Please Help w/Laser Communicator

[Hemlock_Mike]

Tim --

I believe I mentioned input impedance of the audio amp. If high impedance (also called HiZ), .5 mA will drive it if you have the gain. The beam has to hit much of the surface of the solar cell in order for it to produce rated voltage provided it "sees" 405 nM.

HMike

 

[Benm]

If you apply that audio transformer before the driver, the driver may very well be the problem. After all, its job is to provide constant output current regardless of input voltage, and it may very well be quick enough to compensate for the audio signal. Also, it may have buffer caps on its in- and output that cancel the modulation.

The simple laser mentioned in the pdf will not likely have a good driver and will pass audio on the supply side to the laser diode to some degree.

Solution: build a driver that accepts analog modulation.


On the receiving end: its hard to say what is going on exactly. Its easy to diagnose though. Considering this uses linear modulation, you should be able to pick up some rattle or hum from flickering light sources. Energy saving lights or TLs powerd off mains should give you a solid 100/120 Hz rattle, but even light from a regular lightbulb does have some mains flicker to it. Not enough to see by eye, but enough to see using a solar cell/photodiode and scope, so it should translate to a hum in the receiver as well.

 

[Benm]

Hard to believe that a CdS cell could react fast enough for audio! That IS old technology !!!

CdS is slow... On audio frequencies it will still pick something up, but its dampened by the slow response, and will favour lower frequencies over higher ones.

The sensitivity is comparable to that of the human eye - its most sensitive to green, but will still be activated as far as 400 nm, so it should work with bluray to some degree.