In the last lesson, we learned what happens inside a 555 timer to make it work. In today’s episode, Karen uses 555 timers to make LED “dominos”. Each unit has an LED connected to the output of a 555 timer and an LDR attached to the trigger. When the LDR senses light, the 555 is triggered turning the light on for a brief period of time. When multiple units are placed in a row, each one triggers the next. The duration that each light is on is determined by an “RC circuit” made of a resistor and capacitor. Changing their values can change the duration that the light is on.