I saw a demonstration of some old telegraph equipment and thought about how simple the technology was compared to what we have today. The technology to energize electromagnetic devices at a time when electricity was barely understood was the catalyst to many inventions and innovations during middle to late 19th century. This wired technology eventually lead to transcontinental communications, teletypes, telephones, fax machines, internet, email, and finally wireless communications.
This is a tribute to the original texting device that made real-time long distance (beyond line of sight) communications between people a reality.
Currently I have two implementations. One is using an arduino, the other with a Raspberry Pi. For both I have setup a protoboard to host the LCD, contrast pot, a transistor to drive the sounder, 5vDC power supply connector, and keyer connector. I'm working to finalize the configuration and wrap it up in a 3D printed box.
This is my first attempt to capture the schematic using Eagle. I welcome any constructive input on how to improve. I'm using a 12VDC power supply and limiting the current to the sounder using a trimpot. I selected a trimpot so that I can adjust to any sounder since they have varying resistance. I put the Keyer in the same loop as the digital signal so that I can drive the sounder with either an arduino/rpi or with the keyer. I used a 5v voltage regulator to limit the voltage on the bus since it's connected to the microcontroller board. I probably need a diode there, but I have an idea I want to explore, which is to have the wire going to the microcontroller to be used to send commands back to the board.
I've learned (luckily not the hard way) that I need to limit the current going to the solenoids to avoid damaging the sounder. There are a number of references, but I have not yet found a definitive source on what the proper current should be. Each sounder has a known resistance, so I'm going to experiment with a 12VDC Power Supply and will move a way from using the relay shield and replace it with a MOSFET. More to come.