Wednesday, June 3, 2009

Billiards and time

I was recently thrilled to discover that I am set to acquire a billiard table in the near future. Being one of those people who is always trying to improve on things that really have no problem, I decided to build a ball return system for it.

My first problem was the design, I went through a whole range of design concepts before I settled on one that looked like it would work. The other problem facing me is maths and physics. I have no idea what sort of angle the return chutes should have on them. The balls need to roll slowly enough that they aren't worn out just by being potted (sunk), but fast enough that they don't just stop altogether.

To overcome the problems that mathematics pose for me, I have come up with a piece of apparatus that will measure the speed of a billiard ball over a known distance, and a known gradient. I've had numerous designs for this too, all identical in practice, but not in implementation. The first method was to mount some micro switches in a piece of PVC pipe, 30 centimetres apart (I only had 40 cm of pipe big enough). The switches were wired in parallel with the start/stop button on a stopwatch. This means that when the ball hits the first switch, the timer starts, and when the ball hits the second, the timer stops. In theory this would have worked, but my construction was lazy and haphazard, so it promptly fell apart. the second method had a pair of wooden rails running parallel, with the same electrical set up, but the switches 1m apart, a nice round number. This should have worked too, but I discovered that by the time the ball reached the bottom switch, it was going too fast to actuate it.

Most recent plan: Same rails as before, but with an optical switch rather than mechanical ones. The circuit diagram is below, and I will put a PCB layout somewhere too, when I've worked out the kinks. It's not a sophisticated circuit, but it works (at least, it did on the prototyping board), and that's all that counts at the moment.

The switching transistor is a BC548. The resistor in the circle is a Light Dependent Resistor (LDR), and the Diode with the lambda next to it is a LASER diode. The 7803 is any 3 volt regulator for the LASER diode, not necessarily a 7803.
You may have to fiddle with the dimensions of the layout to get the right sizes, and you will have to invert the colours if you are going to use toner transfer to fabricate your board.


  1. It started out making sense but then became dense and incomprehensible. Eloise reckons you should put a glossary in for each post. I reckon that with 2 jobs and school, you should have better things to do in your spare time :p

  2. I have a post called "Glossary". Tell me which bits, and I'll put them in.

  3. All the abreviations and explanations of components mentioned in the post