Pekka,
1: Yes, the noise level is quite high so far, when using the previously mentioned circuits. What comes to electro-optical components, I have a few different types of IR-receivers, which are not yet tested.
And the mechanical construction.. there is always room for improvement, one of the most important being the accurate height adjustment of the sender/receiver pairs. Likely by grub screws.
The current mechanical setup is too narrow for that, but it's just one of the first practising platforms.
2: I got the idea for the modulation from several sources on the net. It seems to be true, that by pulsing the leds, one can get a lot more 'power' out of them, when compared to using plain DC.
At first I thought: "more is better". But when used on a project like this, overly powerful output from the sender IR-led goes way above the receiver's range.
Also, when using something like 50-100KHz square wave for modulation, it attracts all kinds of noises, if not properly shielded. Those noises don't necessarily show on the scope, but they are audible.
As I'm on the testing phase, there are a lot of wires around to act as an antenna.
3: Thanks for the link. I'll check it out.
4: As I've looked info about demodulation, it seems to be a very complex subject. As I see it, to cancel out certain noises/frequencies, one should be able to provide exact, inverted copy of it/them.
Bill, I'm not sure either. When I got the resulting ~50Khz square wave output, the vibrating guitar string was able to modulate it, but with a lot of (audible)noise.
WeldingRod, I'll admit, there is options/settings on the scope, which I'm not familiar yet. I thought about subtracting/cancelling unwanted frequencies, but as mentioned above, it isn't necessarily an easy task to do.
Today, after reading these replies, I decided to test using plain DC for the sender IR-led. I tried it earlier, but it was just too easy to burn out the buggers. One way to see, if they are dead or alive, is to use camera.
Now, that I'm aware(sort of), that the ultimate brightness is to be avoided, perhaps more delicate approach is justified.
Result: the sending IR-led was fed with mild ~3,5mA DC, and the output was strong, not so much on the scope, but audible. And this time there was no noticeable noise at all.
But it wasn't enough for the schmitt trigger. Needs probably more amplification to produce string's frequencies in a square wave form.
I'm led to believe, that audio measurements on the scope could be very tricky. Don't know why, but that seems to be the case.
For audio probe, instead of laptop, I now use a guitar multieffect device, as I guess it's designed to handle better the incoming signals, without adding strain, or loading them down(impedance?).
The device is used without any effects, so only dry input signal appears at the output.