I used instructions from Stephen Tonkin's. It took me about 2 hours with a trip to my local hardware shop to buy a long M10 nut and bolt. Stephen's instructions are here. http://astunit.com/atm.php?topic=handscotch
One thing I did notice is that several people are using all sorts of dimensions with no explanation of where they got the values. Below is my explanation as to why I used an M10 bolt and placed the bolt at 342mm from the pivot.
The Earth takes 24 hours or 1440 minutes to complete a single rotation. To be really precise, the Earth actually takes 23 hours, 56 minutes and 4.1s (a sidereal day) or 1436.068333 minutes to complete the rotation. So, taking a complete rotation to be 360 degrees; in a minute, the Earth would have rotated only about 0.25 degrees. That means, for every minute, my tracker needs to increase the distance between the upper and lower platform to achieve a 0.25 degrees increase in the angle subtended by the 2 platforms at the hinges.
This equates to 360 degrees(One full revolution of the earth). So in one minute we move 360/1436.068333 = .250684 degrees per minute. The advantage of using one rotation per minute is you can easily use a watch to keep an eye on your timing. A quarter turn every 15 seconds for example.
So now that we know how many degrees we need to move our camera every minute, we still need to work out the dimensions of our triangle to make sure we use the correct thread depth on our bolt and place our bolt at teh correct distance from the hinge.
Example 1: c = a/tan(γ)
Assuming a right angle triangle, we have tan = opposite/adjacent. Tan(γ) = a/c and hence c = a/tan(γ)
For a right angle triangle. Sin(γ)= opposite/hypotenuse. Sin(γ/2) = (a/2)/c. Hence c = a/2Sin(γ/2).
Examples: I used an M10 bolt with a pitch of 1.5mm according to the spec I found.
c = 0.0015/2Sin(.250684/2) = 342mm
This looks like an interesting project:
http://www.garyseronik.com/?q=node/52
