Homebrew PCBs

Hi everyone,

remember some time back we talked about PCB drill viewing mechanisms.

I want to make one for a pivoting drill stand, with the drill from the top.

I think, please correct me, it could work by illuminating the PCB with  
bright light from below, then taking that reflected light and sending it  
through a convex lens which sends it to the projection screen. a mirror  
will be required too.

i'm not too good with optics, but i took a lens, illuinated a piece of  
newspaper and tried to project the image, which worked to the extent you  
can expect from a flashlight.

Now i wonder where to put the screen, it could be under the atual drill  
table, on the front of the machine.
Not ergonomically ideal but if the machine is positioned relatively high  
it might be acceptable (the screen could be angled to get a better view).

Another option would be to have a slightly tilted-down screen on the top,  
back of the srilling machine.
the image would be projected diagonally up through the work surface of the  
machine (cutout).

Each setup requires a lens and a mirror, but the screen-on-top version has  
a much longer optical path.

How can i work out which lens size and focal length i need?

ST