Monday, April 1, 2013

Experimenting at 461 THz

I went on YouTube ( ) and found a simple circuit that allows modulation of a pocket laser pointer.  These red, 650 nm devices (461 THz) can be AM modulated with an audio transformer that varies the intesity of the light.  The modulated signal can then be demodulated by a solar cell directly wired to earphones (where voltage is directly related to light intensity.)

I went to my local electronics store and got my parts.  The solar cell was $5 and the audio transformer was about $3.  Depending on the laser pointer, most are 4.5V, you will need a 4-cell battery holder (I used AA-sized) and jump out one of the slots.  My laser pointers are 3V so I opted for a 2-cell holder.  The AA cells are larger capacity than the button cell or AAA cells normally used to power these pointers, so should offer enhanced life per cell during use.

The trouble I am running into with this experiment is that the audio transformer has significant impedence on one side.  Depending on how you wire it, you are attenuating either the laser power or the input audio.  The spec is 72 ohms on the input coil and 0.62 ohms on the other coil.  If you choose to attenuate the laser, your range is greatly diminished (my lasers are rated at < 0.5 mW).  If you attenuate the audio, you need much more audio drive into the circuit. 

While driving the input is easy enough if you are using a music device such as an MP3 player, it is more difficult if you are using a microphone.  The solution will have to be another circuit with an audio amplifier to add to the audio I wish to drive the circuit.  Unfortunately, my initial experiment had insufficient drive to generate the necessary signal. 

So tomorrow it is back to the drawing board (and the reference books) to make myself an audio amplifier that hopefully can be rated at 3 V so that I don't need another set of batteries.


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