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PC/microcontroller “wireless” data transfer (part 1)

UPDATE! >>> [FINISHED PROJECT HERE]

Several days ago I had a crazy idea. I was driving to Orlando to pick my wife up from the airport and it was dark and stormy on the highway and I was thinking about the backlash I got from my Sound Card Microcontroller/PC Communication project, where I used an embarrassingly simple hardware to accomplish the simple task of exchanging a few bytes of data between a PC and microcontroller (in the face of many people who adamantly prefer more complicated “traditional standard” methods). The car in front of me drove with his emergency flashers on, and at times all I could see were his lights. At that moment the crazy idea popped in my head – I wonder if I could use a PC monitor and phototransistors to send data to a microchip? I can’t think of any immediate uses for this capability, but perhaps if I make a working prototype I’ll stumble upon some. Either way, it sounds like a fun project!
DSCN1652

The circuit is as simple as it gets. PHOTOTRANSISTOR MICROCONTROLLER CIRCUITA phototransistor is exactly what it says, a photo (light-triggered) transistor (uses small current to trigger a large current). It’s a photodiode with a small transistor circuit built in. Make sure you give it right polarity when you plug it in! For some reason (likely known to electrical engineers, not dental students) the larger metal piece in the plastic part, which I normally associate as negative for LEDs, should be plugged in the +5V for my photodiode. Again, make sure you hook yours up right. I purchased mine from eBay quite cheaply, but I’ll bet you can find some in RadioShack. Note that the value of the 22k resistor is important, and that your needed value may differ from mine. The resistor relates to sensitivity, the larger the value the more sensitive the device is to light. If it’s too sensitive, it will sense light even when aimed at a black portion of the screen.
hardcode

Initial tests were done using the pins as digital inputs. This was difficult to achieve because, even as transistorized photo-diodes, it took a large difference in light to go from 5V to 0V (even past the 2.5V threshold). After a few minutes of frustration, I decided to use ADC to measure the light intensity. I use only the most significant 8 bits (ADCH). I found that in ambient light the readings are 255, and that white monitor light is around 200. Therefore my threshold is 250 (4.88V?) and I use this for logic decisions. Here’s my setup showing the ADC value of each phototransistor translated into a 1 and 0 for clock (C) and data (D). Both are aimed toward the lamp, so both show a logical 1:
DSCN1651

My first test involved reading the data from the image above. The clock is on the bottom line, data is on the top. Every time the clock transitions from black to white, the value of the data at that point is read (white=1, black=0) and the number is placed on a screen. Here’s what it looks like in action:

Hopefully soon we can get a JavaScript interface going! Rather than swiping I’d like to just point this at the screen and let JS flash some squares for my device to read. This will allow virtually unlimited amounts of data to be transferred, albeit slowly, to the micro-controller. Here’s a preliminary sketch of how to send strings.
string

Remember now we’re using a time domain, not a 2d barcode. I really stink at writing JavaScript, I’m going to have to pull in some help on this one!

About the author

Scott W Harden

Scott Harden has had a lifelong passion for computer programming and electrical engineering, and recently has become interested in its relationship with biomolecular sciences. He has run a personal website since he was 15, which has changed names from HardenTechnologies.com, to KnightHacker.com, to ScottIsHot.com, to its current SWHarden.com. Scott has been in college for 10 years, with 3 more years to go. He has an AA in Biology (Valencia College), BS in Cell Biology (Union University), MS in Molecular Biology and Microbiology (University of Central Florida), and is currently in a combined DMD (doctor of dental medicine) / PhD (neuroscience) program through the collaboration of the College of Dentistry and College of Medicine (Interdisciplinary Program in Biomedical Science, IDP) at the University of Florida in Gainesville, Florida. In his spare time Scott builds small electrical devices (with an emphasis on radio frequency) and enjoys writing cross-platform open-source software.

Permanent link to this article: http://www.SWHarden.com/blog/2011-07-26-pcmicrocontroller-wireless-data-transfer/

4 comments

  1. Jerry

    If you go to my web page… about the 4th page in on the robot project… you may be able to adapt my photo diode circuit. Here is a direct link: http://askjerry.info/trp2010/page04.html

    Anyway… good lick with your project.

    Jerry

  2. Ryan

    Remember the old Timex DataLink watches? (http://en.wikipedia.org/wiki/Timex_Datalink)
    They used a similar flashing screen to transmit data from PC to watch.

  3. raju h sharma

    i want to transfer image from computer to the microcontroller so if u have any idea about it then tell me and mail me.
    it would be very nice of u if i could recieve your reply in as less days as possible. thanxs.

  4. Jared

    I’m not an electrical engineer, but I know a LIKELY reason for why photodiodes have their leads marked like that: photodiodes are really just an LED turned backwards. Forrest Mims noticed decades ago that many (not necessarily all: silicon is apparently poor at actually emitting) semiconductor junctions will emit under one orientation, and detect under the opposite orientation.

    Word of warning: just as soup spoons, strainer spoons, and serving spoons are all suited to very different tasks, so too are individual diode designs. Just because a particular diode CAN detect light (apparently most of the clear-glass encased signal detectors CAN, they just aren’t particularly good at it), doesn’t mean that you should normally use them. LEDs are probably a DECENT replacement for tuned photodetectors, though.

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