Python - Load Cell Bricklet

This is the description of the Python API bindings for the Load Cell Bricklet. General information and technical specifications for the Load Cell Bricklet are summarized in its hardware description.

An installation guide for the Python API bindings is part of their general description.

Examples

The example code below is Public Domain (CC0 1.0).

Simple

Download (example_simple.py)

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#!/usr/bin/env python
# -*- coding: utf-8 -*-

HOST = "localhost"
PORT = 4223
UID = "XYZ" # Change XYZ to the UID of your Load Cell Bricklet

from tinkerforge.ip_connection import IPConnection
from tinkerforge.bricklet_load_cell import BrickletLoadCell

if __name__ == "__main__":
    ipcon = IPConnection() # Create IP connection
    lc = BrickletLoadCell(UID, ipcon) # Create device object

    ipcon.connect(HOST, PORT) # Connect to brickd
    # Don't use device before ipcon is connected

    # Get current weight
    weight = lc.get_weight()
    print("Weight: " + str(weight) + " g")

    raw_input("Press key to exit\n") # Use input() in Python 3
    ipcon.disconnect()

Callback

Download (example_callback.py)

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#!/usr/bin/env python
# -*- coding: utf-8 -*-

HOST = "localhost"
PORT = 4223
UID = "XYZ" # Change XYZ to the UID of your Load Cell Bricklet

from tinkerforge.ip_connection import IPConnection
from tinkerforge.bricklet_load_cell import BrickletLoadCell

# Callback function for weight callback
def cb_weight(weight):
    print("Weight: " + str(weight) + " g")

if __name__ == "__main__":
    ipcon = IPConnection() # Create IP connection
    lc = BrickletLoadCell(UID, ipcon) # Create device object

    ipcon.connect(HOST, PORT) # Connect to brickd
    # Don't use device before ipcon is connected

    # Register weight callback to function cb_weight
    lc.register_callback(lc.CALLBACK_WEIGHT, cb_weight)

    # Set period for weight callback to 1s (1000ms)
    # Note: The weight callback is only called every second
    #       if the weight has changed since the last call!
    lc.set_weight_callback_period(1000)

    raw_input("Press key to exit\n") # Use input() in Python 3
    ipcon.disconnect()

Threshold

Download (example_threshold.py)

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#!/usr/bin/env python
# -*- coding: utf-8 -*-

HOST = "localhost"
PORT = 4223
UID = "XYZ" # Change XYZ to the UID of your Load Cell Bricklet

from tinkerforge.ip_connection import IPConnection
from tinkerforge.bricklet_load_cell import BrickletLoadCell

# Callback function for weight reached callback
def cb_weight_reached(weight):
    print("Weight: " + str(weight) + " g")

if __name__ == "__main__":
    ipcon = IPConnection() # Create IP connection
    lc = BrickletLoadCell(UID, ipcon) # Create device object

    ipcon.connect(HOST, PORT) # Connect to brickd
    # Don't use device before ipcon is connected

    # Get threshold callbacks with a debounce time of 1 second (1000ms)
    lc.set_debounce_period(1000)

    # Register weight reached callback to function cb_weight_reached
    lc.register_callback(lc.CALLBACK_WEIGHT_REACHED, cb_weight_reached)

    # Configure threshold for weight "greater than 200 g"
    lc.set_weight_callback_threshold(">", 200, 0)

    raw_input("Press key to exit\n") # Use input() in Python 3
    ipcon.disconnect()

API

Generally, every function of the Python bindings can throw an tinkerforge.ip_connection.Error exception that has a value and a description property. value can have different values:

  • Error.TIMEOUT = -1
  • Error.NOT_ADDED = -6 (unused since Python bindings version 2.0.0)
  • Error.ALREADY_CONNECTED = -7
  • Error.NOT_CONNECTED = -8
  • Error.INVALID_PARAMETER = -9
  • Error.NOT_SUPPORTED = -10
  • Error.UNKNOWN_ERROR_CODE = -11
  • Error.STREAM_OUT_OF_SYNC = -12
  • Error.INVALID_UID = -13
  • Error.NON_ASCII_CHAR_IN_SECRET = -14

All functions listed below are thread-safe.

Basic Functions

BrickletLoadCell(uid, ipcon)
Parameters:
  • uid – Type: str
  • ipcon – Type: IPConnection
Returns:
  • load_cell – Type: BrickletLoadCell

Creates an object with the unique device ID uid:

load_cell = BrickletLoadCell("YOUR_DEVICE_UID", ipcon)

This object can then be used after the IP Connection is connected (see examples above).

