You forgot to attach a schematic so should we guess?:
1) Is the +5V supply a low resistance?
2) What is the resistance of the load?
3) Does the load connect to +5V?
You show an output from +1V with a lot of ripple. Why doesn't it go down to ground? Why does it have so much ripple? Its high of +3.8V is normal.
preparing to call it a night .. I'l draw the schematic in the morning and upload it, for now here is the short explanation
1. I have no idea what +5V low resistance power supply mean
.. sorry, I said I'm not too good with it, but, I get the same result from USB stolen 5V and from 7805 + 1000uF capacitor.
2. from pin1 (output from op amp) 5K resistor - point where I attach oscilloscope - 50K resistor - ground
3. no .. the load is supposed to be PIC18Fxxxx input pin but for now I'm just attaching the scope or voltmeter
as for the schematic .. sorry again .. but I was soooooooo frustrated ..
basically the output of another op amp (LM833) trough 1000pF and schottky diode is going to LM358 pin3 (+ input) (the output is attached in prev message, that's the 1V with ripple .. btw, I'm not sure if ripple is from the scope or it is really there as I get ripples in this app even when I connect probe to the ground ?!)
so, the thing is .. this output (~1V / ~4V) I want to get to be ~0V and ~4V so I can feed it to the PIC. Mine understanding is that lm358 will behave like comparator without any feedback loop and that I will get nice square output
..
this is not something I "need" ... but it's kinda project I want to use to learn how to deal with op amps and a bit of analog elco as I miss basic in that field (what can I say, I'm software engineer and this is much more difficult then if/then/else
)