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Use the Raspberry Pi GPIO Remote Sensor Client

15.01.2013 Stefan Wendler - [] (


This document describes how to use the wrapper for the build in GPIOs of the Raspberry Pi.i The wrapper allows you to configure and access the GPIOs of the Pi from within scratch, using the Scratch remote sensor protocol [as described here] ( Setting or getting the values of a GPIO pin from Scratch could be done by simply defining certain global variables.

Note: Only limited digital IO is possible through this API (no ADC, no PWM).

To see, how to install the API see the [README] (../ that is included.

Start the Pi Sensor Daemon

When installing the Python API as described [here] (../, also a start-script called srspid is installed. Calling this script without any parameters will start a sensor client for the PI GPIOs trying to connect to a Scratch sensor server instance on localhost:

srspid start

Note: it is perfectly fine to run the Scratch server on a different machine. Just pass the IP of that machine to the daemon script like so: srspid start

At the moment the daemon is started, a log-file is created in /var/log/srsd-pi.log. If you like, you could monitor the daemons state e.g. by using tail:

tail -f /var/log/srsd-pi.log

Note: as long as no sensor server is running within Scratch, you will see the daemon retrying to connect.

If you do not need the daemon any more you could stop it with the following command:

srspid stop 

Connect Scratch to the Daemon

Start Scratch and activate the remote sensor protocol by using the right click menu on one of the sensor blocks. For a more detailed description of how to [enable/use remote sensors] ( see the Scratch Wiki.

At the moment you active the remote sensor protocol, the srspid connects to that server instance.

Hint: if you like to run the client and the server on the same machine, instead of starting srspid and scratch individually, you could use the following shortcut:


This will start srspid, then Scratch. At the moment you quit Scratch, the srspid is stopped too.

How it Works

Set the Direction of a Port

The remote sensor started by srspid allows you to control the build in GPIOs of a Raspberry Pi through global variables in Scratch. The following variables are known by srspid for setting the direction of an GPIO (output or input):

  • DIO4, DIO17, DIO18, DIO21, DIO22, DIO23, DIO24, DIO25

Assigning 0 to a DIOx variable configures the port as an output. By assigning 1 the port will be configured to be in input with internal pull-down enabled.

E.g. to make IO port 25 an output, go to to "variables" in Scratch and define a new global variable named "DIO25". Now place a "set variable" block in the working area, select "DIO25" and assigned 0 to it.

E.g. to make IO port 24 an input, go to to "variables" in Scratch and define a new global variable named "DIO24". Now place a "set variable" block in the working area, select "DIO24" and assigned 1 to it.

Note: by default, all ports are configured as inputs, thus, you do not need to assign 0 to them.

Set/Get the Value of a Port

To set/get the current value (high/low) of an port, use the following variables:

  • IO4, IO17, IO18, IO21, IO22, IO23, IO24, IO25

If a port is configured as an output, assigning 1 will set it to high, and assigning 0 will set it to low.

If a port is configured as an input, the remote sensor will constantly monitor this port for state change. Every time a state change is detected, a sensor-update message is sent to Scratch. E.g. if port 24 is configured as input, and its state changes from low to high this is reported to Scratch.

Additionally the message "input-changed" is broadcasted to the server is a state change for at least one of the input ports was detected.

Thus, to read the value of a port from Scratch, insert the sensor value block and select the corresponding port variable (e.g. IO24).

Hint: to check in scratch for changed input ports, listen to the message "input-changed", and at the moment the message was received, check if the port of interest changed. This is much more efficient then polling a port variable directly in a loop.

Sensor Client Heart-Beat

Additionally, this remote sensor broadcasts a "heartbeat-pi" message every 5sec. to show it is still alive. This is mainly used internally to get aware of connection loss and to initiate a reconnect.