[GreenKeys] Arduino Project on Hackaday

[Harold Hallikainen]

Interesting project. It "bit bangs" the serial data out. It looks like the
relay specified (version 2) is obsolete. The datasheet on it (
https://www.seeedstudio.com/Relay-shield-V2-0-p-1376.html ) says "Do not
operate voltage more than 35V DC." Otherwise, there are no specifications
on the relay.

Relay version 3 at https://www.seeedstudio.com/Relay-Shield-v3-0.html .
This one does have specifications of:

250VAC/30VDC
Switching Current:	15A/7A

With DC, they are concerned about arc quenching, so the voltage rating is
much lower.

The project page says to use a "current limited power supply" of 300V, 20
mA. I'm used to using much lower voltage (maybe 100 to 150) with a series
resistor to set the current. Also, 20 mA is good for series holding
magnets only. For pulling magnets or parallel holding magnets, 60 mA is
required.

On the relay voltage, there are a couple concerns. One is the contact
voltage rating (how high a DC voltage can it break). The other is the
insulation resistance and breakdown voltage to the remainder of the
circuitry. It'd be nice to not send a hundred volts or more into the chip
I/O pin! It might also be nice to add an RC snubber circuit across the
relay contacts to reduce arcing (like the snubber across a Teletype
keyboard).

So, it will probably work, but it would be nice to use components within
their specifications.

Harold
http://w6iwi.org





> Not sure if this was discussed here before, but I'm curious to throw this
> to the wolves, er experts. I was sent this project on Hackaday, and having
> a Model 32 around I slapped it together and tried it out. Lo and behold,
> it
> actually worked great. (To be clear, it's not my project)
>
> That said I understand the cheapo chinese relays on the arduino shields
> may
> not hold up to switching 100+ volts DC, but just curious to ask the
> experts
> if there's any reason -not- to use this, even if just for periodic
> demonstration purposes. I'm wondering if I could take this, and build a
> basic 120v power supply out of an ISO xfmr and put this in a little
> project
> case for rudimentary teletype testing.
>
> I tested it out by putting it in the loop between the teletype and my Hal
> ST-6, with the ST-6 on standby, and save for an occasional glitch
> (teletype
> does need some love), it worked/printed fine.
>
> Project link: https://hackaday.io/project/2836-arduino-teletype
>
> Jeff
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