PTs are sensitive to IR, in the 800 to 950 nm range, and
would ignore the yellow.  Try an IR emitter instead.  Experiment
with different values of pull-up resistors to get the speed/
sensitivity you want.

-----Original Message-----
From:   Hardy e/ou Rafael Pinto [SMTP:cppinto@NETRIO.COM.BR]
Sent:   Wednesday, April 08, 1998 4:18 PM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        [OT] Need help with phototransistor

    Hi guys,

    I need help on using a phototransistor (PT, for short!) to excite an
input pin on a PIC16F84. I am using the typical circuit, that is, the PT
with emmiter grounded and collector connected to the i/o pin. I put a
pull-up resistor of 10k from the i/o pin to Vdd. It didn't worked! I am
using a TIL78 activated by a LED (yellow). Can anyone suggest other
cirucuit?

    Rafael Pinto



           Vdd
          |
          >
          > 10k
          >
          |
          +-------------> RA0
          |
       | /
       |/     TIL78
       |\
       | \|
         -|
          |
          |
        -----
         ---
          -