At 05:17 PM 4/8/98 -0300, you wrote:
>    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
>       |\
>       | \|
>         -|
>          |
>          |
>        -----
>         ---
>          -
I've been doing this for about 3 years without problems. I use a 33k to
100k pull DOWN resistor, instead of a pull up resistor. I think the
threshold voltage, with 5Vcc is about 1.8 volts.