Modulating a diode laser with sound

In summary, the conversation is about creating a "Modern day photophone" using a diode laser, computer power supply, and photo transistor. The speaker shares their experience and asks for help in making adjustments to their version of the photophone. They also mention the difference between a photo diode and photo transistor and request for assistance in troubleshooting their project.
  • #1
0 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 receiving 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)
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  • #2
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
  • #3

Modulating a diode laser with sound is a fascinating concept that combines the principles of optics and acoustics. It is interesting to see how this can be achieved with simple and affordable materials, making it accessible for students and hobbyists to experiment with.

Using a diode laser instead of a traditional laser pointer is a great idea, as it allows for a more precise and focused beam. The use of a computer power supply and resistors to adjust the voltage is also a clever alternative to batteries.

It is important to note that the use of a photo diode or photo transistor for the receiver can affect the sensitivity and performance of the system. A photo diode is specifically designed for detecting light and converting it into an electrical signal, while a photo transistor is a type of transistor that is sensitive to light. The difference in their construction and sensitivity can impact the accuracy and reliability of the modulated signal.

It is also important to ensure that the correct resistors are used for the receiver, as using resistors designed for a 9V battery instead of a 12V power supply can also affect the performance of the system.

Overall, this is a great project to learn about the principles of light modulation and transmission. It is always beneficial to consult with experts, such as your lecturer, to troubleshoot any issues and improve the design. Keep experimenting and exploring the possibilities of modulating diode lasers with sound!

Related to Modulating a diode laser with sound

1. How does modulating a diode laser with sound work?

Modulating a diode laser with sound involves using a piezoelectric crystal to convert an electrical signal from an audio source into mechanical vibrations. These vibrations then impact the diode laser, causing changes in its output intensity and frequency.

2. Why would someone want to modulate a diode laser with sound?

Modulating a diode laser with sound allows for precise control of the laser's output, making it useful in a variety of applications such as laser communication, spectroscopy, and laser printing. It also allows for the laser to be easily synchronized with other devices.

3. What types of sound can be used to modulate a diode laser?

Any type of audio signal can be used to modulate a diode laser, including music, voice recordings, and electronic signals. The important factor is that the signal is converted into mechanical vibrations by the piezoelectric crystal.

4. Are there any limitations when modulating a diode laser with sound?

One limitation is that the frequency of the sound wave must fall within the range of the piezoelectric crystal's capabilities. Additionally, the intensity and quality of the laser's output may be affected by the quality and consistency of the audio signal being used.

5. Can diode lasers be modulated with sound in real-time?

Yes, diode lasers can be modulated with sound in real-time by using a fast-responding piezoelectric crystal and an audio source with a high sampling rate. This allows for precise and dynamic control of the laser output, making it useful in applications such as laser light shows and laser machining.

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