More than ten years ago I created a micro controller base morse code trainer. It was designed to process the morse code that someone entered by pressing a morse key and show the decoded text on a LCD display. A set of three units was used during a kids event and brought a lot of fun to them.
The design was based on an Atmel AVR 8515 controller. Schematics and software were published on my personal homepage until it became hacked some years ago. A few week ago I have been asked from Alfred DJ0GM to send him the technical details and software. His local ham radio club is still using a replica of that morse trainer for public events like recently at the Hamradio 2017 in Friedrichshafen. So they want to build a backup device.
Meanwhile the AVR 8515 is a little bit outdated. So I ported the software to run on an Arduino Uno with an attached 16x2 LCD and buzzer. You can find the old schematics and software as well as the Arduino sketch here: MorseTrainer.zip (235.19 kb).
The picture shows an Arduino Uno with an attached buzzer, morse key and a LCD Keypad shield acting as morse decoder.
Marie Christiano wrote a nice online article titled "How the Space Race Gave Us GPS Technology". It is published here by All About CIRCUITS.
I did some further experiments with AM modulators. First I tried to use a NE612 as modulator. It works quite well, but requires some adjustments. Volker, SM5ZBS, describes such an approach on his German website.
As alternative approach one can make use of a Dual-Gate FET such as the BF961. J. Roschy has proposed such an AM modulator. It is working really nice, has an excellent audio quality, is simple to build and requires only one simple adjustment. Here is my version of this AM modulator::