Electronics > Beginners

74HC14 schmitt inverter issue

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psubond:
I am trying to get more familiar with using a 74HC14, as an experiment i have an ldr set up in a voltage divider configuration and the center tap goes to the input of the 74HC14 input 1.  I have all of the other inputs tied to ground.  everything is soldered together on a perf board.  I have a .1uF, .01uF, .001uF, .0001uF cap on the power pin (probably overkill).  Input voltage is 5v.  The voltage divider and the IC are both powered off the same rail

The IC triggers as expected when light is shined on/removed from LDR.  Below is the waveform I am getting, it is very consistent from test to test.  What I don't understand is why the signal is dropping to -2.4 or so when triggered then staying below 0.  The rising edge has an overshoot to about 6v but is smoother. Any information/guidance would be appreciated.



https://drive.google.com/open?id=1oG6Eh2Gwj9bf4IXGBJ9i5LoJoLvwybN6

wraper:
1. Measuring with long ground connection. For best results you should use short ground and signal connection directly from IC.
2. Probe compensation probably is not properly adjusted.

psubond:
Thanks, will give that a try!

mvs:

--- Quote from: psubond on January 24, 2020, 03:14:50 pm ---What I don't understand is why the signal is dropping to -2.4 or so when triggered then staying below 0.

--- End quote ---
74HC14 edge is typ. 7ns fast (140MHz), you need a quite fast scope to see real signal.

iMo:

--- Quote --- I have a .1uF, .01uF, .001uF, .0001uF cap on the power pin (probably overkill).
--- End quote ---

The recommended ratio for the decoupling capacitors is 1:100 (for demanding applications).

Like
100u || 1u || 10n
10u || 100n || 1n
1u || 10n || 100p

In your case 2 capacitors would be ok.

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