555 timer issue

[IvoS]

I am having problem with 555 timer. Everything looks nice when channel is put to DC coupling, pic #1.
When I switch to AC coupling (pic #2) on my Rigol scope, I see nasty non-square like waves. I am not sure what's going on. What I don't understand is the falling edge goes to -8.8V. I would expect it to be around -5V when AC coupled, kind of symmetrically distributed from the center 0V point, my PSU is set to single +10V. Why don't I see top and bottom edge flat anymore? Any idea?

[Zero999]

It's what you should expect to see when you switch to AC coupling. The capacitor and resistor on the 'scope input form an integrator which causes the classic exponential decay when the capacitor charges and discharges. If you increase the frequency of the 555 to something like 1kHz, the weird curves and spikes will disappear.

[IvoS]

I thought that pin 3 output is just a flip-flop output and therefore should still maintain reasonably square wave. ?

[grumpydoc]

I thought that pin 3 output is just a flip-flop output and therefore should still maintain reasonably square wave. ?

Nothing to do with pin 3's output.

Hero999 is right - you are seeing the effect of AC coupling on what the 'scope shows - usually when you switch in AC coupling the lower frequency response tails off below about 10Hz and this will affect the display of slow square wave signals.

[M0BSW]

I thought that pin 3 output is just a flip-flop output and therefore should still maintain reasonably square wave. ?

Nothing to do with pin 3's output.

Hero999 is right - you are seeing the effect of AC coupling on what the 'scope shows - usually when you switch in AC coupling the lower frequency response tails off below about 10Hz and this will affect the display of slow square wave signals.

I use pin 3 as a trigger pulse output for scope, showing the triggering pulse on the scope, which I learned from a W2AEW video, and I've used it on several other projects . I agree with the others from my limited experience.

[IvoS]

Thanks guys. Had no idea that scope input can mess it up to that extent. So there is nothing wrong with the output and the waves are still reasonably square. I was surprised how quick rise and fall edges were. If I remember correctly the rise time was 130ns and fall time around 15ns in my slow speed configuration.

[Zero999]

It's a high pass filter so the high frequency parts of the signal (rise and fall) are not changed, only the low frequency parts (where the signal is high or low for a long period) are suppressed.