A one-voice single-octave EMR with a vibrato generator, proposed by Instructables under the nickname simon.w.nordberg, has a minimum of controls. It can be used both as a toy and for the performance of the so-called "beeper" music, popular among retrocomputers. Using the tool will become more convenient if you modify it to a two-octave one.
The EMP scheme is shown below. It consists of three parts: tone generator, vibrato generator and UMZCH. The vibrato generator, which can be turned off if desired, interacts with the tone generator through an optocoupler. Feature homemade is an unusual way to adjust the volume: from the output side of the amplifier, not the input, as is usually done. The connection diagram of any two keyboard keys is separately shown below, the rest are connected in the same way.
Since the master only obtains buttons with the same pushers, he removes the pushers from some of them, paints, waits for the paint to dry and sets it back. Then it places all the buttons on the top cover of the case.
Solders trim resistors directly to the pins of the buttons:
Everything parallelizes:
Assembles a circuit on a perfboard type board. With good soldering skills, the use of panels for microcircuits is optional. Optocoupler collects in a tube of electrical tape.
Puts the controls, connector, board, dynamic head in its place:
Configures EMR using, for example, note frequency table and a smartphone with a frequency meter application. At this EMP is ready. Frequencies can creep away over time, requiring re-tuning, but they do not depend on the supply voltage.
You can record interesting tracks and send them to beeper music contests.