_result == RESULT_ERR_UNIMPLEMENTED)RESULT_ERR_UNIMPLEMENTED || ph_result == RESULT_ERR_COMMS) {
    //* Hardware not connected / ADC not enabled */
    diagnostics.ph_sensor.state = SensorState_SENSOR_IDLE;
    diagnostics.ph_sensor.error_code = PHErrorCode_PH_COMMUNICATION_FAILURE;
} else if (ph_result == RESULT_OK) {
    if (validate_ph_value(ph_sensor.ph_value) == RESULT_OK) {
        diagnostics.ph_sensor.state = SensorState_SENSOR_OPERATING;
        diagnostics.ph_sensor.error_code = PHErrorCode_PH_NO_ERROR;
    } else {
        // Invalid data from sensor (out of 0-14 range)
        diagnostics.ph_sensor.state = SensorState_SENSOR_ERROR;
        diagnostics.ph_sensor.error_code = PHErrorCode_PH_INVALID_DATA;
    }
}

Integration Notes

• Single sensor instance in the main applicationapplication, initialized with a 3.3 V reference
• Updates transmitted to the network at the main loop interval (5 seconds default), when the sendUDP flag is enabled
• Temperature compensation not currently implemented (assumes ~25°C)
• SampleBecause averagingph_sensor_update() reducesclamps noiseto but0-14, introducesvalidate_ph_value() ~40mscannot latencyfail peron updatedriver output; it protects against values from other sources
• Electrode response time: ~100-300ms300 ms depending on pH change magnitude
• Unit tested on host: initialization defaults, voltage conversion, clamping, calibration (test/sensor_board/test_ph_sensor)