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From: blpike@uokmax.ecn.uoknor.edu (Brian Lee Pike)
Newsgroups: sci.electronics
Subject: Using computer to switch high current circuit
Message-ID: <1990Jan2.005157.12780@uokmax.ecn.uoknor.edu>
Date: 2 Jan 90 00:51:57 GMT
Reply-To: blpike@uokmax.UUCP (Brian Lee Pike)
Distribution: na
Organization: Engineering Computer Network, University of Oklahoma, Norman, OK
Lines: 24

I am trying to build a circuit that allows my Commodore 64 to connect a
110,000 microfarad (that's not a typo, I mean 0.11 farad), 18 volt capacitor
across a load for a brief period of time (somewhere between about 0.1 second
and 0.75 seconds) and then disconnect the capacitor.  

The Commodore 64 has a joystick port which has a +5 volt pin and a ground.
My intention is to have the joystick signal go high, trip the circuit, and
then drop the signal when the current should be disconnected from the load.
There are a couple restrictions:

1) There must be NO WAY for the capacitor's current to reach the computer.
   I have heard of 'opto-isolators', but I do not completely understand how
   to use them, particularly in a high-current situation.

2) The switching should be as 'clean' as possible, i.e., should approximate a
   square wave.  I suspect that this cancels out using a large relay, although
   please correct me if I am wrong.

I know precious little about electronics.  Please be as specific with your
reply as you can stand to be, e.g., schematics or pinouts would be GREATLY
appreciated.

                                                   Thanks,
                                                   Brian "no signature" Pike