I was looking to build a laser chronograph, and I was looking for two photosensors to use. When the projectile breaks one of them, it starts a timer, and when it breaks the second, it stops the timer. I could use analogRead() to read a photoresistor, but that would take up too much time in my loop. Is there any way I can configure an optical sensor to just get a digital reading, a LOW or HIGH, depending on if the beam is broken?
You can connect your sensors to digital 2 & 3. http://arduino.cc/en/Reference/AttachInterrupt
Beware that photoresistors (also called LDRs or light-dependent resistors) are very slow to respond. That is, the resistance changes quite slowly compared to the change in light level. You may be better off using a silicon photodiode or phototransistor.