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.

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