Supposing I have a glass sheet 50cm x 30cm, with 10 holes (2 lines x 5 columns), with metal/plastic (i can chose) pins inside it. They can be flush (hidden in the hole) or salient (extending 2mm above, like a flat screw tip).
I'd like to be able to detect which pins are salient. But I can't have anything on top or behind the glass. It must be transparent (except for the pins) and totally free from components except on the borders. I'd have a 5mm clearance above the glass or less (I'd like to keep it really slick).
I considered a few solutions, but would like opinions about them. I believe I could use laser diodes + photo diodes, or IR leds + IR receivers. Any other ideas?
I could have 1 emitter per line and column, but it would have shadowing problems. Or 1 per hole/pin, which would be much more complex and expensive (not to say the alignment or elements).
I thought about using one emitter for each column, slightly off the column axis, with 2 detectors (one for each hole/pin). Not sure if it is possible to find cheap leaser diodes with wider enough angle. But may be possible with IR.
Not sure if IR reflections would be a problem, since I'd have at least 5 emitters.
Avoiding shadowing is a plus, not not absolutely required.
Any ideas? Thanks a lot.
Update: I believe it is possible to do it using cheap IR emitters and receptors. However I'd have to use one pair for each switch:
I used red and green colors only to help visualizing the idea. Those are both IR emitters.
Opinions?
Answer
Marcus can't you make more optically re-arranged for side rear view blocking with 1 per row. I have done this on pattern recognition with 5mm LEDs and IRDA detectors recessed 2cm from aperture holes with a unique data pattern on a PIC and detected 8 LEDs in parallel but in TDM patterns so that path blocking was critically dependent on alignment but laser like resolution with cheap parts on board.
In your case emitters on one side IRDA SMD detectors on the other. You can join your paths in the bottom with ribbon wire crimp wire-to-board connectors or flex ZIF sockets.
Nylon washer stress relief on thread mounts both sides and 8mm thick glass
Add Blue LEDs for edge lights or strip led for appearance.
This is a 4D EO problem , a great challenge for EE's with inside and outside the box thinkers, reduced to 2D , X and time for optical path blocking with low error rate. It can be done with 25 cent 5mm IR LEDs. A PIC is not needed here, if you have a Johnson Counter and hex Darlingtons and some logic for your interface. Total current consumption may be 50mA~100mA @ 5V.
TDM serial port of gated OR outputs 1000110001 indicates 4 IRDA paths blocked from 10 footswitch pins.
I successfully built this proof of concept using a 1m path with a wire resistor in its path was enough with care for perfect reception in spite of crosstalk and scattering. Every path must be analyzed for intensity and attenuation to resolve this design that works to get a binary error free result immune to bright light. PCB alignment is guaranteed but the housing design.
You will need a base to secure the fixture.
I used non-reflective holes and with black heatshrink over the emitters.
Take 2
Careful machining is critical and aperture design.
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