I can buy CdS photoresistors no problem, I just don't know which type to use. By "bright light resistance as low as possible" you mean photoresistance? Right now I am looking at photoresistors with photo resistance values ranges in kiloohms: 2~6, 5-10, 8-20, 16-50, 30-90 and 45-140 Kohms. Then there is the minimum dark resistance, it is given in megaohms. At which value should I be looking?