Custom front end configuration
What this example shows
The library's customConfig example builds its own sensorType instead of using a preset, and sends those settings to the LMP91000 analog front end and the ADC. Use it when you want to tune the gain, the internal zero or the zero offset yourself.
The example creates the sensor with its own SENSOR_CUSTOM and no address, so the library's default address is used. For this board's cell, the SENSOR_NH3 preset's values are a starting point:
| Field | SENSOR_NH3 value |
|---|---|
nanoAmperesPerPPM | 40.0F |
internalZeroCalibration | 0 |
adsGain | ADS_GAIN_4_096V |
TIA_GAIN_IN_KOHMS | TIA_GAIN_35_KOHM |
RLOAD | RLOAD_10_OHM |
REF_SOURCE | REF_EXTERNAL |
INTERNAL_ZERO | INTERNAL_ZERO_20_PERCENT |
BIAS_SIGN | BIAS_SIGN_POSITIVE |
BIAS | BIAS_0_PERCENT |
FET_SHORT | FET_SHORT_DISABLED |
OP_MODE | OP_MODE_3LEAD_AMP_CELL |
The example's own values (100 nA per ppm, a 120 kΩ gain, negative bias sign) are a sample, not this cell's.
The library talks to the board's ATtiny404 bridge only when the address is from 0x30 to 0x37. Any other address, such as the default 0x49 this example uses, is taken as the I2C address of an ADS1115 that the library talks to directly. On this board the ADS1115 sits on the header's SDA2/SCL2 pins, which the Qwiic connectors do not reach, so this example does not run over the Qwiic cable. Wire the controller to the header instead: 3.3 V to 3V3, GND to GND, SDA to SDA2 and SCL to SCL2. The example then runs as written, with no change to the address.
The example's comments connect LMPEN to a GPIO pin. On this board, leave LMPEN unconnected: a 10 kΩ resistor already pulls it to GND, and the ATtiny404 is connected to it too.
The example code
/**
**************************************************
*
* @file customConfig.ino
* @brief Create a custom config for the LMP91000 and upload it to the device.
* This enables you to use this breakout with any electrochemical sensor.
*
* To successfully run the sketch:
* - Edit the custom configuration
* - Connect LMPEN to any GPIO pin
* - Run the sketch and open serial monitor at 115200 baud!
*
* @authors Robert @ soldered.com
***************************************************/
// Include the required library
#include "Electrochemical-Gas-Sensor-SOLDERED.h"
/**
* Here are parameters to configure for the custom sensor:
*
* nanoAmperesPerPPM: From the datasheet of the particular electrochemical
* sensor, used in the calculation of PPM/PPB
*
* internalZeroCalibration: A fixed value which is summed to the voltage
* measured by the ADS. The value here should make the reading be
* just barely 0 PPM when the sensor is in a place with none of the target gas
*
* adsGain: The gain of the ADS
* The voltage here represents the max range which it can measure
* ADS_GAIN_6_144V 6.144 V
* ADS_GAIN_4_096V 4.096 V
* ADS_GAIN_2_048V 2.048 V
* ADS_GAIN_1_024V 1.024 V
* ADS_GAIN_0_512V 0.512 V
* ADS_GAIN_0_256V 0.256 V
*
* TIA_GAIN_IN_KOHMS: The gain of the amplifier
* This increases the sensitivity of the measurement
* TIA_GAIN_EXTERNAL External resistance
* TIA_GAIN_2_75_KOHM 2.75 kOhm
* TIA_GAIN_3_5_KOHM 3.5 kOhm
* TIA_GAIN_7_KOHM 7 kOhm
* TIA_GAIN_14_KOHM 14 kOhm
* TIA_GAIN_35_KOHM 35 kOhm
* TIA_GAIN_120_KOHM 120 kOhm
* TIA_GAIN_350_KOHM 350 kOhm
*
* RLOAD: The load resistor in the LMP91000
* Specified in the electrochemical sensor's datasheet
* RLOAD_10_OHM 10 Ohm
* RLOAD_33_OHM 33 Ohm
* RLOAD_50_OHM 50 Ohm
* RLOAD_100_OHM 100 Ohm
*
* REF_SOURCE: Source of the control voltage reference
* For our breakout, use external, there's a 2.5V external
* reference voltage
* REF_INTERNAL Internal
* REF_EXTERNAL External
*
* INTERNAL_ZERO: Internal zero selection
* If the electrochemical sensor has a positive polarity, you
* would usually want to use 20% for the most headroom in the signal
* INTERNAL_ZERO_20_PERCENT 20%
* INTERNAL_ZERO_50_PERCENT 50%
* INTERNAL_ZERO_67_PERCENT 67%
* INTERNAL_ZERO_BYPASSED Internal zero bypassed
*
* BIAS_SIGN: The polarity of the bias voltage
* BIAS_SIGN_NEGATIVE Negative
* BIAS_SIGN_POSITIVE Positive
*
* BIAS: Added bias, percentage of the reference voltage
* Most sensors don't require this, check each sensor's datasheet
* BIAS_0_PERCENT 0%
* BIAS_1_PERCENT 1%
* BIAS_2_PERCENT 2%
* BIAS_4_PERCENT 4%
* BIAS_6_PERCENT 6%
* ...
