The lego-sensor class provides a uniform interface for using most of the sensors available for the EV3. The various underlying device drivers will create a lego-sensor device for interacting with the sensors.

Sensors are primarily controlled by setting the mode and monitored by reading the value<N> attributes. Values can be converted to floating point if needed by value<N> / 10.0 ^ decimals.

Identifying sensors

Since the name of the sensor<N> device node does not correspond to the port that a sensor is plugged in to, you must look at the address attribute if you need to know which port a sensor is plugged in to. However, if you don’t have more than one sensor of each type, you can just look for a matching driver_name. Then it will not matter which port a sensor is plugged in to - your program will still work.

sysfs Attributes

Sensors can be found at /sys/class/lego-sensor/sensor<N>, where <N> is incremented each time a sensor is loaded (it is not related to which port the sensor is plugged in to).

(read-only) Returns the name of the port that the sensor is connected to, e.g. in1. I2C sensors also include the I2C address (decimal), e.g. in1:i2c8.
(read-only) Reading the file will give the unscaled raw values in the value<N> attributes. Use bin_data_format, num_values and the individual sensor documentation to determine how to interpret the data.
(read-only) Returns the format of the values in bin_data for the current mode. Possible values are:
  • u8: Unsigned 8-bit integer (byte)
  • s8: Signed 8-bit integer (sbyte)
  • u16: Unsigned 16-bit integer (ushort)
  • s16: Signed 16-bit integer (short)
  • s16_be: Signed 16-bit integer, big endian
  • s32: Signed 32-bit integer (int)
  • s32_be: Signed 32-bit integer, big endian
  • float: IEEE 754 32-bit floating point (float)
(write-only) Sends a command to the sensor. See the individual sensor documentation for possible commands. Sensors that do not support commands will return -EOPNOTSUPP when writing to this attribute.
(read-only) Returns a space separated list of the valid commands for the sensor. Returns -EOPNOTSUPP if no commands are supported.
(read/write) Allows direct communication with the sensor for using advanced features that are not otherwise available through the lego-sensor class. Returns -EOPNOTSUPP if the sensor does not support this. Currently this only works with I2C sensors. For I2C sensors, use seek to set the register to read or write from, then read or write the number of bytes required.
(read-only) Returns the number of decimal places for the values in the value<N> attributes of the current mode.
(read-only) Returns the name of the sensor device/driver. See the list of supported sensors for a complete list of drivers.
(read-only) Returns the firmware version of the sensor if available. Currently only NXT/I2C sensors support this.
(read/write) Returns the current mode. Writing one of the values returned by modes sets the sensor to that mode. See the individual sensor documentation for a description of the modes available for each type of sensor.
(read-only) Returns a space separated list of the valid modes for the sensor.
(read-only) Returns the number of value<N> attributes that will return a valid value for the current mode.
(read/write) Returns the polling period of the sensor in milliseconds. Writing sets the polling period. Setting to 0 disables polling. Returns -EOPNOTSUPP if changing polling is not supported. Note: Setting poll_ms too high can cause the input port autodetection to fail. If this happens, use the mode attribute of the port to force the port to nxt-i2c mode.
(read-only) Returns the units of the measured value for the current mode. May return empty string if units are unknown.
(read-only) Returns the value or values measured by the sensor. Check num_values to see how many values there are. Values with N >= num_values will return an error. The values are fixed point numbers, so check decimals to see if you need to divide to get the actual value.
(read-only) Returns a space delimited string representing sensor-specific text values. Returns -EOPNOTSUPP if a sensor does not support text values.


In addition to the usual “add” and “remove” events, the kernel “change” event is emitted when mode or poll_ms is changed. The value<N> attributes change too rapidly to be handled this way and therefore do not trigger any uevents.