I am trying to build a hardware debounce circuit after Elliot Williams published here. This is his schematic:


Elliot shows this behaviour:


The right hand trace shows the voltage at the output of the RC stage (yellow) and the trigger (green) when the switch closes, the left hand when it opens again.
What I get is this
Switch closing:



Yellow after RC, blue trigger output. So far so good, but zooming out to see the whole button press:


Can anyone suggest a reason for this behaviour? I don't understand (a) why the Schmitt trigger doesn't latch to +5V for the duration of the button press and (b) how it swings to -5V (ish) when the button is released.
I'm probably being thick (very new to playing with electronics), any education would be most welcome.
Bob.

 

Your Channel 2 input on the oscilloscope is set to AC coupling.
Set it to DC coupling.

 

Thank you so much MrChips. This was driving me nuts. I suspected it was some stupid newbie thing I was doing wrong. With DC coupling:



Don't yet understand exactly why AC coupling produced the effect it did, but I expect a bit of research will reveal all.
Thanks again, Bob

 

Don't yet understand exactly why AC coupling produced the effect it did, but I expect a bit of research will reveal all.

AC coupling places a capacitor in series with the input to block DC.
This means a pulse voltage will decay to zero with a time-constant of that capacitor value (typically in the neighborhood of 100nF) times the scope input resistance (typically 1MΩ).

 

AC coupling places a capacitor in series with the input to block DC.
This means a pulse voltage will decay to zero with a time-constant of that capacitor value (typically in the neighborhood of 100nF) times the scope input resistance (typically 1MΩ).

Thanks. That explains the decay after the trigger goes high I guess. Maybe the negative spike I see when the switch opens and the trigger goes low is something to do with the 'scope differentiating the signal. Shall think on it!
Bob.

 

AC coupling removes DC. Hence it shifts the waveform average to the the zero line. The average voltage is 0 V.

 

Here is something that you need to know.
Traditionally, DC and AC meant Direct Current and Alternating Current.

In signal processing, they take on some additional meanings.
DC refers to signals at 0 Hz.
AC refers to signals with frequency greater than 0 Hz.
All signals contain DC and AC components.

With regards to oscilloscope terminology,
DC means all frequencies, 0 Hz and > 0 Hz, DC + AC
AC means that DC is removed, i.e. the signal is filtered using a high-pass filter.

This is the circuit of a simple RC high-pass filter.