In the past few days I've looked a way to replace the 386-amp board with an op-amp based one. Test circuit(TL072):
It was just thrown together for breadboard, and probably lacks
all of the some important components. But as it is, it gives enough output for testing.
Seems to work better by using a split power supply.
But in general, the lack of delivering enough output to keep the schmitt triggers 'squaring' long enough, to make the output to feel more like guitar-ish, still exists.
Then I had to rethink about the whole thing: I have already an overly powerful(and noisy) 386-based amplifier board, which should be more than enough for the purpose.
But in practice, is that much grunt for amplifying the phototransistor's output really even needed, as the end result is still far from usable?
So it was time to try a bit different approach to the problem. I remembered seeing something like positive- and negative input threshold voltages on the 4093's(quad schmitt trigger ic's, that are used so far in this project ) datasheet.
This picture is actually from 40106's(cmos hex schmitt triggers) datasheet, as it's much more clearer, than the old 4093's one:
One thing, that came to my mind, was to bring the input signal between the Vp and Vn, by using an offset, and then adjusting it, to see how low it would go to 'tickle'(trigger) enough both limits.
There might be something in it, but I'll have to admit, that after trying that with a function generator and a scope, there is much more in it.
No matter, which way I tried to test that out, I just couldn't get my head around, of how it could actually be done.
Threshold voltages, although only approximate ones, are on the datasheet, and as the supply voltage is lowered, they lower also.
So the next thing to test was obvious; decrease the schmitt trigger's supply voltage. At 1,8 volts, the input threshold was under 0,1 volts, when testing with a square wave.
Needless to say perhaps, but at that level, the quality of the output never meets any of the standards.
Besides of that, I kept on going, and tested the bugger with a g-string. Yes, the output was a lot lower, but for some reason, 'squaring' was there, and it just kept on doing it longer, than ever before.
And that was by using the above 072-op-amp based circuit.
So far, cons of using that low supply voltage:
- output waveform has some mutations(although not necessarily lethal), when compared to clean square wave
- needs to be amplified/gated afterwards, to get back to the cmos-level
Then the pros:
-
overall bandwidth of the schmitt trigger chip decreases drastically(down to few kilohertz), which should/could be good enough to reject radio-like interferences- less amplification is needed
There is a drawback, when using optical components, that are extracted from optoswitch(or at least the ones I have), though. The phototransistor doesn't have an ir-filtering in it, so it is easily affected by the visible lighting.