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'snappy transistors'
1999\04\12@183149 by Alice Campbell

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Hi all,
ive just spent the weekend trying to build an infrared comm link
between the pic and the serial port.  The reciever end is a
radioshack photodiode that switches the base of a 2n2222
transistor which runs through a diode to the rs232 Rx line.  The
problem is that the computer cant decode the characters.  it gets the
right number of gibberishes, in the right order, though.  i think
that the resistor is too sloppy to give clean voltage transitiions.
i looked in catalogs and cant find a shottky device with less than 14
pins, and i only need to clean up one signal, and hopefully cram
everything into a d-9 case.  Does anyone have any sound advice on how
to produce cleaner levels for the port using just a few parts?  or is
sending ir ascii a hopeless approach?
thanks,
alice

1999\04\12@184428 by wagnerl

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Dear Alice,

1) What is the comm speed are you using?

2) Which polarity of the RS232 voltage the 2n2222 is switching for bit
'1'? up or down?, it must be down, from zero to -12Volts.

3) I understand you are not using any modulation signal, just pure bit 1
= IR tx led solid irradiation, right?

4) What is the distance from TX ro RX led?

I recommend start this tests in a closed very short distance TX-RX (less
than 2 inches), inside a tube or pipe, to avoid external interferences.
Note that IR can travel through several materials, even black or solid
colors for IR is just as clear as pure glass... :)   Once you have this
"close test" working fine, go for taking them apart and them you will
deal with bigger distances... I bet you would end up using 30-50kHz
modulation pulses for distances bigger than 1 meter.  Remember that
Radio-Shack or Digikey has ready to use IR modules that nicelly remove
the modulation and delivery pure digital ones and zeros.  You just need
to modulate the TX led and data with 30-50kHz.
Wagner

Alice Campbell wrote:
{Quote hidden}

1999\04\12@212418 by Bob Blick

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Hi Alice,

Try the MN1381. You can get it at Digi-key and elsewhere. it's meant as a
voltage detector reset device, but it works fine as a general-purpose
schmitt trigger.

They come in many voltages, but the important thing to pay attention to is
the maximum operating voltage, which I recall being 7 volts.

The CMOS output version is what you want.

It comes in a TO92 transistor package.

Cheers,
Bob

http://www.bobblick.com/

1999\04\12@230337 by Eric Oliver

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Alice,

There is a Basic Stamp application note ( App Note #11 Infrared
Communication ) that implements the very thing Wagner recommended.  It uses
a 555 timer and a few pots to generate the 40 khz signal. The testers claim
the circuit is reliable at speeds of 1200 baud over distances over 10 feet
indoors. Using a higher performance LED could increase the range. Parallax
makes these app notes available to the general public on their website (
http://www.parallaxinc.com ). In fact, in general, the complete TX/RX circuit is
applicable, just use PIC pins instead of BS-I pins <g> !

HTH,
Eric

{Original Message removed}

1999\04\13@020155 by w. v. ooijen / f. hanneman

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> or is sending ir ascii a hopeless approach?
> thanks,
> alice

Sending IR ascii is not hopeless, but your simple circuit will be
extremely sensitive to background IR. Try is in full dark and it might
work. IR remotes use a modulated approach (information often
on-aff modulated on a 30 - 60 kHz carrier). Simple canned
receivers, e.g. siemens SFH506 and sharp exist, but those are
not cheap but much cheaper than the equivalent build from
separate parts. Look on the web for RC5.

Wouter.

1999\04\13@062238 by Russell McMahon

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You can implement a Schmitt trigger with 2 transistors.
Email me privately if you wish to pursue this.

Also. in similar pulse type applications you can often get magical
(*) improvements in pulse shapes with a simple 2 pole active filter
using an emitter follower, 2 R's and 2 C's.

Presumably, "The art of electronics" covers both of the above.


regards


       Russell McMahon

From: Alice Campbell <spam_OUT1502amcTakeThisOuTspamLO.SCSENG.COM>
ive just spent the weekend trying to build an infrared comm link
between the pic and the serial port.  The reciever end is a
radioshack photodiode that switches the base of a 2n2222
transistor which runs through a diode to the rs232 Rx line.  The
problem is that the computer cant decode the characters.  it gets the
right number of gibberishes, in the right order, though.  i think
that the resistor is too sloppy to give clean voltage transitiions.
i looked in catalogs and cant find a shottky device with less than 14
pins, and i only need to clean up one signal, and hopefully cram
everything into a d-9 case.  Does anyone have any sound advice on how
to produce cleaner levels for the port using just a few parts?  or is
sending ir ascii a hopeless approach?
thanks,
alice

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