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Author Topic: What sensor should I use to detect object going through a hole?  (Read 2102 times)
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I'm meandering towards my first ambitious Arduino project and I have no idea what sensor(s) to use for it. The short description is that I need to detect an object going through one of three holes.

The long description is...it's a game with a 4 foot by 1 foot board with three 6 inch diameter holes in it. Points are scored by throwing a flat, metal washer (approximately 3 inches in diameter) through the holes. Different points are allotted based upon which hole the washer goes through. Not sure if this matters, but the game is played outside, in any kind of weather or light. Only one washer at a time is thrown, but multiple washers could go through the same hole in the same round (up to six per round, in theory).

I'm not sure that a motion sensor would work because: A) it's all close proximity (i.e., there's not much space on the underside between the edges of the whole and the sides of the game board), B) the washers will be flying quickly, and C) there are three holes to individually detect.

In theory, I could do a Force-Sensitive Resistor (FSR) if I added baskets to each hole, although that's less preferable.

Any thoughts or input is most appreciated! Also let me know if anything's not clear. Thanks in advance!
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I'd suggest some kind of light beam, get an interrupt when reception of the beam is blocked.
Might have to do several beams across the hole to have the washer block at least one beam.
Or have 1 beam redirected with mirrors
  /\    /\
/    \/    \
so just 1 source and 1 receiver are needed.
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Thank you very much. It is appreciated. Do you (or anyone) have an opinion on using a laser + a photocell or using infrared?
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From doing research, it looks like a laser diode seems to be the way to go, with a simple photocell receiver. If I use mirrors and wrap the \/\/ area with a shield, I shouldn't have to worry about the one laser affecting those for the other two holes. If anyone else has any other input, that's great, but otherwise this seems straightforward enough.

Thanks again for the input!
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Laser would be cool - probably good for outdoor use too.
Can get a really inexpensive  battery powered LED/flashlight as a source:
http://www.mpja.com/Flashlight-8-White-LEDs-with-LASER-Pointer/productinfo/17475%20TL/

I bought 3 or 4 on sale for $1 each - one was missing a  battery spring so it doesn't work.
Nice metal case, very sturdy feeling. Handy belt-case too. Don't know how long the included  batteries will last.
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Thanks again!
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You could use IR light modulated at 38khz and an IR detector.  Helps ignore IR "noise".
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Would IR travel across a couple of reflectors as well as a laser would?
The 6" hole is like 4x the size of the washer, so I think 2 or 3 beams are needed to make sure the washer can't go thru undetected.
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Maybe something looking for a reflected signal along with looking for broken beams.  That's a pretty big hole, so maybe the LASER is the way to go by bouncing the signal back and forth several times across the opening at different angles making a star like pattern.
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That's what I was thinking. I don't know if reflected is needed, if the beam is being reflected then surely the beam is broken to the receiver?
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I was thinking that maybe with the IR, the beam might not be broken if the washer went thru, say,  parallel to the ground or tilted on some plane that slides thru beam undetected.  Another sensor picking up a reflected signal when normally it receives nothing kinda acts like a NO switch.  The others being NC naturally.  Too complicated though, the LASER is simpler and more precise.  Though the extra precision from the narrow beam has its disadvantages too by making it harder to "cover" the hole.  I suppose a "real engineer" would come up with something that worked 99% and cost 9 cents to implement.  smiley-wink

Again, the LASER (diode) has the advantage that, though it's not the cheapest part in the world, you only need one if plan your "route" carefully.  Mirrors are passive and tiny ones must surely be cheap in bulk.
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Here comes the 9 cent solution, well closer anyway.  Inductive pickup from hall sensor or something like a simple metal detector using coils of wire.  When money is no object, then use RFID.
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laser has the advantage that any object can be used for the game.
I could see reflectors being a little trickier to set up and keep maintained.
metal detector has the disadvantage of also detecting if the ring is just sitting above the coil and not actually going thru the hole.
So everything is a tradeoff.
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I may have been wrong with the sizes. The hole diameter may be 4". I'll have to confirm this.

RFID is a very interesting option. It would allow me to identify which player's washer went through, which would be great. But I would need to rule out washers sitting on top of the board (that did not go in the hole). And I would worry about the constant throwing and hitting of the board wrecking the RFID chip.
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IR phototransistors are much more responsive (report light changes more quickly) than photocells. If you try to catch a washer free-falling through the hole with a photocell I guarantee you'll have problems -- even using a laser as a light source. Using a real world example I can say that you can pretty lazily drop your arm through the laser and the photocell won't give enough voltage swing for accurate detection.

The simplest solution would be to put a funnel under the hole so the washer ultimately drops through a restricted space. I'd still recommend using an IR phototransistor and a red LED. An IR LED is not required and makes things more difficult because you can't see it. The other advantage of phototransistors is that you can use them with digital inputs and won't have the problems with the Arduino ADC mux.
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