TweetTelegraph – Part 3 (Telegraph Hardware)

Okay, let me level with ya.  The last two posts were written as I was doing the work described and I had every intention of doing the same for the rest of the project but that didn’t happen.  You know how it is – you get caught up in the project and the documentation suffers.  Anyhoo, from here on out I’m going to do my best to reconstruct events as I best remember them.  For this reconstruction it would be instructive to imagine the role of me being played by Kirsten Vangsness.  No, really. I do it all the time and it feels GREAT.

Shortly after I managed to get the radios working my telegraph package arrived.  The machine was beautiful and everything I had hoped for.  Heavy wood, solid brass fixtures.  Mm.  So nice.  I quickly hooked it up to a battery to ensure the sounder and coil still worked as promised and sure enough they did.  Huzzah!  Of course, now I had to ruin it.  A key / sounder pair is set up in series such that when the key is pressed the circuit is completed and the sounder sounds.  That’s all well and good back in the 1800s but I wanted the input and output from the key and sounder to be split for ease of development.  I carefully severed the connections on the board such that the key and sounder were each on their own circuits.

First up, the sounder.  I connected it to an Arduino loaded with a simple program that alternated HIGH/LOW to a pin at a regular interval and… no dice.  The sounder requires more juice than the output pin of an Arduino can muster.  Instead of hooking the pin directly to the sounder I added a transistor into the mix as a switch to allow the full 5V from the Arduino to hit the sounder.  With this setup the sounder worked reliably (and loudly).

The key is a much simpler matter.  It’s just a switch.  Press the key and complete the circuit.  For a sanity check I hooked the key up to another Arduino app which simply toggled the built in LED based on the switch and thankfully it worked.  My surgery to separate the key and sounder worked.

In addition to the telegraph I needed a switch that I could use to toggle from input to output mode.  Full duplex communication is doable here but it felt like more grief than it was worth. A simple modern looking switch might do but I wouldn’t be very satisfied by that – it wouldn’t match at all with the telegraph.  In the end I settled on a simple knife switch from RadioShack (or The Source now.  Sigh.)  It feels more appropriate than a modern switch but it’s still made of plastic and cheap metal so I’m not really satisfied with it.  I’m still on the lookout for an antique knife switch to really complete the look.

Finally, I wanted an enclosure to put the whole thing in.  Originally I was aiming for an old cigar box and while I did find one it’s unfortunately too small. It fits the Arduino and shield well enough but the telegraph base is too big to fit flush on the top of the box.  While rummaging around at a thrift store I stumbled upon an older wooden breadbox with the rolling door.  It’s nowhere near as old as the telegraph but it does the trick. I can put the whole setup inside the box without a problem.

And that’s pretty much it for hardware.  Other than the expensive antique it’s a fairly cheap project. ;)

See also: TweetTelegraph – Part 1 (Shopping)TweetTelegraph – Part 2 (XBee Hardware)TweetTelegraph – Part 4 (Software)

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  • About Me

    Hi! My name is Kimberly Horne and I have absolutely nothing interesting to say. Unfortunately for you I DO have an overpowering need to tinker with technology which is explains the presence of this journal. I mostly talk about games (video and tabletop), technology, tattoos, and my pets.

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