Audio signal from incandescent bulb into phototransistor receiver
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- เผยแพร่เมื่อ 31 ม.ค. 2018
- Let's see what happens when I drive an incandescent light bulb with an audio signal and receive the signal with a phototransistor. Yes an LED is a better transmitter but I wanted to see the results.
- วิทยาศาสตร์และเทคโนโลยี
This is a very interesting field for experimentation, and a lot of fun. I did a load of work on it back in the 80's, I found that to send intelligible speech or music you have to bias the bulb on sorta bright but not fully- you can use a series transistor with adjustable base bias, then feed the audio onto the base via a capacitor. This makes sense if you think of it like amplitude modulation! It also helps with the frequency response cos the bulb responds faster when hot, but its still going to be very bassy. LEDs should be much better though. Back then, they weren't bright enough. The only way to sent audio over big distances using light was to use a high power bulb, and modulate the light from it using a shutter made from two blades one of which was mounted on the voice coil of a modified (cone removed) speaker, and then use a lens to project it. You could get miles using this setup at night, given a sensitive receiver with a lens focussing the light on to the phototransistor, and biassing the latter on slightly.
You are correct. Don't know why I did not think of that. Someone mentioned that earlier so I made a followup video.
To tune the bias transmit a sine wave and monitor the even harmonics (audio spectrum analyzer). Adjust the bias to minimize the even harmonics and you have the optimum bias point. An asymmetrical waveform creates even harmonics and a symmetrical waveform minimizes them with only odd harmonics remaining. I have done this for broadband amplifiers when output filtering was not possible.
Yes, I think without bias you'd get a frequency doubling, as the filament lights the same positive or negative.
What about high frequency pwm through led?
Isn’t that in a way how optical audio works
Put your videos up unlisted and then if they get demonetised you can contest it before it goes live so you don't lose your main spike of ad revenue!
Man, this is great stuff. I like it a lot. Thanks for the upload John
Alan Cordwell was right. I built an audio link back in the mid '60s using a 12V tail light bulb and a germanium power transistor to drive it. I used a reflector of headlight to focus the beam. At the far end I used a small selenium photocell and a magnifying glass. I got it to work for about 600 feet, but the audio was very heavy on the low frequencies, I had to reduce the coupling caps to roll off the low frequencies. That was before LEDs, even dim red LEDs. Today it is easy to get watts of LED light that can be modulated at 10 megabits per second or more. The only problem is getting it to work in rain, snow, fog, etc.
Really interesting, i thought it would never worked! Very nice.
Congrats on the subscriber growth. Your style, knowledge, and abilities are what draw them to your channel. I wonder what the numbers would be if you were still Vegematic1966.
Speaking of the old channel, do you ever think you might revisit some of the subjects from back then? I think people might like the LM317 amp and maybe your speaker builds. I know, you already said those were gone, but if you do decide to do another one, I know I'd be interested.
Anyway, kudos, and keep the vids coming.
Some of the first sound movies had a photo phonic sound track that worked on a similar principle
True, I couldn't even discern the genre of music. I suggest you try LEDs to modulate the music. Keep trying different colors, as your laboratory could be awash in some colors.
Apparently the first soundtrack on film used a fast bulb, and I recall and old project from ETI (Electronics Today International) the "photon phone"
I can't help but wonder if running a highly compressed song though the bulb would work better, seeing as the output level would be almost the same. But biasing seems to make the most sense as I see in the other comments! Fun little thing to play around with though~
I need a small, low noise mic amp that can boost the signal to line level for a video camera. I'd like to buy one but the only one's I've found are quite expensive. I thought you might have some ideas about how to build one.
Is it possible to build a ss pre and output amp and add.a tube buffer in between to see if there is any measurable difference? Thanks
Cool! Optic fiber next. J/k
This one was interesting.
You've confirmed what I thought might be the case with filament bulbs as light-audio senders. They don't give very good response but may be okay for speech.
Have you tried putting a little dc bias on the bulb to see if that changes things at all?
Also have you tried doing this with LEDs?
Of course with those you use a transistor circuit to set the idle current of the LED and apply audio to that. I think the fidelity might be quite surprising and you could use lenses to send over a considerable distance
Think, one of those cheap 1W LED torches that has a focus used as the sender.
I think I might eventually try this one!
You have the scope to test the upper frequency limit though. You might discover it's high enough to even send sub carriers!
It would be really funny to transmit RTTY using a light bulb :)
Should have biased it to a slight glow and try it that way.
Indeed. Made a follow up video.
Great! Thanks a lot.
I love your channel, keep up the good work.
Greetings from Hungary :)
What's up with that ? like your videos 😂😂😂
I was also surprised at how high the frequency response was. One of these days, I'm going to try it with a 60 watt incandescent bulb.
well this is kinda like those IR data ports early 00's mobile fones had, before bluetooth. i never had one such mobile though.
Doesn't the led mess things up? It throws light on the phototransistor as well
The LED sits lower than the transistor which has black sides. It could reflect back though. It is rather dim to be much of an issue.
shame you didnt show the output spectrum for distortion. it coul give a few interesting sound effects for voice or guitar. photoacoustics could be used for reverb
Perhaps YT algorithm might be triggering on the music you play during first part of amp testing? Copyright questions??
Maybe a new smart form of surveillance!? A small microphone mounted in a LED bulb!
Nearly impossible to detect since it does not transmit any radio waves and impossible to see.
And all you have to do is to point a telescope with a light censor at the light that emits from the room behind the curtains, and it is clears as day to hear. :-)
that's a neat little project/idea but obviously not practical.
I feel contempt towards TH-cam.