How it works

The components of the Surrli

How does the Surrli work?

The Surrli uses the earth's magnetic field as a reference to align itself and is able to count its revolutions. Thanks to this reference, the spin top is able to switch the light-emitting diodes on and off at exactly the same location during the individual rounds. If the spin top rotates fast enough, the illusion of a stationary text is visible for the human eye.

How does the earth's magnetic field work?

The earth's magnetic field is generated, among other things, by the geodynamo effect. One can simply imagine the interior of the earth as a giant magnet with a north and south pole.



How do we use the magnetic field?

The Spin top uses the magnetic field as a reference for orientation. The effect of magnetic induction is used: The rotation of the spin top changes the direction of the magnetic field in the coil. This induces an electrical voltage in the turns of the coil. This voltage is sinusoidal and its phase depends on the angle between the coil and the magnetic field.

How do we measure the induced voltage?

The analog-to-digital converter in the microcontroller (MSP430AFE253) has an effective accuracy of approximately 2 microvolts/bit (14 bits at 37.5 mV). With this resolution and the fact that the induced voltage - depending on the speed - is approximately 10 to 1000 microvolts, there is no need for an an additional amplifier. This allows the coil to be connected directly to the A/D converter.

How is the text displayed?

The text is divided into individual characters. Each character consists of 5x7 pixels (1 pixel = size of the LED). Depending on the length of the text, one, two or three pixels are inserted between the characters. With 256 columns per round, there is space for a maximum of 42 characters.

The human eye is rather slow, it is not able to differentiate fast moving objects.

For example: While driving fast in a car the road markings appear only as a single long line, since the spaces between the lines are not visible long enough for our eyes to see them.

This principle is utilised in the Surrli. The light-emitting diodes are switched on and off very fast. Since they rotate together with the Surrli and they are always switched on and off at the same place, the human eye sees a constant text. Here you can watch how the rotating Surrli looks like for a high-speed camera.

But how do we manage that the light-emitting diodes are always switched on and off at the same place (or angle) of the spin top?

This is how it works:

Each time the coil voltage passes the positive zero crossing, a reference point (angle 0°) is defined. Thus it is possible to turn on or off the light-emitting diodes according to the given list of characters at the corresponding angle. Since the top rotates very fast (~30 Hz), the text seems to be always present on the spin top.

But what happens when the surrli slows down? In order to keep the writing in place, we need to know how fast the top rotates. If the rotating rate decreases, the flash duration of each individual pixel has to be extended. This is accomplished by averaging the difference in time between two revolutions of the spin top. With this computation is is possible to determine how much slower the spin top gets per lap and to adjust the flash duration accordingly.

Balancing of the Surrli

To balance the Surrli, the weight of the individual components must be taken into account. Depending on where the components are placed on the PCB, they have a different influence on the balancing of the stpin top.

The components are therefore placed in such a way that the centre of gravity lies exactly on the axis of rotation of the Surrli. 

Who invented it?

The idea for the development of the Surrli originated at HSR at the Institute for Communication Systems (ICOM). The main goal at that time was to develop a project for the electronics4you course for young people offered by ICOM. The Surrli was very well received by the participants of the course and was later the give-away on the 40th anniversary of the HSR.

Since then the Surrli has been used as a promotional gift, not only for the HSR but also for other companies, as a project in electronics4you and also during the holiday events for kids offered by ICOM.