Re:Electronics
Need more info.
The first is trying to get an understanding of your level of expectation. The eye is lucky to see a fluctuation that lasts under 1/30th of a second. This would imply that you could not reliably see any fluctuations at a frequency of over 30Hz. This is well below the threshold of hearing, so it would not be possible to re-create the sound from the fluctuations in light level. You could at best hope to produce some kind of a brightness to volume relationship. The louder the sound, the brighter the light. Is this what is hoped for? If not, would this be acceptable?
The next question, is how much signal is there to work with. It takes .3 to .6 volts to forward bias a PN junction. V^2 / R = P so V = sqrt(P*R) This means that if you have a 1 watt transistor radio, it will deliver 2.8 Volts. This is more than enough to reliably turn on a Transistor, SCR, or Triac.
I would recommend using a Triac or SCR rather than a Transistor. This will preclude the necessity of rectifying the power to DC. Triacs and SCR's are turned on by a gate current, and turned off when the current through them drops to zero. If you feed them 50hz AC, they can only stay on for 1/100th of a second ( 1/2 of a cycle) and then they turn off. In your application, this would mean that they would be turned on for 1/100th of a second at a time when the sound level exceeds 43 miliwatts. Triacs SCR's are either on or off, so the light bulb will dissapate all of the power and you won't have to hassle with heat sinks. Get an SCR or Triac that is rated at 350V or more to leave yourself a margin of error.
Be careful, get help! 240VAC can be very dangerous.
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