The Photoelectric Effect & My Second Photophone

I succeeded yesterday in picking up sound-modulated light on the homemade photocell I made Sunday:

Here’s the workbench:

I’m outputting a signal from the radio’s headphone jack, adding it to the base voltage the LED is already getting via a 10 µF capacitor.  You can see it pulse if the sound changes drastically enough, or if you change radio stations.  The oxidized copper pickup plate was described in my post “Homemade Photocell” (https://mfleisig.wordpress.com/2011/03/06/homemade-photocell/).

The circuit is not complicated–the voltage regulator and capacitor at the left are merely bringing the voltage down to 5v and smoothing it.  The wires, LED and resistor to the right simply confirm the circuit is on.  The components at the center are producing the sound-modulated light:

Again, I am indebted to the site “Simple Homemade Photocell” (http://www.sparkbangbuzz.com/els/photocell-el.htm) and eHow’s “How to Make an Adjustable Homemade Photocell” (http://www.ehow.com/how_7290427_make-adjustable-homemade-photocell.html) for their great documentation.

I was able to move it roughly 6″ away and still get a signal, and with a simple lens, further still:

Next steps are to throw the signal further with lenses and laser, and to explore options on the transduction of sound to light, and vice-versa.  I also need to explore further the necessity of oxidizing the copper.  I don’t really understand that: the clean copper wire was also giving off electrons, but the effect was stronger when it was in contact with the plate, and most especially when it was in the salt-water solution.