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Topic: multiplexing Ir sensors. would this plan work? (Read 1 time) previous topic - next topic

nutty

Hi!
I've been using the following schematic to use an IR Led and photo-detector transistor for my touch panel (sorry about the bad drawing):


This is a cheap alternative to proximity sensors and works great for finger detection on my Plexiglas panel.

Now I have a project that requires 42 "touch" areas and I'm trying to figure out the simplest way to multiplex them.
My plan (below) is to use shift registers to turn on each IR led one at a time, and take a reading from the respective photo-detectors:



would it be safe to hook all the transistors together or should I power them one at a time also?


I'm still an electronics novice and not sure whether photo-transistors consume power at all.
I don't mind soldering an extra bunch of shift registers if I have to, but I figured the experts might have a better plan to share ;)

Again, sorry about the foggy explanation/schemo and many thanks for any insight!

Best
Alex

MarkT

The issue is that each phototransistor will be picking up ambient light.  Thus they will all be conducting a bit - put lots in paralllel and that becomes a much bigger current.  The signal from the one transistor that is picking up the IR reflected from a finger is now competing with the large current through all the other transistors.

The more photo transistors in parallel the less sensitive each one will become.  However you have 6 analog inputs so you could have 6 groups of 7 and see if that works (try seven on one analog pin and see if its good enough...)

You will probably have to reduce the 3.4k resistors to get the best sensitivity.  Where did you find 3.4k resistor by the way, its not a standard value.
[ I won't respond to messages, use the forum please ]

nutty

Hi MarkT,

Thanks for your insight! I used a 10k trimpot to find the sensitivity "g spot" which turned out to be 3.4k in my case.

I did some thinking which lead to Plan B :


What if I used a 4067 to multiplex GND to each pair, one at a time, would that work?

I only have 3 analog pins left for IR but I can always multiplex the groups with another 4067 I guess...

Cheers

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