Learning how to use a 555 timer - oscilloscope - troubleshooting

[de_light]

Just messing around with a simple 555 astable circuit and having a probe around with my new Rigol 1054Z oscilloscope. Just learning.

I built a circuit and clearly did something wrong. My LED switched on and off at the (slow) rate I set, but initially, the LED was very dim for about a second. I put the probe on the LED and got quite an interesting waveform. It showed lots of high-frequency noise at 11KHz at 5.1V which corresponded to the dim section, before settling down to 3.5V when the waveform was stable. I've attached a picture of the same pattern but AC-coupled.

I then rebuilt the circuit from scratch and problems were solved - nice square waveform pattern.

What went wrong in my circuit the first time to give me all that high-frequency AC signal which made the LED dim?!

[Doctorandus_P]

I'm guessing this capacitor is supposed to be connected to something, possibly pin 5? (I have not looked at a 555 for some time).