IR Headphones

[Ramji1]

Hi all,
        I hv gotta schematic to build a IR headphone set. It simply amplifies the audio output signal from a TV or Computer and transmits through IR Led's. This IR signal is received and given to the head phones wher the original sound can be reproduced. Im plannin to do this as a proj n have some doubts...

1. Firstly will this circuit work? I know it involves simple amplification of the audio signal but i have never dealt with data transmission using IR rays. So is this design good enough to make it work?

2. There is a potentiometer/variable resistor in the transmitter. What is the purpose of it? Is it related to volume control?

3. What do i need to do to increase the range of this design(assumin it works!). And should i tak any precautions for ensurin the correct reception of the transmitted signal(like avoidin interference, ambient lightin  etc)

View attachment 40303

 

[audioguru2]

1. Firstly will this circuit work? I know it involves simple amplification of the audio signal but i have never dealt with data transmission using IR rays. So is this design good enough to make it work?

Hi Ramji,
It is a very old and crude circuit.
It is mono, not stereo.
It doesn't transmit data, its IR beam is AM modulated so its receiver is sensitive to interference.
I think it will be very distorted because the T1 transistor doesn't have any bias current and therefore it will rectify the signal.
Its receiver has DC current running through the headphones which is bad.

Modern (for 20 or 30 years) wireless IR headphones are stereo and use FM modulation to avoid most interference from mains-powered light bulbs. They have very low distortion.

2. There is a potentiometer/variable resistor in the transmitter. What is the purpose of it? Is it related to volume control?

It adjusts the maximum peak LED current.

3. What do i need to do to increase the range of this design(assumin it works!). And should i tak any precautions for ensurin the correct reception of the transmitted signal(like avoidin interference, ambient lightin

 

[Ramji1]

Hi audioguru,
  Thanks for the info. And i get most of wat u meant. I dint know bout AM or FM in IR transmission. Anyway i think i'l drop this circuit and go for a FM design. So can u help me Find a schematic for FM transmission using IR. And also i hv some clarifications regardin some of the points u mentioned..

1. How did u identify the circuit to be AM? are there any distinctive features for AM n FM?

2. I agree that the transistor T1 has no bias current. That means most of the signal im sending (i.e. negative part of the signal to be precise) will be lost. Am i right in that??

3. What is the difference between stereo and mono? Has it got somthing to do with digital and analog audio signals??

 

[audioguru2]

1. How did u identify the circuit to be AM?
are there any distinctive features for AM n FM?

The amplitude of the IR beam is modulated with the sound signal. Incandescent light bulbs and fluorescent lamps flicker at double the mains frequency. The AM receiver will pick up the interference. M modulates the frequency of a high frequency carrier so that flickering lights won't cause interference. The carrier frequency is high so that two channels can be used for stereo.

2. I agree that the transistor T1 has no bias current. That means most of the signal im sending (i.e. negative part of the signal to be precise) will be lost. Am i right in that??

Yes, it causes severe distortion. The LED also causes severe distortion of the signal. It is a horrible circuit.

3. What is the difference between stereo and mono? Has it got somthing to do with digital and analog audio signals??

You have two ears so you hear stereo. Headphones usually have two separate earphones. Stereo has two separate audio channels. You hear sounds from the left, from the right and from any position in between all at the same time. It is similar to the difference between black and white TV and colour TV.
Mono is both channels mixed together to make one signal. Mono sounds dead when it is compared to stereo.

I looked in Google for an FM stereo IR headphones circuit but didn't find one. Instead I found the awful old circuit you had. I have an awful FM IR receiver circuit on my hard drive but the important transmitter isn't shown.