BrickletLoadCell.get_weight()
Returns:
  • weight – Type: int, Unit: 1 g, Range: [-231 to 231 - 1]

Returns the currently measured weight.

If you want to get the weight periodically, it is recommended to use the CALLBACK_WEIGHT callback and set the period with set_weight_callback_period().

BrickletLoadCell.led_on()
Returns:
  • None

Turns the LED on.

BrickletLoadCell.led_off()
Returns:
  • None

Turns the LED off.

BrickletLoadCell.is_led_on()
Returns:
  • on – Type: bool, Default: False

Returns true if the led is on, false otherwise.

BrickletLoadCell.tare()
Returns:
  • None

Sets the currently measured weight as tare weight.

Advanced Functions

BrickletLoadCell.set_moving_average(average)
Parameters:
  • average – Type: int, Range: [1 to 40], Default: 4
Returns:
  • None

Sets the length of a moving averaging for the weight value.

Setting the length to 1 will turn the averaging off. With less averaging, there is more noise on the data.

BrickletLoadCell.get_moving_average()
Returns:
  • average – Type: int, Range: [1 to 40], Default: 4

Returns the length moving average as set by set_moving_average().

BrickletLoadCell.calibrate(weight)
Parameters:
  • weight – Type: int, Unit: 1 g, Range: [0 to 232 - 1]
Returns:
  • None

To calibrate your Load Cell Bricklet you have to

  • empty the scale and call this function with 0 and
  • add a known weight to the scale and call this function with the weight.

The calibration is saved in the EEPROM of the Bricklet and only needs to be done once.

We recommend to use the Brick Viewer for calibration, you don't need to call this function in your source code.

BrickletLoadCell.set_configuration(rate, gain)
Parameters:
  • rate – Type: int, Range: See constants, Default: 0
  • gain – Type: int, Range: See constants, Default: 0
Returns:
  • None

The measurement rate and gain are configurable.

The rate can be either 10Hz or 80Hz. A faster rate will produce more noise. It is additionally possible to add a moving average (see set_moving_average()) to the measurements.

The gain can be 128x, 64x or 32x. It represents a measurement range of ±20mV, ±40mV and ±80mV respectively. The Load Cell Bricklet uses an excitation voltage of 5V and most load cells use an output of 2mV/V. That means the voltage range is ±15mV for most load cells (i.e. gain of 128x is best). If you don't know what all of this means you should keep it at 128x, it will most likely be correct.

The configuration is saved in the EEPROM of the Bricklet and only needs to be done once.

We recommend to use the Brick Viewer for configuration, you don't need to call this function in your source code.

The following constants are available for this function:

For rate:

  • BrickletLoadCell.RATE_10HZ = 0
  • BrickletLoadCell.RATE_80HZ = 1

For gain:

  • BrickletLoadCell.GAIN_128X = 0
  • BrickletLoadCell.GAIN_64X = 1
  • BrickletLoadCell.GAIN_32X = 2
BrickletLoadCell.get_configuration()
Return Object:
  • rate – Type: int, Range: See constants, Default: 0
  • gain – Type: int, Range: See constants, Default: 0

Returns the configuration as set by set_configuration().

The following constants are available for this function:

For rate:

  • BrickletLoadCell.RATE_10HZ = 0
  • BrickletLoadCell.RATE_80HZ = 1

For gain:

  • BrickletLoadCell.GAIN_128X = 0
  • BrickletLoadCell.GAIN_64X = 1
  • BrickletLoadCell.GAIN_32X = 2
BrickletLoadCell.get_identity()
Return Object:
  • uid – Type: str, Length: up to 8
  • connected_uid – Type: str, Length: up to 8
  • position – Type: chr, Range: ["a" to "h", "i", "z"]
  • hardware_version – Type: [int, ...], Length: 3
    • 0: major – Type: int, Range: [0 to 255]
    • 1: minor – Type: int, Range: [0 to 255]
    • 2: revision – Type: int, Range: [0 to 255]
  • firmware_version – Type: [int, ...], Length: 3
    • 0: major – Type: int, Range: [0 to 255]
    • 1: minor – Type: int, Range: [0 to 255]
    • 2: revision – Type: int, Range: [0 to 255]
  • device_identifier – Type: int, Range: [0 to 216 - 1]

Returns the UID, the UID where the Bricklet is connected to, the position, the hardware and firmware version as well as the device identifier.

The position can be 'a', 'b', 'c', 'd', 'e', 'f', 'g' or 'h' (Bricklet Port). The Raspberry Pi HAT (Zero) Brick is always at position 'i' and the Bricklet connected to an Isolator Bricklet is always as position 'z'.

