Weird Science

Photophone: Sound in a Beam of Light

Polish ver­sion is here

Where Did the Idea Come From?

While brow­sing thro­ugh the book Nowo­cze­sne zabawki (eng. Modern Toys) by Janusz Woj­cie­chow­ski, I came across a descrip­tion of devi­ces known as pho­to­pho­nes. The book was publi­shed in 1974, and most of its sche­ma­tics are now quite out­da­ted. These devi­ces relied on com­po­nents that are no lon­ger pro­du­ced, inc­lu­ding ger­ma­nium tran­s­i­stors. Howe­ver, the con­cept of tran­s­mit­ting sound using nothing but a beam of light intri­gued me so much that I deci­ded to con­duct a few expe­ri­ments in this field. Below, I pre­sent the first pho­to­phone model I built years ago. It demon­stra­tes the basic prin­ci­ple of how these devi­ces work and can pro­vide one-way com­mu­ni­ca­tion over a fairly long distance. Altho­ugh I later con­struc­ted more com­plex and advan­ced devi­ces, I believe that this sim­plest model holds the gre­a­test edu­ca­tio­nal value.

Requ­i­red Mate­rials

To build the sim­plest pho­to­phone, you will need the fol­lo­wing com­po­nents:

You might think the most chal­len­ging com­po­nent to obtain is the semi­con­duc­tor laser. Don’t wor­ry—a laser diode from a stan­dard laser poin­ter is more than suf­fi­cient. It also comes equ­ip­ped with a sui­ta­ble col­li­ma­tor. Howe­ver, it’s impor­tant to use a sim­ple red laser poin­ter. Laser poin­ters emit­ting light in other colors typi­cally have built-in minia­ture power conver­ters that make them unsu­i­ta­ble for this pro­ject. The best appro­ach is to care­fully disas­sem­ble the poin­ter and sol­der wires to its power sup­ply ter­mi­nals. Remove the switch inside the poin­ter and replace it with a jum­per. To pro­tect the deli­cate laser module, I cove­red the entire assem­bly with a heat-shrink tube that fits tigh­tly when hea­ted. The pre­pa­red semi­con­duc­tor laser looks as fol­lows:

Ilustracja:

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All the rema­i­ning com­po­nents can be easily pur­cha­sed at an elec­tro­nics store.

Con­struc­tion

As you might expect, the device con­si­sts of two main blocks: a tran­s­mit­ter and a rece­i­ver. The tran­s­mit­ter’s pur­pose is to convert an alter­na­ting cur­rent car­ry­ing audio infor­ma­tion into fluc­tu­a­tions in the bri­ght­ness of the laser beam, while the rece­i­ver converts these fluc­tu­a­tions in light ampli­tude back into an elec­tri­cal signal and then into sound.

Tran­s­mit­ter sche­ma­tic:

Ilustracja

If the cir­cuit is powe­red by a 5V sup­ply, the vol­tage regu­la­tor can be omit­ted. The ope­ra­tion of the cir­cuit is stra­i­ght­for­ward: the tran­s­i­stor's base is bia­sed with vol­tage from the poten­tio­me­ter's wiper. By adju­sting the poten­tio­me­ter, you con­trol the base cur­rent, which in turn affects the col­lec­tor cur­rent that flows thro­ugh the laser diode. An alter­na­ting cur­rent of audio fre­qu­ency, sup­plied for exam­ple by an MP3 player, is also fed into the tran­s­i­stor's base thro­ugh a capa­ci­tor. As a result, the col­lec­tor cur­rent beco­mes stron­gly depen­dent on the alter­na­ting vol­tage car­ry­ing the audio infor­ma­tion. The laser beam’s inten­sity is thus modu­la­ted by the sound signal, albeit to a small but suf­fi­cient degree. The poten­tio­me­ter is used to set the tran­s­i­stor’s ope­ra­ting point and modu­la­tion depth. Always adjust the poten­tio­me­ter as fol­lows: before con­nec­ting the power sup­ply, ensure that the poten­tio­me­ter is tur­ned to the gro­und posi­tion. After powe­ring the cir­cuit, gen­tly and slowly turn the poten­tio­me­ter until the laser diode begins to lase. This moment is reco­gni­za­ble by a sud­den incre­ase in bri­ght­ness. Avoid tur­ning the poten­tio­me­ter any fur­ther, as this may damage the tran­s­i­stor and laser diode. Once adju­sted, you can con­nect the audio source.

Rece­i­ver sche­ma­tic:

Ilustracja

The pho­to­tran­si­stor and resi­stor form a light-con­trol­led vol­tage divi­der. The cur­rent flo­wing thro­ugh the pho­to­tran­si­stor depends on the inten­sity of the light it rece­i­ves. Accor­ding to Ohm's law, the vol­tage at the junc­tion of the pho­to­tran­si­stor and resi­stor will also change in response to fluc­tu­a­tions in light inten­sity. When the pho­to­tran­si­stor is illu­mi­na­ted by laser light modu­la­ted by the tran­s­mit­ter, an alter­na­ting vol­tage appe­ars at this junc­tion, ana­lo­gous to the audio signal dri­ving the tran­s­mit­ter. This vol­tage is then pas­sed thro­ugh a capa­ci­tor and fed into any audio ampli­fier. You can use com­pu­ter spe­a­kers or any other ampli­fier (for exam­ple, this one).

Thanks to the laser diode, the device has a sur­pri­sin­gly long range for such a sim­ple setup. At night, it can easily tran­s­mit over a distance of up to half a kilo­me­ter (about 0.3 miles). Natu­rally, nothing sho­uld obstruct or inter­rupt the laser beam.

Below are pho­tos of the fini­shed tran­s­mit­ter and rece­i­ver. Due to the sim­pli­city of the design, both were built using a "dead bug" or "air-wired" method, without prin­ted cir­cuit boards.

Ilustracja: Transmitter

Transmitter

Ilustracja: Receiver

Receiver

Enjoy expe­ri­men­ting with this edu­ca­tio­nal pro­ject! :)

Fur­ther rea­dings:

Marek Ples

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