 

[Ramji1]

Ya even i hvnt got the FM circuit. But i found that a similar AM IR transmitter has figured in the Jan 2007 edition of EFY magazine. I hv sent the schematic along. It transmits a musical signal thro IR and i suspect it uses AM too. Just hv a look n see if this is a workin design. I kno its similar to the one i sent(maybe a different receiver section) but its given that the circuit has been tested by the magazine(there's a seal which says that). So plz post ur views on it...

ir_fm.pdf

 

[audioguru2]

I don't know if the music IC biases the transistor, it probably does.
The red LED will create distortion.
It is an AM system like the other one and will pickup interference from mains lights.

 

[Ramji1]

Yes the music IC does bias the transistor i think. And it'l cause distortion. How about giving a bias DC current to the transistor to prevent it? Will it minimise distortion? How much of a current do u reckon wud be enuf? And the red LED simply acts as an indicator. So it can be removed and replaced by a resistance. And ofcourse the interference is something i hv not yet been able to address. I have still not been able to find a FM schematic.

 

[Ramji1]

And one more thing. The schematics for FM transmitters of radios are easily available. Can i use those schematics to produce a FM audio signal and then try to transmit using infrared? Again its the transmission part that i dont kno much. How do i transmit such an FM signal thro possibly IR LED's? Will this kind of coupling of an FM transmitter with IR transmitter work? And ofcourse the receiving end also needs to be dealt with... Any ideas or tips??

 

[audioguru2]

And one more thing. The schematics for FM transmitters of radios are easily available. Can i use those schematics to produce a FM audio signal and then try to transmit using infrared?

An FM radio has LC tuned circuits for 100MHz and 10.7MHz. An FM modulated IR headphones system needs only about 100kHz and 200kHz so is much different.
Use the VCO from a CD4046 IC for the transmitter and use the Phase-Locked-Loop from a CD4046 IC for the receiver's FM demodulator. 

How do i transmit such an FM signal thro possibly IR LED's?

The signal must change the frequency of a carrier frequency to produce FM. A VCO does it.

Will this kind of coupling of an FM transmitter with IR transmitter work?

No, IR is completely different from radio.

 

[Ramji1]

ok... I'l hv to read about VCOs n PLL's n then construct the circuit myself. But assumin that i produce a FM audio signal. How do i transmit and receive using IR? Do i use the same LED n photo transistor mechanism?(Isnt that AM then?) Or do u suggest a different mode of transmission??

 

[audioguru2]

With FM, The carrier frequency is changed by the modulation.
You have a VCO that oscillates and drives the IR LED with 100kHz or another ultrasonic frequency from a VCO. Then the frequency of the VCO is changed by the audio modulation. Loud sounds in the modulation cause the carrier frequency to change more. The receiver doesn't care if the carrier is received very strong or if it is weak. The output is the same for either signal strength. The FM demodulator in the receiver rejects AM interference from light bulbs and sparks.

 

[Ramji1]

Because of lack of time i havent been able to design a FM circuit for this.. Instead i have stuck to the AM circuit which i have sent earlier in the file "irfm.pdf" The circuit workd perfectly for me once i modified the transmitter. I replaced the amplifying stage into a 2-stage common emitter with the same transistors shown in the diagram. I just biased them with different resistors. The receiver is the same. I was able to hear the melody clearly but the range is very limited. Its about 2feet. I want to improve my range to about 10-12feet. Any suggestions on how to do it??

Also I have set 5mA as the DC bias current for the current through the transistor BD140 and the IR led's. I heard that increasing this current will improve range. If so how much can i increase the LED bias current to??

Can i use lenses? What type of lenses and any suggestions on type of arrangement?

Can i use more than 2 IR Led's?? Will it improve my range??

Any other suggestions are welcome..... plzz help me out....

 

[audioguru2]

It is mono, not stereo.
It is poor quality and narrow bandwidth AM, not high quality FM.
The receiver must be in the direct path of the IR beam.

Look at the datasheet for your IR LEDs to see their max continuous current rating. It is probably about 40mA so bias them at 20mA then it will be at 40mA at peak modulation. Use more IR LEDs for more range. Focus the IR beam at each end for more range. Use it in the dark for more range.