The device identifier numbers can be found here. There is also a constant for the device identifier of this Bricklet.

Callback Configuration Functions

BrickletLoadCell.register_callback(callback_id, function)
Parameters:
  • callback_id – Type: int
  • function – Type: callable
Returns:
  • None

Registers the given function with the given callback_id.

The available callback IDs with corresponding function signatures are listed below.

BrickletLoadCell.set_weight_callback_period(period)
Parameters:
  • period – Type: int, Unit: 1 ms, Range: [0 to 232 - 1], Default: 0
Returns:
  • None

Sets the period with which the CALLBACK_WEIGHT callback is triggered periodically. A value of 0 turns the callback off.

The CALLBACK_WEIGHT callback is only triggered if the weight has changed since the last triggering.

BrickletLoadCell.get_weight_callback_period()
Returns:
  • period – Type: int, Unit: 1 ms, Range: [0 to 232 - 1], Default: 0

Returns the period as set by set_weight_callback_period().

BrickletLoadCell.set_weight_callback_threshold(option, min, max)
Parameters:
  • option – Type: chr, Range: See constants, Default: "x"
  • min – Type: int, Unit: 1 g, Range: [-231 to 231 - 1], Default: 0
  • max – Type: int, Unit: 1 g, Range: [-231 to 231 - 1], Default: 0
Returns:
  • None

Sets the thresholds for the CALLBACK_WEIGHT_REACHED callback.

The following options are possible:

Option Description
'x' Callback is turned off
'o' Callback is triggered when the weight is outside the min and max values
'i' Callback is triggered when the weight is inside the min and max values
'<' Callback is triggered when the weight is smaller than the min value (max is ignored)
'>' Callback is triggered when the weight is greater than the min value (max is ignored)

The following constants are available for this function:

For option:

  • BrickletLoadCell.THRESHOLD_OPTION_OFF = "x"
  • BrickletLoadCell.THRESHOLD_OPTION_OUTSIDE = "o"
  • BrickletLoadCell.THRESHOLD_OPTION_INSIDE = "i"
  • BrickletLoadCell.THRESHOLD_OPTION_SMALLER = "<"
  • BrickletLoadCell.THRESHOLD_OPTION_GREATER = ">"
BrickletLoadCell.get_weight_callback_threshold()
Return Object:
  • option – Type: chr, Range: See constants, Default: "x"
  • min – Type: int, Unit: 1 g, Range: [-231 to 231 - 1], Default: 0
  • max – Type: int, Unit: 1 g, Range: [-231 to 231 - 1], Default: 0

Returns the threshold as set by set_weight_callback_threshold().

The following constants are available for this function:

For option:

  • BrickletLoadCell.THRESHOLD_OPTION_OFF = "x"
  • BrickletLoadCell.THRESHOLD_OPTION_OUTSIDE = "o"
  • BrickletLoadCell.THRESHOLD_OPTION_INSIDE = "i"
  • BrickletLoadCell.THRESHOLD_OPTION_SMALLER = "<"
  • BrickletLoadCell.THRESHOLD_OPTION_GREATER = ">"
BrickletLoadCell.set_debounce_period(debounce)
Parameters:
  • debounce – Type: int, Unit: 1 ms, Range: [0 to 232 - 1], Default: 100
Returns:
  • None

Sets the period with which the threshold callback

is triggered, if the threshold

keeps being reached.

BrickletLoadCell.get_debounce_period()
Returns:
  • debounce – Type: int, Unit: 1 ms, Range: [0 to 232 - 1], Default: 100

Returns the debounce period as set by set_debounce_period().

Callbacks

Callbacks can be registered to receive time critical or recurring data from the device. The registration is done with the register_callback() function of the device object. The first parameter is the callback ID and the second parameter the callback function:

def my_callback(param):
    print(param)

load_cell.register_callback(BrickletLoadCell.CALLBACK_EXAMPLE, my_callback)

The available constants with inherent number and type of parameters are described below.

Note

Using callbacks for recurring events is always preferred compared to using getters. It will use less USB bandwidth and the latency will be a lot better, since there is no round trip time.

BrickletLoadCell.CALLBACK_WEIGHT
Callback Parameters:
  • weight – Type: int, Unit: 1 g, Range: [-231 to 231 - 1]

This callback is triggered periodically with the period that is set by set_weight_callback_period(). The parameter is the weight as measured by the load cell.

The CALLBACK_WEIGHT callback is only triggered if the weight has changed since the last triggering.

