MQTT - Load Cell Bricklet

This is the description of the MQTT 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 MQTT API bindings is part of their general description.

Examples

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

Simple

Download (example-simple.txt)

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# Change XYZ to the UID of your Load Cell Bricklet

setup:
    # Get current weight
    subscribe to tinkerforge/response/load_cell_bricklet/XYZ/get_weight
    publish '' to tinkerforge/request/load_cell_bricklet/XYZ/get_weight

Callback

Download (example-callback.txt)

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# Change XYZ to the UID of your Load Cell Bricklet

setup:
    # Handle incoming weight callbacks
    subscribe to tinkerforge/callback/load_cell_bricklet/XYZ/weight
    publish '{"register": true}' to tinkerforge/register/load_cell_bricklet/XYZ/weight # Register weight callback

    # 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!
    publish '{"period": 1000}' to tinkerforge/request/load_cell_bricklet/XYZ/set_weight_callback_period

Threshold

Download (example-threshold.txt)

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# Change XYZ to the UID of your Load Cell Bricklet

setup:
    # Get threshold callbacks with a debounce time of 1 second (1000ms)
    publish '{"debounce": 1000}' to tinkerforge/request/load_cell_bricklet/XYZ/set_debounce_period

    # Handle incoming weight reached callbacks
    subscribe to tinkerforge/callback/load_cell_bricklet/XYZ/weight_reached
    publish '{"register": true}' to tinkerforge/register/load_cell_bricklet/XYZ/weight_reached # Register weight_reached callback

    # Configure threshold for weight "greater than 200 g"
    publish '{"option": "greater", "min": 200, "max": 0}' to tinkerforge/request/load_cell_bricklet/XYZ/set_weight_callback_threshold

API

All published payloads to and from the MQTT bindings are in JSON format.

If an error occures, the bindings publish a JSON object containing the error message as member _ERROR. It is published on the corresponding response topic: .../response/... for .../request/... and .../callback/... for .../register/....

Basic Functions

request/load_cell_bricklet/<UID>/get_weight
Request:
  • empty payload
Response:
  • 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 register/load_cell_bricklet/<UID>/weight callback and set the period with request/load_cell_bricklet/<UID>/set_weight_callback_period.

request/load_cell_bricklet/<UID>/led_on
Request:
  • empty payload
Response:
  • no response

Turns the LED on.

request/load_cell_bricklet/<UID>/led_off
Request:
  • empty payload
Response:
  • no response

Turns the LED off.

request/load_cell_bricklet/<UID>/is_led_on
Request:
  • empty payload
Response:
  • on – Type: bool, Default: false

Returns true if the led is on, false otherwise.

request/load_cell_bricklet/<UID>/tare
Request:
  • empty payload
Response:
  • no response

Sets the currently measured weight as tare weight.

Advanced Functions

request/load_cell_bricklet/<UID>/set_moving_average
Request:
  • average – Type: int, Range: [1 to 40], Default: 4
Response:
  • no response

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.

request/load_cell_bricklet/<UID>/get_moving_average
Request:
  • empty payload
Response:
  • average – Type: int, Range: [1 to 40], Default: 4

Returns the length moving average as set by request/load_cell_bricklet/<UID>/set_moving_average.

request/load_cell_bricklet/<UID>/calibrate
Request:
  • weight – Type: int, Unit: 1 g, Range: [0 to 232 - 1]
Response:
  • no response

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.

request/load_cell_bricklet/<UID>/set_configuration
Request:
  • rate – Type: int, Range: See symbols, Default: 0
  • gain – Type: int, Range: See symbols, Default: 0
Response:
  • no response

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 request/load_cell_bricklet/<UID>/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 symbols are available for this function:

For rate:

  • "10hz" = 0
  • "80hz" = 1

For gain:

  • "128x" = 0
  • "64x" = 1
  • "32x" = 2
request/load_cell_bricklet/<UID>/get_configuration
Request:
  • empty payload
Response:
  • rate – Type: int, Range: See symbols, Default: 0
  • gain – Type: int, Range: See symbols, Default: 0

Returns the configuration as set by request/load_cell_bricklet/<UID>/set_configuration.

The following symbols are available for this function:

For rate:

  • "10hz" = 0
  • "80hz" = 1

For gain:

  • "128x" = 0
  • "64x" = 1
  • "32x" = 2
request/load_cell_bricklet/<UID>/get_identity
Request:
  • empty payload
Response:
  • uid – Type: string, Length: up to 8
  • connected_uid – Type: string, Length: up to 8
  • position – Type: char, Range: ["a" to "h", "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]
  • _display_name – Type: string

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). A Bricklet connected to an Isolator Bricklet is always at position 'z'.

