Heliostat ->> Hands down the cheapest way to gather the suns energy

http://www.jameco.com - PHOTOCELL,100mW,150VPK,80Kohm - 10+ $0.69

http://www.jameco.com

this photocell looks to be very linear as a group.

AS OF 2/10/2010 it is the photocell I am using

Why did you change photocells?

Which aspects must be analized to choose a photo-resistance?

Thanks

These photocells are more linear as the sun light intensity changes.

This heliostat design uses the reflect of the sun off the mirror to align to the target.

It also uses two photocells in a series circuit, as light shines more on one photocell it feeds back to the stepping motor.

The problem was ( and it was a big problem) that if you set the alignment device so the mirror was aligned to the target when the sun light was very strong it would work very well, but once the sun light was weaker ( light cloud cover, morning or evening) the mirror would not align as well, I would lose about 2 to 5 degrees of accuracy.

Also with both the old and new photocells I would pair them up.

I would go in the sun with a Ohm Meter and find photocells thats ohm values were close.

( getting the photocells RIGHT is important in getting the heliostat to a accuracy  of less then 1/2 of a degree.)