Hello,
I came across this digram from an instructables page, but it doesn't seem to work (something wrong with the Mosfet wiring??). Do you think this approach will work at all?
I am trying to get 2 arduinos to talk to each other using a focused laser beam. If I can get something working then I plan on using 38khz modulation to help with interference.
Thanks
ycans:
I came across this digram from an instructables page
Oh dear, you used a rude word!
You mean this laser? That stands a fair chance of working; presumably you have the photodiode (very) well-shielded from light other than the laser.
With the mini-laser I cite here, you can actually drive it directly from the Arduino so the MOSFET is not required, but the circuit seems more-or-less correct and the FET will (just) work at this voltage.
Note that if you go to 38 kHz, you will not be able to use a Baudrate much greater than 1200. Also that for longer distances (i.e., outdoors), "plain" LEDs have often been found to be more effective than lasers.
You know it never ceases to amaze me how many ways there are of getting electronics, on instructables, totally screwed.
No that diagram will not work. This is because that p-channel FET is not a logic level FET and so it can not fully turn on with only 5V.
The concept is sound. With the right components it will work.
KenF:
The concept is sound. With the right components it will work.
What a 1M pull down resistor on the photo diode fighting the 1K seriese resistor pulling up from the USB converter chip?
Grumpy_Mike:
KenF:
The concept is sound. With the right components it will work.What a 1M pull down resistor on the photo diode fighting the 1K seriese resistor pulling up from the USB converter chip?
Oh, they've used pins 0 and 1 as well! 
I thought these people that make indestructables worked from their own experience of working projects? I strongly suspect that this one has been created based totally on theory.
Thanks for your replies.
I'll order a logic level Fet, but in the meantime do you think a Tip120 transistor will work instead? (I have a bunch in drawer somewhere) http://www.adafruit.com/datasheets/TIP120.pdf
To get around the 1K USB converter pull up, should I just configure a soft serial on 2 other pins and use those instead of 0 & 1?
Hi, did you read any comments at the end of the instructables to see if anybody got it to work?
Tom.... 
Here is the page... http://www.instructables.com/id/Laser-Transceiver/?ALLSTEPS
It's lacking quite a bit of information and their final PCB appears to have caps. There is a video of it 'supposedly' working, but no-one in the comments has conclusively said the thing works.
The only change to the wiring diagram is the RX, TX connections as it says "If you use Arduino, Pinguino or any other board you can make it easy simply connecting the laser and the receptor to any TxRx pins (hard or soft configured)."
the drawing shows a PNP mosfet.
the TIP120 in an N type.
you would need to bring power to the laser, then put the TIP120 to bring the laser to ground.
FYI, the TIP120 is an NPN Transistor ( darlingon I believe), not a Mosfet.( meaning not an N- channel fet)
Looks like they got something working and then came up with that diagram without checking it. That sort of thing is very common with that site. Also the diagrams not matching what they show as working is also common. That is why the site is totally useless for electonics.
ycans:
The only change to the wiring diagram is the RX, TX connections as it says "If you use Arduino, Pinguino or any other board you can make it easy simply connecting the laser and the receptor to any TxRx pins (hard or soft configured)."
When they said that the were wrong.
The circuit as published needs to use a software serial connection. If you don't then how are you going to communicate with it?
The extra components are the regulator and capacitors and crystal and so on needed to make the chip into a working unit.
I have some of these 5mW 20mA at 4.5V modules and they do work on 5V from Arduino.
These are not just the laser but the power circuit as well.
However I never ran one long enough to test for any kind of lifetime. 30 mins at 5V and still bright.
I have an old 3mW pointer pen and it definitely less bright.
Here is the problem for comms. The dot doesn't get a whole lot bigger with distance and you're aiming at maybe a 5mm bulb sensor. Perhaps a convex or Fresnel lens could improve that?
I can't say for sure but a good superbright led might be effective if you put the detector well behind an aperture or in a long soot-black inside tube. Don't bother choking the light, it's enough to aim the detector. Only rays aimed at the detector will be detected so why have to aim that closely?
Grumpy_Mike:
... that p-channel FET is not a logic level FET and so it can not fully turn on with only 5V.
Datasheet suggests it will work - just - for any current up to an amp.
Grumpy_Mike:
What a 1M pull down resistor on the photo diode fighting the 1K series resistor pulling up from the USB converter chip?
There's a puzzle. The circuit ycans has given is not the circuit in the instructable, which details a custom made PCB using an ATmega and no other peripherals, so it has no problem with being connected to another serial interface and the hardware UART would be just fine. So from where did he get the useless diagram with the UNO?
Clearly you could do this but would need a transistor or HCMOS gate to buffer the photodiode.
Grumpy_Mike:
Looks like they got something working and then came up with that diagram without checking it.
Not defending the general tenor of "instructables", but their circuit does seem to be workable where the one ycans has posted is not.
Other than that, I agree with Mike. 
GoForSmoke:
I have some of these 5mW 20mA at 4.5V modules and they do work on 5V from Arduino.
Well, 20mA at 4.5V allows for the voltage drop when you seriously load the pin, that sounds just fine for direct connection.
GoForSmoke:
These are not just the laser but the power circuit as well.
The "power circuit" would appear to be the same as those I cited before. Just a different lens housing. It is in fact precisely, a "910" or 91 ohm resistor. Given a 2.17V drop across the laser diode, at 4.5V this gives 25 mA and implies 55mW dissipation so if it truly generates 5 mW output, that is one seriously efficient laser!
GoForSmoke:
However I never ran one long enough to test for any kind of lifetime. 30 mins at 5V and still bright.
I have an old 3mW pointer pen and it definitely less bright.
Ah, but how do you know it is 3 mW? (What can we believe?)
GoForSmoke:
Here is the problem for comms. The dot doesn't get a whole lot bigger with distance and you're aiming at maybe a 5mm bulb sensor. Perhaps a convex or Fresnel lens could improve that?
I can't say for sure but a good superbright led might be effective if you put the detector well behind an aperture or in a long soot-black inside tube. Don't bother choking the light, it's enough to aim the detector. Only rays aimed at the detector will be detected so why have to aim that closely?
You need a lens - or a proper parabolic reflector - on the photodiode to achieve any range at all but the gain of a lens is actually quite terrific - probably equates to 30 to 50 dB for a small one. As I said, Ronja figured that efficient non-laser LEDs were much easier to work with (and especially for analog modulation - which is what modems use), using lenses, preferably big ones.
(I recall in my youth, scraping the paint off OC71s or sawing the case off TO-5 or TO-18 transistors and blowing out that white powder - Mike knows what it is.)
Leds be safer too. I'd hate to be in court for having blinded someone. I cringe when some movie-inspired projects get suggested.
I remember a show with a 50's recreation of a fan interrupting light on a semiconductor device causing a mystery signal being someone's big discovery. All transistors are light sensitive, IIRC.
Not defending the general tenor of "instructables", but their circuit does seem to be workable where the one ycans has posted is not.
Yes but in reply #12 I quoted a bit from the page where they said it would work, hence the OP expected it to work. I suspect he made that circuit on the basis of what they said on the page.
Basically they were kids who didn't know fully what they were doing or what they were saying. Look at the no capacitors around the voltage regulators that don't even warrant a mention in the text or diagrams.