Audio modulation of LED chain

[wedgetail]

It is past midnight here and I am very sleepy but would like to post my concept schematic.

Short description it is a standard constant current regulator circuit. I have LF355 OP amps. As I understand that vintage far from rail to rail output. In this case to keep power low in the emitter resistance I have created a negative voltage for the opamp with the 3 diodes so that hopefully the output can go close to no voltage over the emitter resistance.

I am curious will it work properly say perhaps 0-1 Volt drive signal will work?  Not sure what Audio range I may need but probably up to 20 kHz?

I am aware schematic still needs a lot of work. 

Comments welcome

[Infraviolet]

Is this a means of sending audio over an optical channel for communication, or a matter of making an LED panel/strip flash in time as a visual effect to accompany music? In the latter case you might most easily just take the voltage level of the input signal and use that to control the duty cycle of a PWM square wave. Then the LEDs, which will have rise/falltimes of hundreds of ns, or faster, would be driven by an NPN with the PWM signal entering the base, the emitter grounded and the collector going to your series/parallel set of LEDs with appropriate current limitng resistors. Any freqeuncy for the PWM above perhaps 100 Hz would give a visual result indistinguishable to the eye when compared to having the LED brightness modulated by the audio voltage directly. For audio comms over an optical channel though, you'd need a much higher PWM freq to get a signal which , when turned back to audio at the other end, would sound correct to the ear. Definitely in the visual effect scenario, a microcontroller (atmega328- arduino or such) could read the audio voltage on an ADC pin and vary a PWM duty cycle to track this voltage, you might even low pass filter the copy of the audio signal which goes to the ADC pin for this scenario. It could also potentially be done with a 555 to convert voltage to PWM duty cycle, but you might need to amplify the audio voltage before feeding to a 555.

P.S. your schematic seems to lack a connection from the ground of the 3.5mm jack to the ground of the rest of the circuit?

[wedgetail]

Infraviolet

Thanks for replying.

Yes it is transmission of audio frequencies via light waves

I understand your suggestions but I got intrigued by the schematic as shown and want to experiment with the addition of a negative voltage rail. This then started to get me a bit confused because not quite a s simple as I thought.

Initially I thought I just need a rail to rail op amp and I can stay with only +ve and gnd.

The circuit needs more work, yes I need getting a ground for the audio connector.

[Andy Chee]

I don't know how far you're planning to transmit, but perhaps you might be able to get some ideas from these guys:

https://www.modulatedlight.org/Modulated_Light_DX/MODULATED_LIGHT_DX.html

[wedgetail]

Andy Chee

Thank you for the link, great stuff.  I think it may have been 20 or 30 years ago that I plowed through those.  I love it. Spent an hour at noon just reviewing a couple of items.

Ah forgot to mentioned, at present I am only looking for on average I guess 3 m range

[tooki]

I think you’d find this system — amplitude-modulated light with no carrier wave — to be extremely sensitive to noise and ambient influences, since any change in the amount of light received (regardless of its source!) will directly affect your signal.

Commercial light-based analog wireless audio transmission — like the Sony infrared wireless headphones and speakers I had in the 90s — use an FM-modulated carrier. (The Sony system even used Dolby C noise reduction to reduce hiss.) Using FM makes it insensitive to the amplitude of the light, and far less sensitive to ambient light.

Note that with modern PWM-dimmed LED lighting, optical noise becomes an even bigger issue.


Is there any point to this, other than being an interesting experiment? For practical use there are vastly superior options, I would think.

[wedgetail]

tooki
Thanks for replying, I did not at this stage really consider this a problem but with some other information I overheard a comment: " I has to be a Laser "
Well I go ahead and see how bad my little experiment will suffer.

I appreciate the clarification of the importance of a controlled carrier wave.

[tooki]

Lasers are used for free-space optical data transmission over medium distances (tens to hundreds of meters). It won’t work reliably for much farther than that, and for shorter than that, we don’t use lasers, but just infrared. (Think remote controls and IrDA.)

As for the importance of a carrier: it’s so important that even the most basic optical data systems, infrared remotes, use them because it won’t work otherwise. It’s also extremely common for optointerrupters to use a carrier if they’re exposed to ambient light. For example, an infrared beam that, when broken by someone walking through a door, chimes a doorbell. If it simply responded to amplitude, it would probably be oversensitive by day and not sensitive enough at night, since the baseline light levels vary.

A coworker of mine built a device that verifies that an object has exited a dispenser using a series of optointerrupters at various points on the exit chute, and he had to use different carrier frequencies for each one, both to make them insensitive to ambient light, but also to not have crosstalk.