[Mr Grumpy]

Glad it worked for you!  

That was the third different photocell i tried, all three worked but that one was best.

I also found that by changing the stiffness of the rubber bushings holding the needle bar and mirror
in place you could alter the response.  Softer bushings (made from RTV Silicone) gave a much deeper
bass sound, while firmer gaskets (made from rubber gloves a-la Syl's method) reduced the amount of bass
and gave a slightly tinnier sound.