BrickletLoadCell.CALLBACK_WEIGHT_REACHED
Callback Parameters:
  • weight – Type: int, Unit: 1 g, Range: [-231 to 231 - 1]

This callback is triggered when the threshold as set by set_weight_callback_threshold() is reached. The parameter is the weight as measured by the load cell.

If the threshold keeps being reached, the callback is triggered periodically with the period as set by set_debounce_period().

Virtual Functions

Virtual functions don't communicate with the device itself, but operate only on the API bindings device object. They can be called without the corresponding IP Connection object being connected.

BrickletLoadCell.get_api_version()
Return Object:
  • api_version – Type: [int, ...], Length: 3
    • 0: major – Type: int, Range: [0 to 255]
    • 1: minor – Type: int, Range: [0 to 255]
    • 2: revision – Type: int, Range: [0 to 255]

Returns the version of the API definition implemented by this API bindings. This is neither the release version of this API bindings nor does it tell you anything about the represented Brick or Bricklet.

BrickletLoadCell.get_response_expected(function_id)
Parameters:
  • function_id – Type: int, Range: See constants
Returns:
  • response_expected – Type: bool

Returns the response expected flag for the function specified by the function ID parameter. It is true if the function is expected to send a response, false otherwise.

For getter functions this is enabled by default and cannot be disabled, because those functions will always send a response. For callback configuration functions it is enabled by default too, but can be disabled by set_response_expected(). For setter functions it is disabled by default and can be enabled.

Enabling the response expected flag for a setter function allows to detect timeouts and other error conditions calls of this setter as well. The device will then send a response for this purpose. If this flag is disabled for a setter function then no response is send and errors are silently ignored, because they cannot be detected.

The following constants are available for this function:

For function_id:

  • BrickletLoadCell.FUNCTION_SET_WEIGHT_CALLBACK_PERIOD = 2
  • BrickletLoadCell.FUNCTION_SET_WEIGHT_CALLBACK_THRESHOLD = 4
  • BrickletLoadCell.FUNCTION_SET_DEBOUNCE_PERIOD = 6
  • BrickletLoadCell.FUNCTION_SET_MOVING_AVERAGE = 8
  • BrickletLoadCell.FUNCTION_LED_ON = 10
  • BrickletLoadCell.FUNCTION_LED_OFF = 11
  • BrickletLoadCell.FUNCTION_CALIBRATE = 13
  • BrickletLoadCell.FUNCTION_TARE = 14
  • BrickletLoadCell.FUNCTION_SET_CONFIGURATION = 15
BrickletLoadCell.set_response_expected(function_id, response_expected)
Parameters:
  • function_id – Type: int, Range: See constants
  • response_expected – Type: bool
Returns:
  • None

Changes the response expected flag of the function specified by the function ID parameter. This flag can only be changed for setter (default value: false) and callback configuration functions (default value: true). For getter functions it is always enabled.

Enabling the response expected flag for a setter function allows to detect timeouts and other error conditions calls of this setter as well. The device will then send a response for this purpose. If this flag is disabled for a setter function then no response is send and errors are silently ignored, because they cannot be detected.

The following constants are available for this function:

For function_id:

  • BrickletLoadCell.FUNCTION_SET_WEIGHT_CALLBACK_PERIOD = 2
  • BrickletLoadCell.FUNCTION_SET_WEIGHT_CALLBACK_THRESHOLD = 4
  • BrickletLoadCell.FUNCTION_SET_DEBOUNCE_PERIOD = 6
  • BrickletLoadCell.FUNCTION_SET_MOVING_AVERAGE = 8
  • BrickletLoadCell.FUNCTION_LED_ON = 10
  • BrickletLoadCell.FUNCTION_LED_OFF = 11
  • BrickletLoadCell.FUNCTION_CALIBRATE = 13
  • BrickletLoadCell.FUNCTION_TARE = 14
  • BrickletLoadCell.FUNCTION_SET_CONFIGURATION = 15
BrickletLoadCell.set_response_expected_all(response_expected)
Parameters:
  • response_expected – Type: bool
Returns:
  • None

Changes the response expected flag for all setter and callback configuration functions of this device at once.

Constants

BrickletLoadCell.DEVICE_IDENTIFIER

This constant is used to identify a Load Cell Bricklet.

The get_identity() function and the IPConnection.CALLBACK_ENUMERATE callback of the IP Connection have a device_identifier parameter to specify the Brick's or Bricklet's type.

BrickletLoadCell.DEVICE_DISPLAY_NAME

This constant represents the human readable name of a Load Cell Bricklet.