IR Receiver/Demodulator [Design]

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From: Joerg on 19 Mar 2010 12:49

Jim Thompson wrote:
> On Fri, 19 Mar 2010 08:50:01 -0700, Joerg <invalid(a)invalid.invalid>
> wrote:
>
>> markp wrote:
> [snip]
>>> Other possibilites for the receiver might be to use a tube, cone or even a
>>> parabolic reflector made from IR reflective material. If you use the
>>> reflector you'd place the receiver pointing inwards at the focal point. This
>>> effectively produces gain as it captures power from a larger area. An IR
>>> transmissive filter on the receiver (worthless to ad that to the
>>> transmitter!). The LED already has a tight beam angle so no need to modify
>>> that.
>>>
>> The tube is an absolute minimum. I don't think the scheme will work at
>> all during the day if the photodiode gets direct sunlight.
>
> You still may need a DC loop to get the receiver from saturating on
> ambient. Here in AZ, that seems an absolute necessity.
>

Well, that's why I suggested an inductor in the TIA FB path earlier.
That is your DC loop. You really need to get rid of DC immediately
following the photodiode.

--
Regards, Joerg

http://www.analogconsultants.com/

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

From: eeboy on 19 Mar 2010 13:30

>So put a capacitor across the battery[1] and it will supply the current
spike
>to LED. Duty cycle can be very low for LED drive, as it's the peak signal
power
>that provides contrast (signal) at the receiver, not average power that
gets
>swamped by ambient light.
>
>If there's room you could stack a couple coin batteries for 6V to get more
LED
>peak current from cap. Or, perhaps a voltage double charging the
capacitor?
>Lots of options.
>
>[1] you might want to disconnect capacitor in between message sequences to

>improve battery life.

The battery delivers about 240mA based on the voltage which develops across
the 1 ohm resistor. I've thought of placing the capacitor across the
battery but that leakage current would shorten the life of the battery as
you point out. True it could be disconnected at the cost of another
component. Might be worth looking into...

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

From: Grant on 19 Mar 2010 16:57

On Fri, 19 Mar 2010 12:30:08 -0500, "eeboy" <jason(a)n_o_s_p_a_m.n_o_s_p_a_m.jasonorsborn.com> wrote:

>>So put a capacitor across the battery[1] and it will supply the current
>spike
>>to LED. Duty cycle can be very low for LED drive, as it's the peak signal
>power
>>that provides contrast (signal) at the receiver, not average power that
>gets
>>swamped by ambient light.
>>
>>If there's room you could stack a couple coin batteries for 6V to get more
>LED
>>peak current from cap. Or, perhaps a voltage double charging the
>capacitor?
>>Lots of options.
>>
>>[1] you might want to disconnect capacitor in between message sequences to
>
>>improve battery life.
>
>The battery delivers about 240mA based on the voltage which develops across
>the 1 ohm resistor.

3V gate drive doesn't turn the MOSFET on very hard?

> I've thought of placing the capacitor across the
>battery but that leakage current would shorten the life of the battery as
>you point out. True it could be disconnected at the cost of another
>component. Might be worth looking into...

More I think about it, the more I like using an inductor for energy store
and discharge into the LED or a chain of LEDs for more bang.

Extra components are the RS latch and comparator for peak current -- 32k
clock sets latch and MOSFET on, and peak current resets latch, ready for
next edge. But there's already half a quad gate package there for
oscillator and gated drive now, so maybe it's doable?

Grant.

From: Randomly on 19 Mar 2010 22:08

It's already been mentioned but using 32khz crystals for filters works very
well if you don't care about response time. Typical Q's are around 50,000
so bandwidth is less than 1 Hz. Too narrow for the op. Inverter oscillators
are cheap, but suck current so if current consumption is a concern avoid
them, they are especially poor at 32khz. If you must use something like a
4069UB you can cut the current consumption a lot with an appropriate
resistor in series with Vcc to limit the totem pole currents, with the
caveat that you can't use any spare inverters for other things to source
current. This lowers the stage gain so test this over voltage and
temperature to make sure it's robust. 32 Khz crystal need very little drive
energy and are quite breakable if over driven.

Another important note on using 32Khz crystals for TX and RX is that they
both need to be in the same resonance mode, either serial or parallel.
Because the bandwidth is so narrow the frequency difference between
operating one in serial resonance and the other in parallel resonance means
you take a big hit in sensitivity from being off frequency from each
other.

Back more toward the op problem. My own experience agrees with previous
suggestions - optical filters help a great deal for sunlight situations. As
mentioned optimize your optical path with filtering and directionality as
much as possible.

Then toss it all for an RF link when you get frustrated enough.

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From: JosephKK on 20 Mar 2010 02:30

On Fri, 19 Mar 2010 11:19:42 -0000, "markp" <map.nospam(a)f2s.com> wrote:

>
>"JosephKK" <quiettechblue(a)yahoo.com> wrote in message
>news:nhj6q5pmo80u528u70f27u4b8sv50lmgkg(a)4ax.com...
>On Thu, 18 Mar 2010 18:17:19 -0000, "markp" <map.nospam(a)f2s.com> wrote:
>
>>
>>"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.
>>
>First OP would need a battery that can deliver the current pulses, cr2032
>cannot.
>
>True, but a capacitor can. The CR2032 would only be topping up the capacitor
>at relatively low current (depending on mark/space ratio). I've no idea
>whether this would work though in practice.
>
>Mark.
>

From the datasheets:

http://data.energizer.com/PDFs/cr2032.pdf
http://www.renata.com/pdf/3vlithium/CR2032_v06.pdf
http://www.houseofbatteries.com/pdf/CR2032
http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+BA+3+AAA4003+CR2032+7+WW

Is able to provide about 200 uA continuous. OP's pulses were about 80 ms at 200 mA.
Duty cycle should be less than 0.1% max. Or about 80 seconds between transmissions.
A 5% droop at 3 V is 0.15 V required C follows naturally. About 1.33 Farad.

OP clearly, is not getting the output level expected, by about 1000 x.

I suggest 3 AAA in series to get the voltage and the current capability desired.