[GreenKeys] Affordable current loop adapter

Sun May 18 12:35:45 EDT 2025

The Model 15 unit can typically be configured to work with a Model 28. The board can drive the selector magnet directly or the coils of a relay based selector magnet driver as long as you separate the keyboard and printer loops on the M28. I believe David Stickleman’s unit actually had a solid-state selector magnet driver and we got it working with minor wiring changes on the 28 and not stuffing the snubber/voltage capture parts on the interface board.

The Model 33 interface board won’t do you any good. It requires the host to run at the TTY baud rate, requires a solid state SMD, won’t convert between ASCII and Baudot, and expects the Model 33 style Molex connector that includes a 48v power supply on it.

Mike

> On May 18, 2025, at 11:21 AM, Jeffrey Golas <jeffg at junknet.net> wrote:
> 
> Which one?
> 
> Jeff
> 
>> On May 18, 2025, at 11:09 AM, John, W9DDD <w9ddd at tapr.org> wrote:
>> 
>> It's been tested with a 28.
>> 
>> 
>>>> On 5/18/2025 8:21 AM, Jeffrey Golas wrote:
>>> I believe he said the model 15 one depends on the actual 25 selector coils so it wont work with others except maybe a 14.
>>> 
>>> The 33 one may work but Its prob hard wired for 110 baud?
>>> 
>>> A 20ma current loop transmitter may work if you can find a way to feed it data, say from fldigi.
>>> 
>>> The fastest cheapest easiest test would be the arduino one on Hackaday using a relay. It works!
>>> 
>>> I think this one: https://hackaday.io/project/2836-arduino-teletype
>>> 
>>> Jeff
>>> 
>>> 
>>> 
>>>>> On May 18, 2025, at 8:19 AM, John, W9DDD <w9ddd at tapr.org> wrote:
>>>> 
>>>> 
>>>> 
>>>> Actually the M15 adapter would be a better fit.  Same designer, same source.
>>>> 
>>>> 
>>>> https://deramp.com/model15_interface.html
>>>> 
>>>> 
>>>> Tested on one of my 28s by Mike, don't know whey I didn't think of it when the question was originally posted.
>>>> 
>>>> 
>>>> On 5/18/2025 2:20 AM, Erik Grant wrote:
>>>>> While this one is for the M33, it might be worth while to contact this guy @ https://deramp.com/tty_adapter.html <https://deramp.com/tty_adapter.html>
>>>>> Teletype RS-232 Adapter - DeRamp.com <https://deramp.com/tty_adapter.html>
>>>>> The Teletype RS-232 adapter comes fully assembled and tested as shown in the pictures on this page. A telephone cord and modular DB-25 connector are not included.Version "B" of the board is now shipping.
>>>>> deramp.com
>>>>> 
>>>>> 
>>>>> ~KF7WAI
>>>>> 
>>>>> ------------------------------------------------------------------------
>>>>> *From:* greenkeys-bounces at mailman.qth.net <greenkeys-bounces at mailman.qth.net> on behalf of Tim McNerney <mc at media.mit.edu>
>>>>> *Sent:* Thursday, May 8, 2025 9:52 AM
>>>>> *To:* greenkeys at mailman.qth.net <greenkeys at mailman.qth.net>
>>>>> *Subject:* [GreenKeys] Affordable current loop adapter
>>>>> I need to test my 28RO.
>>>>> Most of the RS-232 to 20 mA current-loop adapters are "wicked expensive."
>>>>> Plus nowadays, RS-232 is not exactly easy to find as an interface.
>>>>> So I'll be up to $200 before I finally connect to a computer via USB.
>>>>> And of course USB to current-loop... good luck finding that adapter...
>>>>> 
>>>>> Can anyone recommend a circuit for a simple 20 mA current loop transmitter?
>>>>> I have EE and soldering chops.  Happy to build my own prototype.
>>>>> 
>>>>> BTW, I've noticed lots of affordable 4-20 mA current loop transmitters
>>>>> for remote industrial sensors.
>>>>> Can these be adapted to serve as a 20 mA digital transmitter?
>>>>> Intuition makes me think yes, but I want to double-check.
>>>>> Maybe 4 mA does not equal "0".
>>>>> 
>>>>> Thanks.
>>>>> --Tim McNerney
>>>>> 
>>>>> 
>>>>> 
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>>>> 
>>>> --
>>>> ---
>>>> ----
>>>> The above comments or recommendations are SWAG. Use at your own risk.
>>>> 
>>>> John, W9DDD
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>> 
>> --
>> ---
>> ----
>> The above comments or recommendations are SWAG. Use at your own risk.
>> 
>> John, W9DDD
>> 
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>>>> Jordan Spencer Cunningham's GreenKeys Search Tool: https://teletype.net/gksearch
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> 
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