-----Original Message-----
From: Dwornik, Sebastian <SDwornik@CRSROBOTICS.COM>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Tuesday, November 30, 1999 5:02 PM
Subject: PIC i/o interface controller


>I am designing a pic based (16F84) digital i/o controller; 8 outputs, 2
>inputs; all optically isolated and being able to switch 24V @ 250 - 1000
>mA. My initial thought was to go for some opto-isolators; but research
>has lead me to discover solid state relays. Can anyone recommend which
>choice would be better and possibly a specific part number?


Opto-isolators will do fine if you add a power transistor in Darlington
manner.

                C     power transistor
                ----------+------------+
              |/          |          OUTPUT
            -B|           |
              |\          |C
                |E      |/
   Optocoupler  +------B|
                |       |\
                \         |E
                /         |
                \ 1kOhm   |
                |         |           GND
                ----------+------------+

Current ratio of a typical power transistor is about 40. So maximum output
current of optocoupler should be 1 A / 40 = 25 mA. Practically any
optocoupler is enough.

Regards
_

Nikolai Golovchenko, Electrical Engineering Student
National Mining University of Ukraine www.nmuu.dp.ua
Dnepropetrovsk, Ukraine
E-mail: golovchenko@mail.ru