[Pytor (OP)]

Many years ago I saw a TV documentary about Arthur C. Clarke where on his farm he built a device which converted speech into light pulses which were picked up at the other end of the farm by his brother holding a device which converted the light pulses  back into speech .Does anyone know how he did this and can it be ( cheaply) replicated.

[Colin2B]

Does anyone know how he did this and can it be ( cheaply) replicated.

I don’t know how he did it, but I know how I did it in my teenage.
I connected the output of an amplifier across a low voltage bulb in a torch (to get the reflector). Light shon on a photo detector, in this case a transistor with the paint scraped off (that was before the manufacturers got wise and filled them with opaque material, they could then sell more expensive phototransistors, which were actually the same thing but without the opaque!). Amplified the sound wasn't bad, but range was across the lab, doubt it would do across a farm without better reflector and collector.
Nowadays you should do much better by modulating a cheap laser chip.

[evan_au]

The Photophone was invented in 1880 by Alexander Graham Bell.
- For the transmitter, it used a reflective film which is deflected by speech, deflecting light from the Sun.
- In the detector it used a light sensor and electricity to amplify the signal.

But it was soon overtaken by wireless transmission of speech, which didn't need such careful alignment (and worked at night and indoors, too!)...

See: https://en.wikipedia.org/wiki/Photophone