Questions about infrared cordless headphones

[vindicadi198]

Hi! I was planning to build the infrared cordless headphone project given on the website, I was wondering if I could substitute the transistor amp stages with IC amps like the tDA2822 or the LM386?
And is there any way to make a stereo version of this? Coz I am worried if I just duplicate the circuit there will be interference between the LEDs of the two channels..

 

[vindicadi198]

just found out that i need to provide the link to the project..so here it is:
http://www.electronics-lab.com/projects/audio/016/index.html

 

[audioguru2]

just found out that i need to provide the link to the project..so here it is:
http://www.electronics-lab.com/projects/audio/016/index.html

It is a terrible project! I think it is copied from an Indian online magazine.
The first transistor in the transmitter has no base bias current so it acts like a rectifier which causes severe distortion all the time.
The second transistor also has no bias current and has an LED on its base that limits its max output which causes even more distortion.

The receiver puts DC through the headphones!
The receiver needs to have plenty of gain and might work with an LM386 amp IC (that has an output capacitor to prevent DC in the headphones) with its gain set to 200 like my attachment. I also show an IR transmitter that has no distortion.

IR stereo headphones use FM modulation (this lousy project uses AM) and a 100kHz carrier in one channel and 200khz modulation in the other channel.

 

[vindicadi198]

i too had doubts about the headphones connected to the battery. thanks for your reply, glad i didnt start work on it...
it turns out i have many remote controlled toys at home, so i was wondering if i could use the circuit to build the cordless headphone instead of the IR one

 

[audioguru2]

Remote control is on-off. If it is used for audio then the sound will be click-click or buzz-buzz.

 

[Hero999]

I was going to say how crappy this project is but decided to leave it to audioguru who I knew would enjoy it more and be more constructive than me.


AM is also bad because the response of the photo-diode photo-transistor won't be matched so there will be non-linearities which will distort the signal. To be picky no headphones use FM which would involve changing the optial frequency of the radiation emitted from the LED. The correct term is PFM (Pulse Frequency Modulation) which means the frequency of pulses of infra-red light light is modulated. PFM is better than AM because the LED is either on or off so the non-linearities don't distort the signal.

Another simpler method is to use PWM which is easier because the signal can be demodulated using a simple low pass filter but it only works for mono unless you shift one fo the audio channels to a higher frequency band but that's more complicated than using FM.

 

[audioguru2]

Hero, please ask Mixos to remove that crappy project from this site. The guy who posted it simply copied it from a site in India without even trying it.

I looked for Infrared Wireless Headphones in Google and found: http://www.scribd.com/doc/31211623/Infrared-Wireless-Headphones
a final year project from students at a university in India using exactly the same transmitter as in this crappy circuit.
They fed a pure sine-wave into the input of the transmitter and showed the output into the IR LEDs. It is extremely distorted.
They said, "The quality of the music received was exceptionally good" even though they had a transistor connected backwards and the photo-diode in the receiver was also connected backwards. Here is their oscilloscope waveform:

View attachment 41460

 

[Hero999]

Here's an idea for an IR transmitter which uses PWM at 90kHz.

The two LM393s should be replaced with a single LM339 to save cost. I just used the LM393 because there's a model for it in LTSpice.

I've not tested it. If you want a receiver I'll have a go at designing one.

View attachment 41461