Giorgos Lazaridis writes:
Some time ago i uploaded a breathing LED circuit with the 555 timer chip. It became very popular and i received many comments and emails with people that made this circuit and worked fine, as well as comments with people that had troubles converting it to operate at 12 volts supply. It was designed to operate with 5 volts, because i plan to use it for a future PC mod. Since the PC power supply has 5 volts output, and since the LEDs that i plan to use require 3.8 volts to operate, choosing 5 volts for supply was the best choice to minimize power dissipation on the transistor.
Converting the original circuit to operate at 12 volts is not a big deal, but it requires some transistor knowledge. The only parts that should be changed were the biasing resistors R4 and R5. But i decided to take it one step further. I decided to make a more flexible breathing circuit. The new version as 2 more functions: It has an adjustable voltage oscillation amplitude, and an adjustable output DC offset. What this means is that it can be easily adjusted to operate with different LEDs. If for example the load is a 12V LED strip with operating voltage range 8 to 12 volts, the circuit can be adjusted to provide exactly this: 8 to 12 volts output. Similarly, it can be adjusted to operate with a high brightness LED that has voltage range from 2.4 to 3.8 volts.
Flexible 555 LED Pulsing (Breathing) Circuit - [Link]