Find all your DIY electronics in the MakerShed. 3D Printing, Kits, Arduino, Raspberry Pi, Books & more!

Make: Projects

Simple Laser Communicator

Talk in secret over your own private laser beam link.

Simple Laser Communicator

How would you like to talk over a laser beam? In about 15 minutes you can set up your own laser communication system using a cheap laser pointer and a few parts from RadioShack. The audio signal from a microphone varies the power feeding the laser, so that its brightness changes, following the shape of the original sound wave. At the receiving end, a solar cell or photo-resistor converts the oscillating light signal back into the original sound.

The communication is completely private, with no wire connection to tap into. Only you will be able hear what comes over the secret laser link.

Check out more Weekend Projects.

Steps

Step #1: Assemble the transmitter.

PrevNext
Simple Laser CommunicatorSimple Laser CommunicatorSimple Laser CommunicatorSimple Laser Communicator
  • It is recommend that you solder this project, but initially, it’s easier to make it and test it out using alligator clip leads.
  • Remove the batteries from the laser. Connect the external battery pack to the laser’s power contacts with 2 alligator clips. Usually you’ll connect one lead to the battery case and the other to the spring inside.
  • Some laser pointers are easy to disassemble; you can remove the circuit board and see the power contacts conveniently marked with a plus and a minus. If it doesn’t light, try reversing the power; this won’t harm the laser.
  • Figure out how to hold the laser’s button down with a rubber band, wire, or tape.

Step #2: Connecting the LED.

PrevNext
Simple Laser CommunicatorSimple Laser CommunicatorSimple Laser CommunicatorSimple Laser Communicator
  • Remove the batteries. Following the schematic, connect the bicolor LED across the 1,000-ohm (1kΩ) side of the transformer. With the transformer shown here, this means connecting it to the green and blue wires. We don’t need the tap wire (the black wire in between).
  • The LED protects the laser from high voltage spikes, since so many cheap lasers nowadays have no onboard protection circuit. If you see the LED flash, that indicates a spike.
  • Since we're using a bicolor LED, we don't have to worry about polarity and can connect it either way.
  • Connect the same two transformer leads in-line between the battery pack and the laser.

Step #3: Finish up the transmitter.

PrevNext
Simple Laser CommunicatorSimple Laser CommunicatorSimple Laser Communicator
  • Connect two leads to the earphone plug. Unscrew the cover, solder leads to the plug terminals, and replace the cover.
  • Connect the earphone plug to the 8Ω side of the transformer using alligator clips.
  • That’s it! We have a laser transmitter, in just a few minutes!
  • Plug the headphone plug into a transistor radio, which will be our signal source for testing.

Step #4: Assemble the receiver.

PrevNext
Simple Laser CommunicatorSimple Laser CommunicatorSimple Laser Communicator
  • For the receiver, connect a 9V battery across a cadmium sulfide (CdS) photoresistor and an earphone, so that battery, earphone, and photoresistor are all parallel.
  • The photocell changes its resistance proportional to the amount of light hitting it. Paired with a battery, this acts like a solar cell.
  • Add a 220Ω resistor in series with the battery to reduce power consumption and prevent heating of the photoresistor.

Step #5: Set up and test.

PrevNext
Simple Laser CommunicatorSimple Laser CommunicatorSimple Laser CommunicatorSimple Laser Communicator
  • We’ll test the system by transmitting a radio signal and amplifying the receiver so we can hear it across the room. First, replace the earphone of your receiver with an audio plug (or just clip the audio plug to the earphone wires), and plug it into an amp or stereo. Here we use a mini portable amplifier.
  • With the radio off, plug in the transmitter. Turn up the volume on the amplifier until you hear a hiss, then turn it down until it isn’t noticeable.
  • With the lights low, aim the laser across the room so it hits the solar cell or photoresistor. You may hear some clicks or pops. Now turn on the radio and adjust its volume until you hear it across the room.
  • If you don’t hear it, try increasing the amp’s volume before you turn up the radio. If you pull out the earphone plug, the radio should be just audible.
  • Depending on your signal source, you also might want to reverse the transformer. Some devices, like iPods, don’t have enough power to drive 8Ω speakers, so you should connect them across the 1kΩ side. This arrangement will dim the laser, but won’t affect its range much.
  • When you can hear the radio, break the laser beam with your hand, and notice that the music stops. Try chopping up the audio with your fingers.

Step #6: Your laser com system is ready!

PrevNext
Simple Laser CommunicatorSimple Laser CommunicatorSimple Laser Communicator

To send secret voice communications, move the amp from the receiver to the transmitter and plug in a mic. You’re ready for the field; just be careful with the volume, to protect the laser.

Conclusion

This project first appeared in MAKE Volume 16, page 67.


Comments

  1. Simon Quellen Field says:

    All of the component specifications listed appear correct to me.
    What problems were you having?

    This project is based on this one:
    “http://sci-toys.com/scitoys/scitoys/light/light.html#laser_communicator”
    and the kit can be found here:
    “https://www.scitoyscatalog.com/product/LASERVOICEKIT.html”

  2. Simon Quellen Field says:

    The second link has the transformer you need.
    Scitoys.com ships all over the world.

  3. Simon Quellen Field says:

    The whole kit is available here:
    http://www.scitoyscatalog.com/product/LASERVOICEKIT.html

    The transformer by itself is available here:
    http://www.scitoyscatalog.com/product/XFORMR.html

  4. Simon Quellen Field says:

    Check the voltage required by your green laser, and the voltage actually delivered.

  5. Simon Quellen Field says:

    You’d still want the transformer to mix the signals.
    But a pair of resistors would probably work also.

    1. Darren says:

      Can I use the audio output transformer with these specs?
      AC resistance @10khz : 2KΩ-0-2KΩ (primary), 95Ω (secondary)

      Inductance @10khz : 19.2mH-0-19.2mH (primary), 0.88mH (Secondary)

      Turns ratio approx : 22:22:1

      DC resistor approx : 39Ω-0-39Ω (primary), 1.5Ω (secondary)

  6. Van Haute says:

    If I understand you, you suggest to lower the full output of the radio by using resistors ? Right you are old chap ! What is your location ? I am calling you from Antwerp Belgium. So long !

  7. Margot Paez says:

    Can I use some cheap earphones in place of the crystal earpiece? I have a pair that I don’t need and that’s easier than finding a place that carries the crystal kind…

  8. Mike Keith says:

    I’m Confused. I can’t get the simple input circuit to work. When I add the transformer in series with the batteries the laser pen no longer works. Works A-OK if I short the transformer while it’s in the circuit. All versions of this experiment reference a 4.5v battery source but my pen is 3.0v. Does the laser type (4.5v vs 3.0v) or battery voltage make a difference? If so, you’d think this would have been mentioned along the way…
    Thanks for help, Mike

  9. […] Magazine - http://makezine.com/projects/simple-laser-communicator/ Their Youtube Video for this can be found […]

  10. Margot Paez says:

    This seems unjustified. I used this guide successfully to build my own laser com and it worked on the first try. I was even able to use my iPhone headphones, too.

    It would better more productive to post where you are having problems in understanding the tutorial than to unfairly flame someone. I feel like flaming should have gone out of fashion when AskJeeves did.

    - Margot.

  11. Margot Paez says:

    Oh. I am still groggy. No wonder this didn’t make any sense. This is a spam post. Someone fix the spam catcher! :)

    Sent from my iPhone