In the figure we have an interesting circuit in which an LED is used as a photoelectric sensor. When this LED is lit, the circuit starts and it starts to flash at a frequency determined by R1, R2 and C2. These components can have their values changed over a wide range of values. As the 7555 is a CMOS type, the circuit consumption is extremely low. It is an interesting application to be used in signage. The circuit will also work with a conventional 555 (bipolar), but its consumption will be higher. Equivalent diodes can be used in this configuration.
