... 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.
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.
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.
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.
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!
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?)
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.)