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PICList Thread
'[OT] Phototransistors'
1998\12\20@032854 by Lynx {Glenn Jones}

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hello, sorry this is off topic, but i plan to use this with a pic

what is the usual value for the resistor in the following circuit, or how
can it be calculated?

   /\  Vcc
    |
    |
    |
   <
    >  Resistor
   <
    |
    X-------> Signal out
    |
   /
 |/
 |       Photo transistor (NPN, emmittter pointing towards ground)
 |\
   \
    |
    V     Ground

Also, ive tried a 3.3K resistor, but it didnt seem very sensitive, while a
56K resistor was so sensitive that even in complete darkness the signal
would not get very close to Vcc. Im planning on using this for a wireless
RS232 style tranciever, should i capacitatvily couple the output to the
input of the comparitor (which i am using to detect whether the signal is
high or low)?? Thanks for any help you can provide.

------------------------------------------------------------------------------
A member of the PI-100 Club:
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1998\12\21@101738 by lilel

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> hello, sorry this is off topic, but i plan to use this with a pic
>
> what is the usual value for the resistor in the following circuit,
> or how can it be calculated?

I've used values around 1 meg with good results.  I was looking for
an infrared LED in standard office lighting, and shining the LED
through an object to analyze its composition.  It would punch
through a piece of bread or a bagel pretty well.  I was making a
gizmo that could tell a piece of bread from a bagel.

I don't calculate it.  Mostly my calculations get me in the
ballpark, and the lab is where I actually pick component values that
are critical.   Just build it and test it.  As Steve Ciarcia Sez:

"Solder is my favorite programming language."


-- Lawrence Lile

"Nyquist was an optimist."

=> Median Filter Source Code
=> AutoCad blocks for electrical drafting

at:  http://home1.gte.net/llile/index.htm

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