Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!pasteur!ucbvax!decwrl!labrea!glacier!jbn
From: jbn@glacier.STANFORD.EDU (John B. Nagle)
Newsgroups: sci.electronics
Subject: Re: Computer Controlled Switch?
Message-ID: <17571@glacier.STANFORD.EDU>
Date: 20 Jul 88 03:08:41 GMT
References: <362@ucrmath.UUCP>
Reply-To: jbn@glacier.UUCP (John B. Nagle)
Organization: Stanford University
Lines: 38


      I'd get the modem fixed.  "Brownout conditions" sounds like
utter bullshit to me.  

      One useful thing to know is that pin 20 of an RS-232 port, Data
Terminal Ready, can be turned on and off by the computer.  If you have
a subroutine library for the PC's serial ports, there's probably a way
specified to turn it on and off.  By convention, "ON" is -12v, and 
"OFF" is +12.  DTR just tells the modem that the computer is up and talking,
so it's a reasonable way anyway to turn the modem on and off.  I assume
that it's your own program running the modem, since you want to control
the modem's power when you're running unattended, so presumably you aren't
doing anything else with DTR.

      So, if you built a suitable cable, one that passed all the pins
from the serial port to the modem but also brought out wires from 
pins 20 and 7 (7 is ground), you'd have a signal you could turn on and
off.   The next step is to get that signal to do this.

      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.

      Better to switch the low voltage from the wall transformer than
the 120v power line.  If you do switch the power line, take appropriate
precautions, including fusing, and make sure you have a relay with enough
muscle for the job.  Something like Magnecraft part W101MIP-2 (Newark
stock number 23F5578, price $4.96 each) should do it.  That relay can
switch up to 200v at 0.5A, and is rated for 1500v breakdown voltage
between the contacts and coil, so it should provide sufficient isolation.
You can probably get something at Radio Shack, but I wouldn't trust it.

      This is an awful hack, you know.

					John Nagle