Friday, March 31, 2017

Telegraph Sounder Player

Yesterday I put the finishing touches on my latest project, the telegraph sounder player.

Video link first if you want to see it in action before you read:

The device on the left is the telegraph sounder player, and the device on the right is the telegraph sounder. The telegraph sounder itself is a vintage telegraph module. The player allows for up to four sounders to be plugged in to it, and plays back random snippets from text files loaded up on an SD card. It uses American Morse Code (as opposed to international). I loaded the SD card with excerpts from my favorite novels as well as full novels from Project Gutenberg.

The project was made for the owner of the telegraph sounder, Don. Don used to work as a telegrapher for a railroad company (US land based telegraph operations used American Morse code). He had a first version of the player built for him during the most recent Build Madison event and came back to Sector67 with notes for some features he would like for the next version.

Features I included in this telegraph player include: SD card playback, 4 sounder support, knob for speed of playback, separate knob for inter-sentence interval length, status led and ON/OFF toggle, CNC milled hardwood box with brass corners, battery power (est. 10 hours of playback), battery charging circuit onboard.

Design process in brief:

I had four 5V relays I found in a bin here at Sector, so I chose to support up to 4 sounders and to use a 5V Arduino for the brains. The relays are each switched by a BJT transistor circuit, and when switched they connect the sounder port to the battery pack.

The ports are barrel jack connectors with a 10uF capacitor across the leads and a high power rating. low resistance resistor. The resistor is mostly just in case the leads going to the telegraph are placed so that they are touching each other.

I had been playing around with trying to minimize the currents induced by the sounder turning off (the clack, as opposed to the click). A large ferrite choke close to the telegraph with many windings of the connector cable worked, but was a little unwieldy. The capacitor doesn't entirely solve the problem, but helps make the clack sharper. Too large a value and the clack actually bounces presumably as the capacitor charges on the induced current, and then discharges back into the sounder and re-enables it somewhat. The 'problem' is in any case limited to the Arduino resetting only when connected to a computer during the disconnection of the sounder, so it was possible to ignore. 

The SD card is connected through a bi-directional level converter. The Arduino has limited memory, so the code handles things by grabbing small chunks of the files into four small buffers for the four ports. The Morse codes are stored as 2-byte unsigned int objects, with the exception of a few special 4-byte unsigned long objects for some of the punctuation. The code can be found here: Arduino Code.

Access to the Arduino is through the bottom of the box, which is attached with screws to corners that are glued inside the box.

The project uses two battery cells that each have overcurrent and undervoltage protection built in. The charging circuit is as described here.

You can find more photos of the project and an additional write up here:

And a video here:

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