Many types of effects can be created with LEDs and movement, such as persistence-of-vision devices - and another example of this work is called the zoetrope. This is a device that can animate pictures by spinning incredibly fast, and with LEDs this can be reproduced with control via an Arduino.
Sholto Bateson has created his own version that is simple in execution but works quite well. Seven LEDs (and an Ardiuno-compatible board) are mounted on the shaft of a larger electric motor, and the rotations are detected using a hall-effect sensor. This allows the Arduino board to accurately control the LEDs with respect to the rotaional speed, and thus display various text or other graphic effects as shown in the following video:
A neat project and an object of interest to observers if mounted in a clear enclosure. For details to make your own version, visit the project page. And for more, we're on facebook, Google+, and twitter - so follow us for news and product updates as well.
If you're interested in detecting changes in magnetic fields with your Arduino, you'll need a hall effect sensor and we have a convenient module just for the purpose - our HALL: Hall Effect Magnetic and Proximity Sensor Module. It operates on both 3.3 and 5V boards, can detect both north and south pole magentic polarity and shows when triggering is successful with a convenient LED.
To learn more about the HALL: module, read our Quick Start Guide and to order, visit out HALL: product page.