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Modulating a diode laser with sound

  1. Feb 25, 2005 #1
    http://www.eweek.org/site/DiscoverE/PDFs/middle/First you see it Then You Hear It.pdf

    In my Photonics & Fibre Optics class today, our lecturer showed us a "Modern day photophone" he created for an easy $15 AUD (1AUD = 80US cents)

    I did not have time to ask him how to make one myself, so I went home and emailed him but when waiting for a reply I found another version on the net - the link above.

    Going down to the local electronics shop, I picked up a Diode Laser in place of a pointer, and in place of the batteries needed for the laser and recieving circuit, I used a computer power suply and a few resisters to try and get the right voltage.

    Trying to set the lens for the diode laser was tough. Blu Tak (like plasticeine for sticking posters up) comes in very handy for minute adustments of the lens to achieve a pretty narrow beam.

    After both the Modulator and Demodulator were complete, I tested it out. The speakers made a loud crackling sound when the laser was not pointed at the reciever, and went quiet when the laser was pointed directly at it.
    Moving it around, sound eventually came out of the speakers..but it seemed that it would only be when part of the laser beam was incident on the receiver.

    The link above sais to use a Photo Diode for the reciever but instead i used a Photo Transister, being the only component that said it worked with the visible spectrum as well as infared.

    Also, the resisters used on the Reciever are for a 9V battery, where I am using a 12v DC power suply. The resisters on the Transmitter should be ok.

    If someone could tell me the difference between a Photo Diode and Photo Transister, it would be great. The transister also has 3 prongs..a base a collector and an emmitor. I am using the Collector and Emitter..with the emitter connected to the positive coming from the power suply.

    I am going to see the lecturer soon to see his idea that seemed to work flawlessly. But if anyone could tell me what might be going wrong with my version, i would be very appreciative.

    Thanks heaps,
    BSc(Photonics) & BEng(Telecommunications & Internet Technologies)
  2. jcsd
  3. Feb 25, 2005 #2


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    A photo transistor is just a normal transistor, consisting of two back-to-back diodes made from a sandwich of either npn or pnp semiconductors. The only significant differences between a phototransistor and a regular transistor is that the phototransistor usually has an enlarged base-collector junction and is packaged in a transparent package.

    Rather than applying a base voltage to forward-bias the base-emitter junction, the photoelectric effect is used to produce free electrons in the base. The transistor thus doesn't normally need to have its base connected to anything.

    It sounds to me like the most obvious problem is that you have your transistor connected upside-down. You have the emitter connected to the positive supply; normally the collector is connected to the positive supply. In a perfectly symmetrical transistor this would be a moot point, but it might be a significant problem with an asymmetrical phototransistor.

    You may want to provide us with more information -- for example, the manufacturer and part number of your phototransistor. You might want to look at the datasheet for the part to see if there are any application circuits you could examine.

    - Warren
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