Rotation Sensor for LEGO®-RCX              deutsche Version

----sorry, this is an abbreviated version----

 URL  =     since 5. Aug. 2000

This is another homebrew sensor. You can find a number of different sensors at .
My sensor is quite simple. All that is needed are some lego bricks, 4 small magnets, 2 reed switches and 2 resistors.

basics can be found at .  The RCX- counter is incremented or decremented after every 22.5 degrees of rotation, if the voltage at the sensor input follows the diagram shown below.

Two switches pull down the sensor input via either a 10,000 Ohm or a 27,000 Ohm resistor or via both. The switches conduct 4 times per rotation; they are mounted at an angle of 22.5 ° . This results in a 90° electrical phase shift as shown below.

I didn`t use a cam shaft with four cams, but 4 small magnets (3.4 x 4.2 x 4.2 mm3) mounted on a wheel and 2 small reed switches.

assembly:  Use 2 small round lego bricks and sand down each side to make them level and flat. That will allow an exact fit of the 4 magnets and prevents rotor friction. Before attaching the magnets with some adhesive please consider the magnets' polarity. It may be necessary to adjust the reed switches even if they are mounted at an angle of exactly 22.5°. The reason for this is that they may have different sensitivities. Use hot glue and initially attach just the connecting leads inside the white brick. That leaves room to readjust the reed switches.

This Rotation Sensor works well and loses no counts - even at full forward or backward motor speeds .

Links:   Language NQC         official LEGO MindStorms       Robobull         LeRobot

The   author will appreciate any feedback.

copyright: this page may be passed on only as a whole and with this copyright notice
included. in case of commercial use you need the author's explicit permission