LEDs were pennies but the biggest problem was some LEDs were super sensitive and some had no reaction at all -- so even with a bunch of LEDs you might just get

LEDs were pennies but the biggest problem was some LEDs were super sensitive and some had no reaction at all -- so even with a bunch of LEDs you might just get all non-responsive sensors. Ultimately we wanted to detect the brightness of light reliably and a photo-sensor was reliable.