[sdiy] Using a PIC micro as an Octal schmitt buffer with programmable threshold levels..

[Roman Sowa]

Lower the optos pull-up supply voltage to 4V and it should improve a bit

Roman

W dniu 2024-11-01 o 18:37, Jean-Pierre Desrochers via Synth-diy pisze:
> Hi list,
> 
> Long post here..
> 
> Since I often asked help and suggestions on this forum for many issues,
> 
> I thought: Here is an idea I'd like to share..
> 
> Some months ago I talked about a project I was working on
> 
> to read old piano paper rolls.
> 
> https://www.youtube.com/watch?v=ZhH9kftuT_A&t=2s 
> <https://www.youtube.com/watch?v=ZhH9kftuT_A&t=2s>
> 
> This project works well.. but with some paper holes reading issues I had 
> to address.
> 
> Let me explain:
> 
> The 88 paper holes are read using IR photo transistors with 100k pullup 
> resistors
> 
> with the help of 74HCS165 SPI buffers through a PIC micro.
> 
> A wide Infrared LEDs band is in front of the scrolling paper that has holes
> 
> punched in it (played notes). Each hole lets the IR beam reach an 
> aligned Phototransistor
> that has a ‘light pipe’ to ‘conduct’ the light beam to it.
> 
> Some of the light pipes has more difficulties to get the light flow to 
> its PT..
> 
> This arrangement causes some PT’s ‘ON’ low state to get too high in 
> voltage values
> read by the 74HCS165 SPI readers. The PT OFF state does not cause any 
> problem
> 
> since  it goes high impedance up to 5vdc. Ok. But the ON low state 
> values can go from 0.31v up to 1.7v !!
> _Because of the very variable 74HCS165 threshold voltage values from 
> chip to chip_
> 
> this sometimes causes reading problems and loss of some paper hole notes..
> 
> I tried to use 470k pullup resistors instead of 100k but the system
> 
> starts to get in an unreliable floating state when no notes detected.
> 
> Here is my idea..
> 
> Why not use a small PIC micro to work as an octal schmitt input buffer
> 
> that has programmed threshold levels at fixed specific values ?
> Here are the HCS165 threshold levels compared with my PIC threshold :
> 
> Now the PT’s could have up to 2.5v in their ON (low state) and still
> be read as a low state by the still in place HCS165 SPI readers..
> 
> 88 notes means 11 x PIC16F1829..
> 
> The use of 8 x ADC inputs, applying threshold values while reading
> to finaly output 8 x standard digital I/O.
> 
> Voilà !
> 
> 
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