IR Receiver/Demodulator [Design]

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From: Martin Brown on 18 Mar 2010 05:23

eeboy wrote:
>> Are you using the proper emitter one that is wavelength matched to you
>> TSOP?
>
> I am using a LTE-5228A which has a peak at 940nm. The data sheet on the
> TSOP part appears to have a peak at roughly the same spot.
>
>> I agree about the optics but unless your building these in large
>> quantity you likely don't have any optics.
>
> I am currently using a plastic fresnel lens in front of the device.
> However, the fresnel lens is being shared with another optical component
> which has been given 'optical priority'... meaning the other component is
> at the focal point so the TSOP device sits about .25" below. I could

That isn't a good geometry unless you intend that the remote operator
levitates. Your sensor is pointed skywards and the sky is bright. To one
side or the other would be better assuming that the thing is in roughly
the same plane as the controlling transmitter.

> probably have (and should have) pushed that device up closer to the other
> device (within .100"). Perhaps I'll run some tests where I slide the
> fresnel lens down to see if the situation
>
>> Are you hammering the LED with it's max peak current are close to it?
>> Keeping in mind max package power dissipation for your LED in the
>> worst case ambient etc..
>
> I believe I am currently driving it past the datasheet spec. My transmitter
> is powered by a CR2032. I am utilizing a 1 ohm resistor in the path which
> theoretically results in a 1.8A current for up to 80ms. The datasheets
> states the maximum as 2A for 10uS. However, I am certain the small 7mil
> trace width on the transmitter PCB add a good bit of resistance to the path
> as well.
>
>> You can adjust the duty cycle to keep the average current to
>> acceptable limits. Like 10% for the carrier modulated at 40% for
>> continuous transmission at 1A pulses this would be about 40mA average
>> current. You could reduce this further by using discontinuous
>> transmission i.e transmit intermittently in bursts .
>
> I am utilizing a 50% duty cycle. I had no idea I could get away with that.
> Definitely going to adjust the duty cycle downward.

It is only worthwhile if you can increase the current proportionately
without going outside the operating envelope. The optimum duty cycle is
around a 37% pulse width at correspondingly higher current (puts a shade
over 10% more power into the fundamental carrier frequency).

Regards,
Martin Brown

From: eeboy on 18 Mar 2010 13:11

>
>2kHz may be a bit much for this application. Can't you use a watch
>crystal to get the uC stable? Modern ones have internal digital loops to
>give you 8MHz or thereabouts of clock.
>
>Then use a watch crystal on the receive end as filter. Cheap, but may
>not work well in really frosty temps. And the crystals have to be very
>accurate because the bandwidth will be in the order of 10-20Hz. Anyhow,
>just as an idea to play with in case all else fails.
>

Spent a day playing around with the optics which yielded no major
improvements. The only thing I was unable to try was the optical filter (as
I don't have anything suitable on hand). So, while I wait to get my hands
on something I thought I'd try a few of the other suggestions... if nothing
else it would be a learning experience.

First I am making a change to my transmitter so that I can use a watch
crystal as the time base of the modulation. I've basically added a Pierce
Oscillator with its output going to one input of an AND gate. The other
input of the AND gate is tied back to the existing microcontroller acting
as an enable. Upon enabling the signal is fed to the gate of a FET
controlling the LED. Here is a snippet of the schematic...
http://img717.imageshack.us/img717/8467/transmitter.jpg . I've never
actually constructed a Pierce Oscillator. From what I've been reading they
may be a bit tough to get going with a buffered inverter (my case). My
values were derived based on the crystal manufacturers load capacitance of
12.5pF. Comments?

I am working on the receiver end now and have a few questions. The basic
topology would be a reverse biased pin photo-diode loaded with a shunt
resistor. The voltage that develops would be run through non-inverting
amplifier (perhaps multiple stages) and then through a crystal filter.
Again, I have no real analog skills but I can see that loading it with the
resistor will lead to saturation from ambient light. What if I was to
replace the resistor with the LC tank that someone had suggested. The
ambient light is likely to show as DC right? So, I now have a low impedance
path for the DC current but my modulated current shows quite nicely. Also,
the thought is that I could try the receiver with a crystal filter
(extremely high Q) and then without (Q of the LC tank). I am sure there are
some flawed thoughts here... any comments?

