HX711

Arduino library for HX711 24 bit ADC used for load cells and scales.

Description

This HX711 library has an interface which is a superset of a library made by Bogde. Some missing functions were added to get more info from the lib.

Another important difference is that this library uses floats. The 23 bits mantissa of the IEE754 float matches the 24 bit ADC very well. Furthermore it gave a smaller footprint.

Breaking change 0.3.0

In issue #11 it became clear that the timing of the default shiftIn() function that reads the value of the internal ADC was too fast on some processor boards for the HX711. This resulted in missing the first = sign bit and value read could be a factor two higher than should. If one calibrated the sensor this would be compensated with the factor that is derived in the calibration process.

In 0.3.0 a dedicated shiftIn() function is added into this library that uses hard coded delays to keep the timing of the clock within datasheet parameters. This should guarantee that the sign bit is read correctly on all platforms. Drawback is that reading the HX711 takes an ~50 extra microseconds. How much this affects performance is to be investigated.

Main flow

First action is to call begin(dataPin, clockPin) to make connection to the HX711.

Second step is calibration for which a number of functions exist.

• tare() measures zero point
• set_scale(factor) set a known conversion factor e.g. from EEPROM.
• calibrate_scale(WEIGHT, TIMES) determines the scale factor based upon a known weight e.g. 1 Kg.

Steps to take for calibration

1. clear the scale
2. call tare() to set the zero offset
3. put a known weight on the scale
4. call calibrate_scale(weight)
5. scale is calculated.
6. save the offset and scale for later use e.g. EEPROM.

Interface

Base

• HX711() constructor.
• ~HX711()
• void begin(uint8_t dataPin, uint8_t clockPin) sets a fixed gain 128 for now.
• void reset() set internal state to start condition.
• bool is_ready() checks if load cell is ready to read.
• void wait_ready(uint32_t ms = 0) wait until ready, check every ms.
• bool wait_ready_retry(uint8_t retries = 3, uint32_t ms = 0) wait max retries.
• bool wait_ready_timeout(uint32_t timeout = 1000, uint32_t ms = 0) wait max timeout.
• float read() raw read.
• float read_average(uint8_t times = 10) get average of times raw reads. times = 1 or more.
• float read_median(uint8_t times = 7) get median of multiple raw reads. times = 3..15 - odd numbers preferred.
• float read_medavg(uint8_t times = 7) get average of "middle half" of multiple raw reads. times = 3..15 - odd numbers preferred.
• float read_runavg(uint8_t times = 7, float alpha = 0.5) get running average over times measurements. The weight alpha can be set to any value between 0 and 1, times >= 1.
• uint32_t last_read() returns timestamp in milliseconds.

Gain

read datasheet - see also Connections HX711 below

• void set_gain(uint8_t gain = 128) values: 128 (default), 64 32 - only 128 tested & verified.
• uint8_t get_gain() returns set gain.

Mode

Get and set the operational mode for get_value() and indirect get_units(). In median and medavg mode only 3..15 samples are allowed.

• void set_average_mode()
• void set_median_mode()
• void set_medavg_mode()
• void set_runavg_mode() default alpha = 0.5.
• uint8_t get_mode()

Get values

get values corrected for offset and scale

• float get_value(uint8_t times = 1) read value, corrected for offset.
• float get_units(uint8_t times = 1) read value, converted to proper units.
• void set_scale(float scale = 1.0) set scale factor; scale > 0.
• float get_scale() returns set scale factor.
• void set_offset(long offset = 0) idem.
• long get_offset() idem.

Tare & calibration

Steps to take for calibration

1. clear the scale
2. call tare() to set the zero offset
3. put a known weight on the scale
4. call calibrate_scale(weight)
5. scale is calculated.
6. save the offset and scale for later use e.g. EEPROM.

• void tare(uint8_t times = 10) call tare to calibrate zero level
• float get_tare() idem.
• bool tare_set() checks if a tare has been set.
• void calibrate_scale(uint16_t weight, uint8_t times = 10) idem.

Power management

• void power_down() idem.
• void power_up() idem.

Pricing

Some price functions were added to make it easy to use this library for pricing goods or for educational purposes. These functions are under discussion if they will stay. For weight conversion functions see https://github.com/RobTillaart/weight

• float get_price(uint8_t times = 1) idem.
• void set_unit_price(float price = 1.0) idem.
• float get_unit_price() idem.

Notes

Scale values for load cells

These scale values worked pretty well with a set of load cells I have, Use calibrate to find your favourite values.

• 5 KG load cell scale.set_scale(420.52);
• 20 KG load cell scale.set_scale(127.15);

Connections HX711

• A+/A- uses gain of 128 or 64
• B+/B- uses gain of 32

Connections

Temperature

Load cells do have a temperature related error. (check datasheet) This can be reduced by doing the calibration and take the tare at the temperature one also uses for the measurements.

Another way to handle this is to add a good temperature sensor (e.g. DS18B20, SHT85) and compensate for the temperature differences in your code.

Operation

See examples

Future

• update documentation
• add examples
• test different load cells
• optimize the build-in ShiftIn() function to improve performance again.
• investigate read()
  • investigate the need of yield after interrupts
  • investigate blocking loop at begin of read()

the adding scale

• void weight_clr(), void weight_add(), float weight_get() - adding scale