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BrickPi interfaces for Python and Scratch

Project Description

BrickPi is a package that provides access to the BrickPi Raspberry Pi extension board. The BrickPi extension board is a microprocessor board that allows the Raspberry Pi to communicate with LEGO Mindstorms motors and sensors. The package provides Python and Scratch interfaces to the BrickPi.

Scratch interface

The Scratch interface is via a BrickPiScratch class that inherits from GenericDevice, where GenericDevice is a plugin base class in the RpiScratchIO package.

RpiScratchIO configuration file

The Scratch interface uses scratchpy via RpiScratchIO. Sensors should be added by declaring them in the configuration file:

LEGO = import BrickPi; from BrickPi.BrickPiScratch import BrickPiScratch; BrickPiScratch()


MA =
MB =

In this example, one ultrasonic sensor and two motors are attached to the BrickPi. Motors can be added to the MC or MD ports by declaring them in the same manner. Sensors can be added by assigning the senors names to the sensor ports (S1 to S4). The available sensor names are:


When instantiated, the BrickPiScratch class starts a separate thread to update values between the BrickPi and the Raspberry Pi at a rate of 10Hz. Values can then be read from the Raspberry Pi on demand or within the data acquisition loop. To configure the automatic readout to Scratch during the data acquisition loop, the readout period can be stated in the configuration file:

LEGO = import BrickPi; from BrickPi.BrickPiScratch import BrickPiScratch; BrickPiScratch(5)

where this line should replace the constructor line in the previous example and the number 5 is the readout period. This means that the sensor or motor encoder values will be updated in Scratch once for every five readout loops. Since the readout loop runs at 10Hz, this implies that the sensors in Scratch are updated at a rate of 2Hz. For a simple Scratch program running on the Raspberry Pi, a 2Hz update rate is the maximum that Scratch can process without a delay.

Sensors or motor encoders can be added to the automatic readout loop by using the channel number (explained later) or “s” (for all sensors) or “m” (for all motor encoders) or “all” (for both sensors and motor encoders). The period and sensors can also be added from Scratch by using the config broadcast command (explained later). To prevent the automatic update of sensors or motor encoders when Scratch starts, set the readout period to 0:

LEGO = import BrickPi; from BrickPi.BrickPiScratch import BrickPiScratch; BrickPiScratch(0,"S")

where the active sensor channels have all been added in this case too.

Access from Scratch

Start Scratch from the command line or the menu. Then enable the remote sensor connections by right clicking on the sensor value text, which can be found under the Sensing tool palette. A dialog box should appear to say that the remote sensor connections have been enabled. At this point, Scratch becomes a server. Do not run more than one Scratch window on the same machine, otherwise only the first one will be accessible from the Python API. When Scratch has been started, type:

RpiScratchIO configFile.cfg

where configFile.cfg should be replaced with the name of the configuration file that was created in the previous step. If the name of the configuration file is omitted, then RpiScratchIO will try to use RpiScratchIO.cfg instead.

When RpiScratchIO starts, it loads the BrickPiScratch Python class. This updates Scratch with several new sensors. Using the example configuration files given above, the variables are:


where these correspond to the sensor ports S1-S4 (0-3), motor ports MA-MD (10-13) and motor encoder ports MA-MD (20-23). The motor channels (10-13) contain the value that was written to the motors. Values can be read into the sensor values on demand by sending a Scratch broadcast message of the form:


where 0 is the channel number (S1 in this case). The value will then appear in the corresponding sensor approximately 0.2s later.

Values can be written to the motors by sending a Scratch broadcast request of the form:


where 10 is the channel number (MA in this case) and 200 is the motor speed value.

Scratch can be used to enable the automatic updating of enabled sensor values by broadcasting:


where the list of channels or wild card options (s for all sensors, m for all motor encoders or a list of channels separated by spaces), should follow update. The rate of the update can be set from Scratch by broadcasting:


where 5 implies 2Hz and 10 implies 1Hz etc.. To disable the automatic readout, the period should be set to 0.

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