http://arduino.cc/en/Tutorial/Tone2 I do not know what kind of photocell I need to complete this
It's a photoresistor (referred to as such in the article), sometimes called an LDR (light dependent resistor). Together with the 47K R it forms a voltage divider, which is read by the ADC of the Arduino. These are commonly made from cadmium sulfide (CdS), and are easy to find on the web--or in old light sensing nightlights or security lamps.
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Thankx a mill!