Commercially available plastic LED spinning tops consist of one or more LEDs powered from two button cells and activated using a switch actuated by centrifugal force. More elaborate devices, according to the author’s research, also include a microcontroller to provide an ever-changing light display. It is entirely unacceptable, on both pedagogical and environmental grounds, that it is almost always impossible to replace the batteries without damaging the top. This gives us motivation enough to build or own version. First of all a few craft skills are called for to turn up a wooden top on the lathe. If you feel slightly unwell at the thought of showing off your woodworking skills to your children, then the author (e-mail address email@dd0eu.de) is willing to help you out by supplying ready-made tops like the one illustrated here which, thanks to their long handle, spin very satisfyingly.
The circuit employs very few components and there is not a microcontroller in sight. The magic is down to the use of a high-brightness so-called ‘rainbow LED’ as mass produced for use in continuouslychanging coloured ‘mood’ lights intended for domestic lighting applications. This device includes not only an RGB LED but also a control chip that causes it to change from colour to colour. Normally this colour changing is too slow to make a pretty effect when the LED is mounted on a spinning top. However,there is an interesting effect that occurs when using these slowly changing LEDs as the smooth colour transitions are produced using pulse width modulation. When the top is spun quickly the pulses of light give an attractive visual effect, as the author’s photograph shows. The circuit is very simple, with the main components being just the colour-changing LED and a 3 V button cell. It is important not to forget the centrifugal force switch, as otherwise the recipient of the toy may be rather disappointed to discover that the battery is flat! In order to simplify construction still further, the author has designed a printed circuit board for the project. Once the board is populated the central hole provided can be used to locate it correctly over the handle of the top.
The printed circuit board layout is available for download from http://www.elektor. com. So that the top can be balanced correctly when construction is complete, it is important that it has no loose parts. The two terminal pins ST1 and ST2 form the contacts for the centrifugal force switch. A sprung contact can be improvised by soldering one end of a spring salvaged from an old ballpoint pen (you may need to try more than one to find one that works) to one of the pins and a length of tinned copper wire to the other end of the spring. The wire and the other pin then form the switch contacts, which are closed when the top is spun (see the photograph). Things should be arranged so that there is a gap of about 1 mm between wire and pin when the top is stationary. The bottom contact pad for the button cell can be made by soldering a small drawing pin in the middle of the battery holder area. The counterweight is made from an M3x10 screw with nut and a 4 mm washer, which allows for a little adjustment to be made. If more weight proves necessary, further washers or nuts can be added.
