A couple more notes:Those LEDs are rated at only 30 mA continuous, whereas this circuit runs them at around 50 mA. Not a problem for a low duty cycle with short pulses, but this circuit as you intend has a high duty cycle. Where you might run into a problem is that you're right at the maximum capacity of the output pin (200 mA). You might want to have the output pin drive a transistor to source the LED current.With the resistors you've selected, the LEDs will be on about half the time, and flashing a bit over once per second. This is why Rick recommends something more like a 1MOhm resistor, to slow the pulse rate down. What I would recommend is that you replace the 470 KOhm with a 1 MOhm and then put a diode across it. This allows the cap to charge only through the 1K resistor, resulting in a very quick flash, followed by a relatively longer off period as the capacitor is forced to discharge through the 1M resistor. You should, of course, play around with the resistor values until you find a flash that is what you want,Good luck!