IR Receiver/Demodulator [Design]

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From: Hammy on 18 Mar 2010 15:52

On Thu, 18 Mar 2010 19:29:49 GMT, Jan Panteltje
<pNaonStpealmtje(a)yahoo.com> wrote:

>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.

http://canada.newark.com/microchip/mrf49xa-i-st/rf-transceiver-ic/dp/08R1857?Ntt=MRF49XA-I/ST

From: markp on 18 Mar 2010 16:53

"Hammy" <spam(a)spam.com> wrote in message
news:j315q5t6flb0o0nemdrv2ngh5ig9jjesc8(a)4ax.com...
> On Thu, 18 Mar 2010 19:29:49 GMT, Jan Panteltje
> <pNaonStpealmtje(a)yahoo.com> wrote:
>
>>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.
>
> http://canada.newark.com/microchip/mrf49xa-i-st/rf-transceiver-ic/dp/08R1857?Ntt=MRF49XA-I/ST

Or if you wanted to avoid the impedance matching stuff and layout issues use
the RFM12B module. Similar price, good performance. That module is available
in 868 and 915MHz variants as well with a minor firmware change. Actually
this Microchip part seems identical to the Si4421 chip used on the RFM12B
with just some signal name changes!

Mark.

From: whit3rd on 18 Mar 2010 16:51

On Mar 18, 10:11 am, "eeboy"
<jason(a)n_o_s_p_a_m.n_o_s_p_a_m.jasonorsborn.com> wrote:

[ on sending and receiving pulses with variable time duration on
modulated IR]

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

One idea, common to all TV remotes, hasn't been incorporated yet; you
can add a start pulse as lead-in to the sequence, and look at the
modulated received signal for a few milliseconds, and set
an AGC level. Then, blink the light off, then back on for the
timed duration you want to determine.

Any demodulation high-Q filter will need the long start pulse to lock
to the
phase of the sender, and the phase shifts (and associated timing
errors)
are negligible AFTER you make each time-critical determination
on the light-turn-off signal (rather than determining both an ON and
OFF time).

From: Joerg on 18 Mar 2010 17:11

eeboy wrote:
>> 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?
>

Better to use an unbuffered inverter.

> 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. ...

I'd use a TIA with an inductor across the FB resistor so there's no DC gain.

> ... The voltage that develops would be run through non-inverting
> amplifier (perhaps multiple stages) and then through a crystal filter.

Two things: Best not to have too much gain before the filter. If a
reflection from somewhere saturates the stages before the filter then
the filter ain't doing you any good during that time. Also, someone (I
think John) pointed out that you data rate may be a bit high for a watch
crystal. The BW is usually not much more than 10Hz.

> 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? ...

You've got to cut DC right at the beginning. In a TIA via an inductor
across the feedback resistor, if you want to use the shunt method then
via a coupling cap.

The cutoff should be as high as you feel comfortable with. Yes, ambient
light shows up as DC but, for example, a hornet buzzing by, birds,
reflections by cars and so on, that won't be DC.

> ... 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?
>

If you need 20msec pulses to make it through you should have a few (very
few) hundred Hertz of BW. Looks like LC to me.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

From: Martin Brown on 18 Mar 2010 17:18

Jan Panteltje wrote:
> 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.

I agree. That or one of the generic cheap model car control modules
which claim ranges roughly in the right ball park. Without error
correction on the data sent this thing is headed for a fairly major
train wreck.

Meade stock was once pumped up in the .com (con) boom on the basis of
high bandwidth building to building coms using 8" SCTs as the optics.

http://www.space.com/businesstechnology/business/meade-telescope_000317.html

Regards,
Martin Brown