Infra-red headphones in car....Odd problem

[Gaz]

Hi all. Here's a problem that's stumped me. Any thoughts are most
welcome.

I recently installed a DVD player in my car (people mover with 3 rows
of seats) and got some infra-red headphones for the kids in the back.

The transmitter for the headphones is mounted in the middle of the
roof with the screen, and contains 8 transmitter LED's.

Problem is that when sitting in the rear-most seats the headphones
were unreliable and faded in and out according to your head position.
I assumed they were too far from the transmitter.

So I bought some surface-mounted infra-red LED's (six) and tapped into
the board that housed the original transmitter LED's. I found they
were driven by a transistor and had the requisite current limiting
resistor.

I just tapped into the output of the transistor, added a 47Ohm
resistor and ran six LED's in parallel. I mounted them in the car just
in front of the rear-most passengers, 3 on each side.

Now I find there are even more "dead spots" in the car where the
headphones don't pick up a signal. And if you put the headphones into
one of these dead spots and cover up either the original transmitter
or one of the ones I added the headphones come to life again.

It seems to me like an interference phenomenon but surely the
wavelength of infra-red light is too short for such a large-scale
phenomenon. By definition the LED's are in phase because if they were
connected reverse-polarity they would not emit. I have checked with a
digital camera and all the LED's are emitting.

Any ideas?

 

[Daniel Watman]

"Gaz" <gaz@gaz.com> wrote in message
news:9tsba0l3ui72qf6aflsb23mcgjiji74bq9@4ax.com...

Hi all. Here's a problem that's stumped me. Any thoughts are most
welcome.

I recently installed a DVD player in my car (people mover with 3 rows
of seats) and got some infra-red headphones for the kids in the back.

The transmitter for the headphones is mounted in the middle of the
roof with the screen, and contains 8 transmitter LED's.

Problem is that when sitting in the rear-most seats the headphones
were unreliable and faded in and out according to your head position.
I assumed they were too far from the transmitter.

So I bought some surface-mounted infra-red LED's (six) and tapped into
the board that housed the original transmitter LED's. I found they
were driven by a transistor and had the requisite current limiting
resistor.

I just tapped into the output of the transistor, added a 47Ohm
resistor and ran six LED's in parallel. I mounted them in the car just
in front of the rear-most passengers, 3 on each side.

Now I find there are even more "dead spots" in the car where the
headphones don't pick up a signal. And if you put the headphones into
one of these dead spots and cover up either the original transmitter
or one of the ones I added the headphones come to life again.

It seems to me like an interference phenomenon but surely the
wavelength of infra-red light is too short for such a large-scale
phenomenon. By definition the LED's are in phase because if they were
connected reverse-polarity they would not emit. I have checked with a
digital camera and all the LED's are emitting.

Any ideas?

It is probably an NPN transistor pulling the cathode of the LEDs to ground,
so the "output" you connected to would be the collector? If you connected
that to the base of another transistor, when the "output" is high the
original LEDs will be off but your new LEDs will be on and vice versa, so as
you thought, the 2 sets are out of phase.

Thats what I think anyway...

-Daniel

 

[KLR]

On Sat, 15 May 2004 20:52:05 +1000, Gaz <gaz@gaz.com> wrote:

is it the same in day or night ? I believe sunlight isnt really good
for IR units


You might consider some 3.5mm jacks and sockets instead - no batteries
- no problems - only need the cheap walkman phones.

Works well on commercial aircraft.

Hi all. Here's a problem that's stumped me. Any thoughts are most
welcome.

I recently installed a DVD player in my car (people mover with 3 rows
of seats) and got some infra-red headphones for the kids in the back.

The transmitter for the headphones is mounted in the middle of the
roof with the screen, and contains 8 transmitter LED's.

Problem is that when sitting in the rear-most seats the headphones
were unreliable and faded in and out according to your head position.
I assumed they were too far from the transmitter.

So I bought some surface-mounted infra-red LED's (six) and tapped into
the board that housed the original transmitter LED's. I found they
were driven by a transistor and had the requisite current limiting
resistor.

I just tapped into the output of the transistor, added a 47Ohm
resistor and ran six LED's in parallel. I mounted them in the car just
in front of the rear-most passengers, 3 on each side.

Now I find there are even more "dead spots" in the car where the
headphones don't pick up a signal. And if you put the headphones into
one of these dead spots and cover up either the original transmitter
or one of the ones I added the headphones come to life again.

