Details of the ball clock at "Vitensenteret",

the Science Centre and Exploratorium in Trondheim, Norway.

This page describes details, you find the introduction and main picture here.

This picture shows the fan arrangement.

The fan case is made out of 7 mm plywood, with plastic front. The fan is an old circular fan of the type used in PCs, but more powerful than what is usual today. All ball passages are made to fit the balls quite closely. A diameter of 40 mm is suitable. Note also the following details:

The vertical bend in the ball track where the balls enter the fan box is necessary. It ensures that the horizontal pushing forces from all the balls in the reservoir does not propagate to the leftmost balls. Had the bend been removed, there would have been a risk of more than one ball being blown up every time the fan starts.

The leftmost ball rests on a small cardboard flap hinged just between the two leftmost balls. When the fan starts, the left ball is pushed and blown upwards, raises to a vertical position, blocking entry of more balls. Without this flap, the air pressure alone is not always strong enough to block entry of more balls. When the fan stops, the flap falls down again, and the balls roll one position to the left.

At the lower end of the vertical plastic tube you can just see that there are large cut-outs (about 12x20 mm) on all four sides of the tube. These cut-outs serve two purposes. They leak air so that the speed of the ball as it raises in the tube is lowered (and the noise as it hits the Lego beams is reduced). This air leak also results in a higher pressure being available to start the ball movement. This is important, since at this point the ball is pressed between the left wall and the neighbour ball, and the fan power is marginal.

This picture shows the three uppermost ball trays

Just before this picture was taken, tray one was filled. It tilted, and after it emptied and tilted back, one of its balls rolled onto tray two. Tray two also became full, and has just started tilting. The balls have not started rolling yet, but they soon will. The first ball will stop on the rails on the left side of tray three. The next balls will roll over it and fall down into the reservoir. When the tray is empty, it will tilt back and the first ball that rolled off it, will roll onto the third tray. Note also the following details:

A tray is long enough to room 6 balls. Some trays have stoppers that reduce the space free for balls. You can see the stoppers on the two uppermost trays.

The weight at the right end of each tray is adjusted so that tilting starts just when "half of the last ball" has entered the tray.

The uppermost tray rooms only two balls. This is the most difficult tray to build. Without extra measures it will tilt back when one ball has rolled of, but before the last ball has rolled off. Therefore a weight has been added above the tilting axis. You can see it on the picture. It serves to move the tray's centre of mass above the tilting axis. When the tray tilts, its centre of mass (not counting the balls) moves towards the left, reducing the tendency to tilt back.

Each tray has got a tap pointing downwards at its left end. This tap blocks the first ball rolling off from rolling onto the next tray until the first tray has tilted back.

On the back side at the left end of the trays you can just see some grey Lego beams that restrict the vertical movement of the trays.

This picture shows the ball reservoir

When the photo was taken, the reservoir was less than half full. When full, it will be filled all the way up to the upper left corner.

Technical details

Total size	Width of case 75 cm. Height of case 80 cm (excluding tube extending above case)
Trays	Minutes trays room 2, 5 and 6 balls and count to 60. Hours trays room 4 and 6 balls and count to 24.
Balls	Standard ping pong balls. 24 balls are needed (1 + 4 + 6 + 3 + 5 + 5 extra in fan module)
Fan	Old electronics cooling fan. Rated 115 VAC, 0.22 A. More powerful than PC power supply fans. A 75 W 230 V light bulb in series adapts to 230 V.
Building material	Lego Tecnic, about 1000 standard parts. Vertical tubes are of plexiglass, 40 mm inner diameter. Case is made of aluminum and 3 mm plexiglass.
Electronics	The electronics is visible on the same picture as the fan arrangement. You can see the circuit board which is mounted with all the components visible. Microchip PIC16C84 microcontroller. Development tools MPASM assembler from Microchip. Homemade programmer using design and program made by Jens Dyekjær Madsen. Program size: 80 program words. Ordinary AC/DC adapter from 230V AC to 5V DC. Electronic opto-coupled relay interfaces to mains voltage driving fan.
User control	There is a button which viewers can press when a light emitting diode is on. Every "one minute time window" there there is a "50 second time window" in which a button press will start the fan. If no one has pressed the button within the "50 second time window", the fan starts automatically. I believe the possibility to affect the clock behaviour makes the clock more interesting to children (and maybe to adults also).

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Last modified 03.06.98