The Pink Princess Laser Modulator

The pink laser princess box projects a red beam on the receiver boxLast weekend, a bunch of us from the Vienna Wireless Society set up a table for a Sally Ride Science Festival held at George Mason University. This is annual event, which promotes science, math, and engineering for girls in grades 5 to 8. Part of the fair takes place outdoors in the morning and early afternoon, with seminars held indoors in the afternoon.

Since it was drizzly, we set up on three plastic folding tables under tarps.  We brought a smattering of equipment for show-and-tell including a portable satellite station, an HF station, an electronics demonstration, and some of the equipment from our most recent high altitude balloon launch.

My instructions on the event were to bring stuff that would be attractive to kids, so I asked with my kids about what kind of presentation would catch their interest. They suggested demos that would either make noise or “be shiny”. They also suggested candy.

To cover the noise angle, I brought a few morse code keys and paddles hooked to a code practice oscillator. I made a little cheat sheet for sending morse code and handed them out. For the demo, I printed a few sheets of five letter words, chopped them up, and put them in a tupperware bowl. I put another bowl right behind it full of candy and a sign: “send code. get candy. 5 letters = 5 Calories”. I ended up handing out 75 pieces of candy, and some of the kids had good fists. A couple used the paddles in addition to the key.

While I was shopping for the candy at the store “Five Below”, which caters to girls of just this age bracket, I noticed that they were selling laser pointers for $2 each. I figured the little batteries were probably worth that much alone, so I bought four lasers to play with. It occurred to me that I could cover the “shiny” criterion with some sort of laser project.

I had seen on the net a description of simple circuits for amplitude modulation of a laser by an audio signal, so that’s what I put together. Line level output enters through 1000:8 ohm transformer and directly modulates the supply voltage to the laser. A couple diodes are thrown in to protect the laser diode from spikes, but that’s about it. A couple months back, the Dollar store had been selling overruns of Disney Princess puzzles in nice tins, a dollar each, so I had bought a bunch of them for projects that didn’t quite fit the Altoids form factor.

The day before the event, I recorded the audio from my radio tuned to 14.070 USB, to capture USB conversations. I had the 2.4 kHz filter on, so there were quite a few PSK31 conversations in the audio band pass. I saved the sound as a *.wav and transferred it to my ipod. The ipod then plugged into the Pink Princess Laser Modulator.

On the receiving end, the circuit was even simpler, all that stood between a 1.5v battery and the audio output jack was a photoresistor. I knew that I had one kicking around, but couldn’t put my finger on it for most of the day, until I remembered that a CdS photoresistor is included with the Arduino experimenter kit as an optical sensor.  The receiver went in an Altoids box, and the output ran into an audio breakout box that allowed me to split the signal to a speaker and to a computer. I duct taped the speaker to the awning of the demonstration booth and let visitors click on individual psk31 streams to decode them.

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