It seems to me like an interference phenomenon but surely the
wavelength of infra-red light is too short for such a large-scale
phenomenon. By definition the LED's are in phase because if they were
connected reverse-polarity they would not emit. I have checked with a
digital camera and all the LED's are emitting.

Any ideas?

 

[Bushy]

It seems to me like an interference phenomenon but surely the
wavelength of infra-red light

Somewhere around 800 to 1000 nanometers.

is too short for such a large-scale
phenomenon.

Wave motion. Try throwing two rocks in a pond and watch the ripples. At some
angles they will add together and at other angles they will subtract from
each other. Where they add together you will get a stronger signal, but at
all other angles the signal will get weaker and weaker until it drops under
the signal to noise threshold. At the extreme angle they will totally cancel
each other.

A similar wave effect will occur with visible light, infra red light,
x-rays, ultra violet light, radio waves (remember all those antenna
radiation patterns?) and any other type of wave. This effect is used to
scientific advantage in a machine called a spectrometer. Even stereo sound
can do the same and the sound can cancel out.

Shield your roof mounted units so from any seat the headphones can only see
one or the other LED array. This could be a simple piece of cardboard or
plastic mounted between the roof and the unit and bent to shape.

Hope this helps,
Peter

 

[Daniel Watman]

"Bushy" <please@reply.to.group> wrote in message
news:c8558i$ep5$1@bunyip.cc.uq.edu.au...

It seems to me like an interference phenomenon but surely the
wavelength of infra-red light

Somewhere around 800 to 1000 nanometers.

is too short for such a large-scale
phenomenon.

Wave motion. Try throwing two rocks in a pond and watch the ripples. At
some
angles they will add together and at other angles they will subtract from
each other. Where they add together you will get a stronger signal, but at
all other angles the signal will get weaker and weaker until it drops
under
the signal to noise threshold. At the extreme angle they will totally
cancel
each other.

A similar wave effect will occur with visible light, infra red light,
x-rays, ultra violet light, radio waves (remember all those antenna
radiation patterns?) and any other type of wave. This effect is used to
scientific advantage in a machine called a spectrometer. Even stereo sound
can do the same and the sound can cancel out.

Shield your roof mounted units so from any seat the headphones can only
see
one or the other LED array. This could be a simple piece of cardboard or
plastic mounted between the roof and the unit and bent to shape.

Hope this helps,
Peter

LEDs are not coherent so this effect will not happen on a big enough scale
to have the effect he is describing.

-Daniel

 

[Wayne Reid]

"Bushy" <please@reply.to.group> wrote in message
news:c8558i$ep5$1@bunyip.cc.uq.edu.au...

It seems to me like an interference phenomenon but surely the
wavelength of infra-red light

Somewhere around 800 to 1000 nanometers.

is too short for such a large-scale
phenomenon.

Wave motion. Try throwing two rocks in a pond and watch the ripples. At
some
angles they will add together and at other angles they will subtract from
each other. Where they add together you will get a stronger signal, but at
all other angles the signal will get weaker and weaker until it drops
under
the signal to noise threshold. At the extreme angle they will totally
cancel
each other.

Sounds like you were listening to the teacher during year 11 physics bush.
Shows the benefit of a good education.

WR

 

[Phil Allison]

"Daniel Watman" <pywo@optushome.com.au>

LEDs are not coherent so this effect will not happen on a big enough scale
to have the effect he is describing.

** Absolutely correct. The light coming from a LED is not a single frequency like a radio wave from an antenna. The only kind of light that has that characteristic comes from lasers.

Most IR headphone transmitters first modulate the current to the LED array so the light output from the LEDs is pulsing. This might be at any frequency between about 150kHz and 2.5 MHz. The IR receiver merely converts this pulsing light into a signal at the pulse frequency. The audio is transmitted to the receiver by frequency modulating the LED pulse rate up and down by a few percent - so the recovered signal is a frequency modulated. The rest of the circuit in the receiver is similar to an FM radio receiver.

Using multiple LED transmitter arrays to enhance the range of a IR audio system is normal practice - it is only necessary to make sure that all the LEDs are either on or off at the same instant in time. If this is not the case and some LEDS are off while others are on then the essential pulse modulation of the light will be reduced or even lost completely.




............. Phil

 

[Phil Allison]

"Daniel Watman" <pywo@optushome.com.au>



** Apologies for the HTML - I dunno what happened there.




........... Phil