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From: phd@speech1.cs.cmu.edu (Paul Dietz)
Newsgroups: sci.electronics
Subject: Re: Computer Controlled Switch?
Keywords: There are easier ways...
Message-ID: <2324@pt.cs.cmu.edu>
Date: 20 Jul 88 15:03:47 GMT
References: <362@ucrmath.UUCP> <17571@glacier.STANFORD.EDU>
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In article <17571@glacier.STANFORD.EDU> jbn@glacier.UUCP (John B. Nagle) writes:
>By convention, "ON" is -12v, and "OFF" is +12.
>
>      You can probably take 5 or 10 mA of power off the DTR line.  
>So look for a 12v relay that requires very little current to drive
>it, and has enough current capacity at the contacts to switch the power
>to your modem.  Put a diode in series with the relay, and connect the
>result to the wires from pin 20 and 7.  This should switch when you 
>change the state of DTR from software.  If you reverse the diode, you
>will reverse the state for which the relay is on.

Several comments:
1) I didn't realize that there were any 12V relays that worked reliably
off of 5mA.

2) While +/-12 V is what's supposed to be on the line, it's frequently
not what you get, besides which, not all machines let you muck with
this. (i.e. it's hard wired on some, and plain non-existant on others.)

3) In addition to the series diode, you also want a free wheeling diode
in parallel with the relay coil, pointing the other direction. (Though
one could probably debate wether this was absolutely necessary given the
other diode...)

4) There's an easier, much cheaper solid state solution. Go to your
local Radio Shack and get an opto-isolator with triac output, and
a standard 600V triac (overkill, but dirt cheap...). The opto-isolator
can easily be driven by these sorts of currents (it just looks like
an LED from that end) and then you can easily use the triac output
to control the big triac. Then you can control reasonably big
120 VAC loads so long as they're not too inductive (no motors, etc.).
Total cost should be under $10, and that includes the pretty box.
If I remember correctly, the little card that the opto-coupler comes
on gives the schematic (these two parts, and two resistors) which
should make life trivially easy. Another advantage is that it's
easy to try out before you commit to building the thing. Just take an
LED and a resistor (say 2k for sake of argument) and connect it to
these pins and see if you can get the software to work...

Hope this helps...

Paul H. Dietz                                        ____          ____
Dept. of Electrical and Computer Engineering        / oo \        <_<\\\
Carnegie Mellon University                        /|  \/  |\        \\ \\
--------------------------------------------     | | (  ) | |       | ||\\
"If God had meant for penguins to fly,             -->--<--        / / |\\\  /
he would have given them wings."            _________^__^_________/ / / \\\\-