Hi, I built a standard 555 timer circuit for a square wave output signal of 10 Hz. It is the standard circuit for the astable mode, as given in Wikipedia: C = 1 uF, R1 = 6.8 kOhm, R2 = 68kOhm, in my case a 100 kOhm potentiometer. The circuit was mounted on a breadboard and drives a LED with a current of 3 mA. The power source is a 9 V battery.
Oscilloscope reading at the LED resistance shows a heavily distorted square wave, see screenshot attached. Fourier analysis reveals the presence of the third and fifth harmonics.
Where is my error, what to do to obtain a true square wave?
Remark: the output becomes a true square wave when changing the R, C values so that the frequency is above 70 Hz (driving the same LED with resistance).

 

We need to see a Schematic ..............
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Set your oscilloscope CH1 input to DC, not AC.

 

Your squarewave appears to have phase shift caused by a reduced low frequency response.
You did not show your schematic but it is probably caused by the setting of the oscilloscope to AC that cuts low frequencies.
It should produce a perfect squarewave if the oscilloscope is set to DC.

 

AC coupling on an oscilloscope does not have a low enough frequency response to faithfully reproduce a low-frequency square-wave, giving the drooping of the top and bottom that you see.

Generally you only use AC coupling on an oscilloscope if you are looking at a small AC signal riding on a DC bias, and you want to block the DC.

 

Set your oscilloscope CH1 input to DC, not AC.

Yes, that is a typical beginner mistake. With this setting, I see now a nice square wave. Thank you