This is my very simple LED fader design. I didn't have any TLO72P dual op-amp ICs in my bits box today, so I've used two 741s instead. I also didn't have a 560k resistor, so (almost) made one using two 220k resistors and one 100k. The 560k resistor biases the output transistor such that it just stays on enough to keep the LED light even at the oscillator's lowest voltage output, giving a more pleasing effect. A portion of the value of the 560k could be made adjustable to allow fine tuning of the 'breathing' effect.