Using an audio signal to control the brightness of an incandecent bulb

Thread Starter

BobRobertson

Joined May 22, 2019
2
This is old-school, but I don't want to re-invent the wheel here. I'm sure there is a schematic somewhere for a circuit that does this. It is very similar to a color organ that was popular in the '70s, but in this case, it is one channel and we don't care about filtering frequencies. The only thing I want is for a light bulb (Not an LED) to grow brighter and dimmer in sync with the amplitude of an audio signal. No audio, no light. Full volume audio, Light at full brightness, and everything inbetween, with the light reacting as the audio plays. I don't want to use a mic pickup for this. I would prefer a line level audio input, but most of these seemed to use a fully amplified signal to drive it. That's OK, just not my first choice. I found this long discontinued kit still listed on a website: http://www.omnitronelectronics.net/phpstore/html/K4738-120VAC-Single-Channel-Color-Organ-Kit-solder-kit-723.html. Somewhere I found a video of this working, and the effect is exactly what I'm looking for. The owner of the website says it's discontinued, so if you go there and it's not showing up, he may have taken it down after being alerted to it. I want the incandescent bulb because of it's natural tendency to smooth out the variations in brightness due to its relatively long fading due to the filament. Something that would take a much more complicated circuit to reproduce with LEDs. Can anyone guide me in the right direction?
 

danadak

Joined Mar 10, 2018
4,057
Fading is a pretty easy task these days with PWM control in a UP.
Many micros have this capability.

That being said you can use an ordinary audio power amp and drive
a 12 V or 6V marine filament bulb, or the like.

Regards, Dana.
 

DickCappels

Joined Aug 21, 2008
6,374
Simply connect a light bulb of the appropriate voltage rating across the output of an audio amplifier. It is one way of seeing whether the amplifier is working.

There is also an informative video showing a one transistor amplifier on YouTube. Start watching around 8:50 for the interesting part.
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Thread Starter

BobRobertson

Joined May 22, 2019
2
These are great suggestions! I never though of just connecting a light bulb to the output of an amplifier. I remember years ago (decades, actually) as a High School Science project I built a "light Transmitter" and transmitted sound over a light source. It was even simpler than the video showed. I used a red bulb in a flashlight so I would have the reflective cone behind it. Connected that bulb to the output of a small mono amp. At the other end in a darkened space, I mounted a magnifying glass to focus the light on a small photocell, and plugged that output directly into a little radio shack amp that they made at the time. It just had a mic jack, a volume control and a speaker. I set it up at a science fair and it worked surprisingly well. Much better results than the guy in the video got, but I kept the area where the light and the receiver were fairly dark and focused the light, so there was much less loss or interference. It was set up so spectators could slide their hands in front of the beam and stop the sound. I had completely forgotten about that experiment. All we need to do is run the amp, measure the voltage coming out during the loudest passages and get a bulb close to that rating.
 
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