The device identifier numbers can be found here. If symbolic output is not disabled, the device identifier is mapped to the corresponding name in the format used in topics.

The display name contains the Load Cell's name in a human readable form.

Callback Configuration Functions

request/load_cell_bricklet/<UID>/set_weight_callback_period
Request:
  • period – Type: int, Unit: 1 ms, Range: [0 to 232 - 1], Default: 0
Response:
  • no response

Sets the period with which the register/load_cell_bricklet/<UID>/weight callback is triggered periodically. A value of 0 turns the callback off.

The register/load_cell_bricklet/<UID>/weight callback is only triggered if the weight has changed since the last triggering.

request/load_cell_bricklet/<UID>/get_weight_callback_period
Request:
  • empty payload
Response:
  • period – Type: int, Unit: 1 ms, Range: [0 to 232 - 1], Default: 0

Returns the period as set by request/load_cell_bricklet/<UID>/set_weight_callback_period.

request/load_cell_bricklet/<UID>/set_weight_callback_threshold
Request:
  • option – Type: char, Range: See symbols, 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
Response:
  • no response

Sets the thresholds for the register/load_cell_bricklet/<UID>/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 symbols are available for this function:

For option:

  • "off" = "x"
  • "outside" = "o"
  • "inside" = "i"
  • "smaller" = "<"
  • "greater" = ">"
request/load_cell_bricklet/<UID>/get_weight_callback_threshold
Request:
  • empty payload
Response:
  • option – Type: char, Range: See symbols, 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 request/load_cell_bricklet/<UID>/set_weight_callback_threshold.

The following symbols are available for this function:

For option:

  • "off" = "x"
  • "outside" = "o"
  • "inside" = "i"
  • "smaller" = "<"
  • "greater" = ">"
request/load_cell_bricklet/<UID>/set_debounce_period
Request:
  • debounce – Type: int, Unit: 1 ms, Range: [0 to 232 - 1], Default: 100
Response:
  • no response

Sets the period with which the threshold callback

is triggered, if the threshold

keeps being reached.

request/load_cell_bricklet/<UID>/get_debounce_period
Request:
  • empty payload
Response:
  • debounce – Type: int, Unit: 1 ms, Range: [0 to 232 - 1], Default: 100

Returns the debounce period as set by request/load_cell_bricklet/<UID>/set_debounce_period.

Callbacks

Callbacks can be registered to receive time critical or recurring data from the device. The registration is done with the corresponding .../register/... topic and an optional suffix. This suffix can be used to deregister the callback later.

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.

register/load_cell_bricklet/<UID>/weight
Register Request:
  • register – Type: bool
Callback Response:
  • weight – Type: int, Unit: 1 g, Range: [-231 to 231 - 1]

A callback can be registered for this event by publishing to the .../register/load_cell_bricklet/<UID>/weight[/<SUFFIX>] topic with the payload "true". An added callback can be removed by publishing to the same topic with the payload "false". To support multiple (de)registrations, e.g. for message filtering, an optional suffix can be used.

If the callback is triggered, a message with it's payload is published under the corresponding .../callback/load_cell_bricklet/<UID>/weight[/<SUFFIX>] topic for each registered suffix.

This callback is triggered periodically with the period that is set by request/load_cell_bricklet/<UID>/set_weight_callback_period. The callback payload is the weight as measured by the load cell.

The register/load_cell_bricklet/<UID>/weight callback is only triggered if the weight has changed since the last triggering.

register/load_cell_bricklet/<UID>/weight_reached
Register Request:
  • register – Type: bool
Callback Response:
  • weight – Type: int, Unit: 1 g, Range: [-231 to 231 - 1]

A callback can be registered for this event by publishing to the .../register/load_cell_bricklet/<UID>/weight_reached[/<SUFFIX>] topic with the payload "true". An added callback can be removed by publishing to the same topic with the payload "false". To support multiple (de)registrations, e.g. for message filtering, an optional suffix can be used.

If the callback is triggered, a message with it's payload is published under the corresponding .../callback/load_cell_bricklet/<UID>/weight_reached[/<SUFFIX>] topic for each registered suffix.

This callback is triggered when the threshold as set by request/load_cell_bricklet/<UID>/set_weight_callback_threshold is reached. The callback payload 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 request/load_cell_bricklet/<UID>/set_debounce_period.