---------------------------------------
Posted through http://www.Electronics-Related.com

From: markp on 18 Mar 2010 14:17

"eeboy" <jason(a)n_o_s_p_a_m.n_o_s_p_a_m.jasonorsborn.com> wrote in message
news:woydnU4jer8kwz_WnZ2dnUVZ_oadnZ2d(a)giganews.com...
<snip>
> Spent a day playing around with the optics which yielded no major
> improvements. The only thing I was unable to try was the optical filter
> (as
> I don't have anything suitable on hand). So, while I wait to get my hands
> on something I thought I'd try a few of the other suggestions... if
> nothing
> else it would be a learning experience.
>
> First I am making a change to my transmitter so that I can use a watch
> crystal as the time base of the modulation. I've basically added a Pierce
> Oscillator with its output going to one input of an AND gate. The other
> input of the AND gate is tied back to the existing microcontroller acting
> as an enable. Upon enabling the signal is fed to the gate of a FET
> controlling the LED. Here is a snippet of the schematic...
> http://img717.imageshack.us/img717/8467/transmitter.jpg . I've never
> actually constructed a Pierce Oscillator. From what I've been reading they
> may be a bit tough to get going with a buffered inverter (my case). My
> values were derived based on the crystal manufacturers load capacitance of
> 12.5pF. Comments?
<snip>

As an aside, if you're bandpass filtering 32KHz, could you use a large
capacitor from the junction of the resistor and LED to ground, such that
when the FET turns on you get a high powered shorter spike? It seems you
can't drive more than about 200mA with the LED shown as it's a 50/50
waveform. You're allowed up to 2A though with a 10us pulse. If your
photodiode can respond fast enough to that shorter pulse it might mean you
can turn the gain of the receiver down and reduce background noise effects.
You'd need to do a fourier analysis of the 50/50 waveform at lower power for
the 32KHz content and compare to the 32KHz content of a shorter pulse but at
higher power. Alternatively use a monostable to create a controlled pulse
width and up the current. I'm curious whether that would work or not...

Mark.

From: markp on 18 Mar 2010 14:24

"markp" <map.nospam(a)f2s.com> wrote in message
news:80f8pgF6q7U1(a)mid.individual.net...
>
> "eeboy" <jason(a)n_o_s_p_a_m.n_o_s_p_a_m.jasonorsborn.com> wrote in message
> news:woydnU4jer8kwz_WnZ2dnUVZ_oadnZ2d(a)giganews.com...
> <snip>
>> Spent a day playing around with the optics which yielded no major
>> improvements. The only thing I was unable to try was the optical filter
>> (as
>> I don't have anything suitable on hand). So, while I wait to get my hands
>> on something I thought I'd try a few of the other suggestions... if
>> nothing
>> else it would be a learning experience.
>>
>> First I am making a change to my transmitter so that I can use a watch
>> crystal as the time base of the modulation. I've basically added a Pierce
>> Oscillator with its output going to one input of an AND gate. The other
>> input of the AND gate is tied back to the existing microcontroller acting
>> as an enable. Upon enabling the signal is fed to the gate of a FET
>> controlling the LED. Here is a snippet of the schematic...
>> http://img717.imageshack.us/img717/8467/transmitter.jpg . I've never
>> actually constructed a Pierce Oscillator. From what I've been reading
>> they
>> may be a bit tough to get going with a buffered inverter (my case). My
>> values were derived based on the crystal manufacturers load capacitance
>> of
>> 12.5pF. Comments?
> <snip>
>
> As an aside, if you're bandpass filtering 32KHz, could you use a large
> capacitor from the junction of the resistor and LED to ground, such that
> when the FET turns on you get a high powered shorter spike? It seems you
> can't drive more than about 200mA with the LED shown as it's a 50/50
> waveform. You're allowed up to 2A though with a 10us pulse. If your
> photodiode can respond fast enough to that shorter pulse it might mean you
> can turn the gain of the receiver down and reduce background noise
> effects. You'd need to do a fourier analysis of the 50/50 waveform at
> lower power for the 32KHz content and compare to the 32KHz content of a
> shorter pulse but at higher power. Alternatively use a monostable to
> create a controlled pulse width and up the current. I'm curious whether
> that would work or not...
>
> Mark.

Whoops, had a brain freeze, I meant kHz not KHz. I could see there was
something wrong but couldn't figure out what. Strange thing is word
blindness..

From: Jan Panteltje on 18 Mar 2010 15:29

On a sunny day (Thu, 18 Mar 2010 12:11:21 -0500) it happened "eeboy"
<jason(a)n_o_s_p_a_m.n_o_s_p_a_m.jasonorsborn.com> wrote in
<woydnU4jer8kwz_WnZ2dnUVZ_oadnZ2d(a)giganews.com>:

>
>Spent a day playing around with the optics which yielded no major
>improvements.

I wanted to say this before, it is a weird thread, but at that distance use a 430 MHz transmitter receiver module.