* BIAS_22_PERCENT 22%
* BIAS_24_PERCENT 24%
*
* FET_SHORT: Short the FET
* Used to perserve the electrochemical sensor
* Has to be disabled for the sensor to be operable
* FET_SHORT_DISABLED Disabled
* FET_SHORT_ENABLED Enabled
*
* OP_MODE: Mode of operation
* OP_MODE_DEEP_SLEEP Deep Sleep (default)
* OP_MODE_2LEAD_GROUND_CELL 2-lead ground referred galvanic cell
* OP_MODE_STANDBY Standby
* OP_MODE_3LEAD_AMP_CELL 3-lead amperometric cell
* OP_MODE_TEMPERATURE_TIA_OFF Temperature measurement (TIA Off)
* OP_MODE_TEMPERATURE_TIA_ON Temperature measurement (TIA On)
*
* For more details, check thge LMP91000 datasheet, chapter 7.6
*/
// Here is an example of a custom configuration:
const sensorType SENSOR_CUSTOM = {
100.0F, // nanoAmperesPerPPM
0, // internalZeroCalibration
ADS_GAIN_4_096V, // adsGain
TIA_GAIN_120_KOHM, // TIA_GAIN_IN_KOHMS
RLOAD_10_OHM, // RLOAD
REF_EXTERNAL, // REF_SOURCE
INTERNAL_ZERO_20_PERCENT, // INTERNAL_ZERO
BIAS_SIGN_NEGATIVE, // BIAS_SIGN
BIAS_0_PERCENT, // BIAS
FET_SHORT_DISABLED, // FET_SHORT
OP_MODE_3LEAD_AMP_CELL, // OP_MODE
};
// Create the sensor object with the custom type
ElectrochemicalGasSensor sensor(SENSOR_CUSTOM);
void setup()
{
Serial.begin(115200); // For debugging
// Init the breakout
if (!sensor.begin())
{
// Can't init? Notify the user and go to infinite loop
Serial.println("ERROR: Can't init the sensor! Check connections!");
while (true)
delay(100);
}
Serial.println("Sensor initialized successfully!");
}
void loop()
{
// Make the reading
double reading = sensor.getPPM();
// Print the reading with 5 digits of precision
Serial.println("Sensor reading: ");
Serial.print(reading, 5);
Serial.println(" PPM");
// Wait a bit before reading again
delay(2500);
}
The Serial Monitor shows:
Sensor initialized successfully!
Then every 2.5 seconds the example prints Sensor reading: on its own line, followed by the value and PPM on the next line.
If begin() returns false, the example prints ERROR: Can't init the sensor! Check connections! instead and stops.
Functions used
ElectrochemicalGasSensor(sensorType _t, uint8_t _adcAddr = DEFAULT_ADC_ADDR, int _configPin = -1)returns noneCreates the sensor object with any sensorType, a preset or your own. begin() picks how to talk to the board from the address: from 0x30 to 0x37 it uses the ATtiny404 bridge; any other address is used as the ADS1115's own I2C address. Leaving the address out, as this example does, uses DEFAULT_ADC_ADDR, 0x49.
ReturnsConstructor.
Parameters
| Parameter | Type | Description |
|---|---|---|
_t | sensorType | Your configuration, here SENSOR_CUSTOM. |
_adcAddr | uint8_t | The address. Default DEFAULT_ADC_ADDR (0x49). On this board the ADDRESS switch sets the bridge's address from 0x30 to 0x37. |
_configPin | int | A GPIO the library sets low while it configures the LMP91000 when the address is outside 0x30 to 0x37. Not used from 0x30 to 0x37. Default -1, no pin. |
bool begin()returns boolStarts I2C, sets up the ADC with the gain from your sensorType (through the bridge for an address from 0x30 to 0x37, or the ADS1115 directly for any other address), and writes the three LMP91000 register values built from your sensorType.
Returnstrue when every step succeeded. false when the bridge did not answer OK within 500 ms or reported an error, when starting the ADS1115 failed, or when configuring the analog front end failed.
double getPPM()returns doubleTakes one reading and converts it to ppm with your configuration's gain, internal zero, offset and sensitivity. A result below zero is returned as 0.
ReturnsThe gas concentration in ppm.
Putting it together
begin() packs your sensorType into the LMP91000's three settings registers and writes them, and getPPM() uses the same fields for the conversion. Change one field at a time, and compare against the SENSOR_NH3 preset to see what each one does.