[GreenKeys] printing to KSR 28 was

[John Nagle]

> Date: Fri, 14 Nov 2014 18:57:23 -0800
> From:epvgk at limpoc.com
> To: Jim Haynes<jhhaynes at earthlink.net>
> Cc: John Foust<jfoust at threedee.com>, Green Keys
> 	<greenkeys at mailman.qth.net>
> Subject: Re: [GreenKeys] printing to KSR 28 was Re: OT: Radio Shack
> 	near bankruptcy
> Message-ID:<20141115025723.GA26931 at limpoc.com>
> Content-Type: text/plain; charset=us-ascii
>
> On Fri, Nov 14, 2014 at 08:26:07PM -0600, Jim Haynes wrote:
>> >I'm not familiar with the Black Box things being offered, but there
>> >is this caution.  Some 20/60 ma devices like Models 32, 33 and 35
>> >operate at low voltage, so the converter circuit is pretty simple.
>> >Models 15, 28, etc. normally operate with much higher voltage, so
>> >the converter has to be able to withstand the voltage.
>> >
>> >We used to use bipolar transistors like MJE-340 but then the "sold
>> >state relay" chips became available, offering both high voltage and
>> >optical isolation.  I've attached someone's document on the subject;
>> >ask me by email if the attachment doesn't come through the system.
>> >
>> >Jim W6JVE
> Haha, I'm that someone.
> it's on my site athttp://heepy.net

I have a simple board 60mA loop design using an optoisolator here:

    http://www.aetherltd.com/connecting.html

There are lots of optoisolators that work.  The Omron
G3VM-401B, around $2 each, is known to work. I recommend
using some R/C snubbing as shown if you're using a Model 14
or 15; the inductive kick from the selector magnet can
be a few hundred volts, and has to go somewhere.  The Omron optoisolator 
can withstand 320V continuous.

    Unless you want a polar relay for historical reasons,
don't bother.

    This still requires a 120VDC supply and a 2K ballast
resistor. There are some more modern designs that generate
the 120VDC from 5VDC with a boost converter.

    If you just need a 20mA current loop, buy a commercial
converter.

			John Nagle