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Help with light detection keyboard

flavitie

Feb 26, 2015
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Hi everybody, I am not sure if this post follows the guidelines of the forum nor if it is in the right section, I apologize if not.
I have no clue of electronics design (I work on software design) and I need help to solve the following:

I have to build a sort of fingertip keyboard using light detecting sensors, i.e. to detect when a finger covers up a senor.
The sensor (photo resistor or diode or similar) is mounted on a plastic box as depicted in the attached image where>
yellow is the sensor, cyan is the plastic box, dark grey is opaque coating, blue is water (right part of the image is outside, left part inside the waterproof box), pink is fingertip.
the sensor should be connected to a digital Input port of the raspberry Pi, when the finger covers the sensor the port should detect a a logic High, otherwise low.
there are 10 of such sensor in the box.

Could anyone suggest a design? I am afraid I would need a complete schematic and component list since I have no electronics knowledge...

Thanks!
 

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Harald Kapp

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Welcome to electronicspoint.

The Octolively open source kit uses IR sensors to detect the approcimation of objects. The octolively uses active IR sensors where an emitter and a receiver are paired. That way the Octolively is independnt from external IR lighting. this may be a good idea for your project, too.
 

flavitie

Feb 26, 2015
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Welcome to electronicspoint.

The Octolively open source kit uses IR sensors to detect the approcimation of objects. The octolively uses active IR sensors where an emitter and a receiver are paired. That way the Octolively is independnt from external IR lighting. this may be a good idea for your project, too.

thanks for your reply, the box will be operated underwater (sorry for not mentioning it in previous post) at a depth of 40meters where most of IR light is absent, I guess that IR sensors would not work, is this correct?
 

Harald Kapp

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I guess you'r guess is correct. At least not passsive sensors. The active sensors used in the example should work.
 

CDRIVE

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It better be a well designed enclosure because you're going to have about 58 PSI of water pressure on it at that depth.

Chris
 

hevans1944

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I don't think this project is practical without some experimentation. Depending on a single sensor to detect light at 40m depth is asking a bit much. You will most likely need an emitter/sensor pair.

There is one thing that might be used to advantage here: total internal reflection at the plastic/water interface. You can observe the phenomenon easily in a fish aquarium by looking at an adjacent wall from the outside to see a mirror reflection on the adjacent wall. Pressing a finger to that mirrored wall from outside the aquarium makes the finger visible.

Perhaps an active emitter, paired next to an IR or visible sensor, with both "looking" outward toward the plastic/water interface, will allow the finger to be detected when it is underwater and pressing against the plastic. The finger might cause enough absorption of the emitter radiation to create a measurable change in the sensor output.

Problems that might occur:

Total internal reflection is angle dependent. If you look through a fish aquarium from two parallel sides there is no mirror effect on the opposite side. The idea here is to reflect most of the emitter radiation into the sensor, except when the plastic/water interface is disturbed by a finger pressing on the plastic. There needs to be a "critical angle" between the emitter and the detector before any appreciable internal reflection occurs. Placing an opaque optical barrier between the two and moving them closer to the plastic window (to increase the included angle between them) may be all that is necessary. IR tends to zip right through most plastics, so a visible emitter may work better than an IR emitter.

Here is another thought:

Place an emitter in the plastic, outside the sensor hole with its opaque wall, and use the "light pipe" effect to illuminate the sensor through the plastic window. I have no clue whether or not this will work, but it is easy to try. Don't forget to submerge the window in water while testing. I think the plastic/water interface is essential to detecting a finger pressed against the plastic window. There is not enough difference in index of refraction between air and finger versus water and finger to have much effect.

The key to solving your finger sensing problem is to find out, experimentally, whether interrupting the water-to-plastic window interface with a finger will yield a detectable response from the sensor behind the window, assuming appropriate illumination. If so, an electronic interface to your Raspberry Pi is easy.

I strongly suggest that you get a clue about basic electronics theory and design. It will benefit you greatly to have some knowledge of the hardware your software will control.

If it is available in Italy, you could start with the Electronics Learning Lab Kit, sold by Radio Shack and also available through Amazon. There are other learning kits available, but this one I happen to be familiar with because I purchased one for a grandchild last year. It may no longer be available, since Radio Shack is having financial difficulties. Otherwise, you can gather for yourself a small stock of components (resistors, capacitors, LEDs, light sensors, solderless breadboard, 24 AWG jumper wires, some dry-cell batteries or a small 5 V power supply, perhaps some switches, and a small set of hand tools, along with a few beginner tutorials. Total investment in this learning experience will probably be less than €100.

Begin the journey! We can help here at Electronics Point once you get the basics nailed down.
 

CDRIVE

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It better be a well designed enclosure because you're going to have about 58 PSI of water pressure on it at that depth.

Chris
Oops, I forgot to add the initial air pressure at sea level. That would be 14.7PSI + 58PSI = 72.7PSI.

Chris
 

hevans1944

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thanks for your reply, I will get some sensors and do some tests.
Please let us know how this turns out. I would like to know if you are able to exploit "total internal reflection" at the plastic-water interface by pressing a finger (under water, of course!) against the plastic. No need to submerge to great depths. The index of refraction of water does not change (much) with pressure because water is virtually incompressible.

I found some integrated infrared LED + photo-transistor modules in my "junk box" of parts I keep around, just in case something turns out to be useful. These are Texas Instruments TIL 139 reflective photosensors, a GaAs IR LED emitter (TIL 32) and a silicon photo-transistor (TIL 78) mounted side-by-side in an epoxy package. See attached PDF file. I think the TIL 139 is obsolete now, and perhaps its component parts are too, but it may be convenient to use for test purposes. If I have time, I will experiment with it a little to see if it can be used to detect the presence and absence of a finger held underwater while pressing (or not) on plastic.

Actually, since the TIL 139 is a reflective infrared sensor, maybe all I have to do is point it at a partially submerged piece of Lexan and find out if it can "see" the presence of my finger under water... more on this later if I can find time to do the experiment...
 

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Gryd3

Jun 25, 2014
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Couple of considerations/ideas that may help the final outcome.
Chorded keyboards... require far less keys, at the expense of a learning curve to use it.
They are commonly used in court (http://en.wikipedia.org/wiki/Stenotype)
Although... the stenotype keyboard requires much more training than the chorded keyboard layouts available for PC, and Android devices.

Additionally, flex sensors may be employed and fastened to a glove.. this again would require some additional training, but the wearer would 'type' with gestures.
I am unsure however what kind of effects a flex sensor would experience under pressure though.
 

hevans1944

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@Gryd3 the OP only needs ten "keys" but the logic can process these any way desired, including chorded keying if that makes any sense while working underwater.
 
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