Bdoop21

Hi Electro Tech,
I need an optical sensor that can be "tripped" when a thin piece of paper moves past it.
Can anyone suggest a type of small optical sensor to start looking into?

My goal is to make a system where:
When the optical sensor is "tripped" it tells, through use with an arduino board (arduino.cc), a fan to move.
Any suggestions on where to start???
Thank you so much!
-MBE

Torben

A beam-break detector can do it if you can put an LED on one side and a phototransistor or photodiode on the other. That would be a so-called "dark alarm". If everything has to be on the same side of the paper, you might be able to bounce light off the paper and detect that--a "light alarm" circuit. Depending on ambient light conditions, you may very well need to modulate the light beam so that it's not giving false triggers all the time. I've had good luck by ripping the laser diodes out of $2 laser pointers and using those as light sources to detect a foosball ball going past (cheaper than just buying the laser diodes, at least the ones I could find). You haven't filled in your location information so I don't know where or whether you can easily find a phototransistor, but they're often sold in matched pairs with an IR LED. If you can order from DigiKey, Mouser, or similar, that would probably be your best bet.

Do you need to be able to distinguish a thin piece of paper from thick piece of paper or a cat walking by or something?


Torben

Bdoop21

Thanks Torben,
I think if I have a phototransistor on one side and a photodiode on the other that might do it. BUT I have used Infared phototranisistors for a pulse detector. I think it is basically the same principal, right?
The only thing about using an infared sensor is that it seems SO sensitive. It seems to be affected by ambient light more than (i think) a red laser light would be. Is that right?
I think I would prefer if I could actually see the laser light and only if and when that very specific laser was interrupted my fan would turn on.

I said a thin piece of paper, because I want it to be pretty sensitive, but not so sensitive that a cat would blow the paper as it walked by. I'm sure a thick piece of paper would be good enough for my needs, actually maybe even better.
I will fill in my profile, I have just been so excited reading all the posts and asking questions that I havent gotten to that part. But, I am in Seattle Washington and have ordered plenty from Digikey and Mouser in a very short amount of time. I am VERY new to electronic work and dont really know anything...
So thank you for anything and everything you all have to offer.
-MBE

Torben

Basically, depending on what your pulse detector was detecting pulses of. But a phototransistor and photodiode both do essentially the same thing: detect light. So a detector on each side probably isn't what you're looking for. A photodiode and an LED are not the same thing: an LED gives off light, while a photodiode detects it. The picky will note that an LED can in fact be used to detect light.

Well, the laser will work somewhat better in general simply because when it's shining on the detector, it is delivering a whole lot of light, so the difference between it being on and off is much greater and therefore is easier to detect. However, ambient light is always going to be an issue unless you can somehow bury the detector. For instance, in my foosball scorer, the detector is about an inch deep inside a black plastic void, which is itself inside the black goal cup--not much ambient light gets in there, so I never have problems with ambient light shining on the detector. But if you can't do that, then AFAIK you pretty much *have* to modulate the light beam you're sending from the one side and have the detector side only trigger when it can no longer sense a beam of that frequency shining on it. Otherwise when your beam is blocked, the detector could easily still receive enough environmental light to make it think the beam had never been blocked.

Yep, that's the idea. If you are better with the physical stuff than the electronic, and if your design allows for it, you could maybe get away with burying the detector. I mean make it really hard for ambient light to get to it. Maybe take a Bic pen, remove the ink tube and end cap, coat the pen inside and out with matte black paint, and shove the phototransistor in the narrow end with the leads sticking out. You'll also need to block light from hitting the back side of the detector where the leads come out, since that can be enough to register. The laser is then aimed so it shines in the other end, so that it has to be aimed carefully so it shines down the whole length of the pen before striking the phototransistor. The matte paint inside should help keep ambient light from bouncing down the tube onto the detector, depending on how bright the area is where you'll have the setup.

So what exactly are you trying to do? Have a piece of paper fall into the beam, which turns on the fan, which blows the paper up, which turns the fan back off and lets the paper fall again?


Hope this is at least giving you some ideas. I'm posting this now but I'll try to dig up some references on the modulation in case that's needed. (You might have guessed that I'm a hobbyist and not an EE or anything. I'm learning too but I have a few projects under my belt.)


Torben

Bdoop21

Hey Torben,
I guess I meant an LED and a phototransistor then. Sorry, the electronic terms are still not concrete in my mind.
What I am trying to do is:
A person will blow into a tube. They will blow on the piece of paper, which will trip the sensor. The sensor will then turn a heating element and some fans on.
SO
The tube that they blow into can be dark, so ambient light, I guess isnt the biggest problem...thankfully!
So if I use the bic pen idea (thank you, sounds good) then the laser will be infared or not?
I think I know what to buy if it is infared light we are talking about, but I dont really know what to buy if it is not infared. Do you have any suggestions?
The pulse detector that I made is for detecting heart rate using an infared LED and a phototransistor. I just didnt really think that infared is necessary in this case. What do you think?
Thanks for your time,
Molly

Torben

OK, so far, so good. . .

The darkened pen tube would be separate from the tube they're blowing down, unless I misread what you wrote earlier. See the attached image: does that show what you're thinking? (Sorry about getting the ears uneven on the blower-dude).

If you want to use lasers, don't use infrared. They are not visible, and won't cause a blink/avert reflex if something happens and one ends up shining where it can get in someone's eye. An infrared phototransistor will have enough bandwidth to use the light from a red laser diode.

Buy a couple of $1-$2 laser pointers at a dollar store. You only need one, but they are cheaply made and easy to destroy so get 2--one to practice with. Carefully disassemble them and remove the laser diodes. Discard the 3 button cells (safely. Or keep them for another project). If there is a current-limiting resistor, keep that. Power the laser diode from 4.5VDC. A cheap laser pointer won't have enough light output to be dangerous unless you stare into it, which would be a challenge because your eye won't want to let you. Don't do it anyway.

If you're underpowering the diode it may give off red light, but won't actually lase. To get it to lase, gently boost the voltage up to around 4.5V and suddenly the light will turn noticeably "speckly"--this is a result of the laser action.

Nope, I don't think infrared is required. One thing I do worry about is that if my drawing is correct and the laser needs to be blocked by the paper edge-on, the paper may not be thick enough to block the beam completely. You may be able to weigh down one of the hanging corners of the paper so that it twists slightly when blown upon, thereby presenting more blocking surface area to the beam.

No problem. I'm not a pro like some of these guys but the help I get here spurs me to try to give back what I have learned.


Torben

Attached Images

blown_paper_project.png (7.3 KB, 18 views)

Bdoop21

OK!! I'm going to the dollar store!
You're awesome!