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noise discriminator

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throbscottle

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I found this http://www.scarpaz.com/documents/an-366.pdf - on page 23 there is a "noise discriminator" circuit. It looks like just the thing to eliminate (or at least, improve) noise from mechanical vibration going into the photo-interrupter on The Ridiculously Complicated PCB Drill. It has a very vibratey spindle motor, I'm thinking of getting a better one but even smooth motors still cause issues.

I was wondering if there's an easy way to avoid having to use a D type flip-flop in there?

I also found http://www.ti.com/lit/an/scla014/scla014.pdf?ts=1597861624556 on page 17, which might be the better option - the Fairchild document has a different one on page 26.

The TI circuit would need changing to PNP if I used it.

I'm already using a 74HC123 monostable to solve some issues so it's part-way there.

What I actually have is potentially some spare xor gates and potentially a spare monostable. Though I wonder if there's a better way using our old friend the 555?

They all seem to do pretty much the same thing anyway. I just want to get rid of the short pulses caused by vibration. I don't really want to have to rename the project as The Very Ridiculously Complicated PCB Drill... Though it might be fun :)

(edit - wouldn't need to change the TI version, I'm just a dummy...)
 
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Looks like it might be better if I make the mono non-retriggerable.
 
The Fairchild version looks super easy and perfect for short-pulse noise elimination.
 
Can you just use the 555 as a schmidt trigger (pins 2/6 together as input, 3 as output), with an RC LPF on the input signal? You can put a diode across the resistor if you need the output to change quickly for a low input (like in the app note). Note that the 555 will obviously invert the output level.
 
I tried the TI circuit, works well enough, flakey breadboard connections notwithstanding (add a ton of vibration to that too...). Had to add the other half of the '123 onto it because I need the output to always default to low (and the input source can rest low or high).

I didn't want to add another chip for the Fairchild circuit but yes it does look just right. TBH it might come down to pin counts - 2 transistors + resistors, cap and another RC net adds more legs than a 14 pin dual d type. Sad but true.

I must investigate the 555 route. My first line of attack was a slow LPF but in this case it doesn't help. Slow it down enough to filter out the extra pulses and I get other problems. But I would not be surprised if in the 100's of 555 circuits out there, there is one that does what I want.
 
Yay, got it working quite well on a little perf-board. Needs a little bit of tuning. Put an xor gate to provide an invert-or-don't function between the discriminator and the next monostable, which was convenient to add a resistor and small cap to the output of to get rid of some spikes that were appearing because the input isn't a very good square wave. Almost completely ignoring vibration now :D

So now I'm going to pull it apart and try the one from the Fairchild app note, since it's simpler and should handle the less-than-perfect input a lot more graciously! Only trouble is I would have to get some single D types. grrrr....
 
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