A simple musical instrument can be made in less than half an hour. Of course, the range of its sound, frequency, and as a result, the tone is very different from real professional instruments, but due to its simplicity it will be an excellent device for assembly to a beginner electronics engineer.
The basis of the circuit is the well-known and mega-popular 555 microcircuit, its period, and from here it is possible to control the frequency using the values of some resistors' resistors and capacitor capacitance.
As you can see, many resistors with different ratings are leaving us, so by pressing a certain key you turn on a resistor of a certain resistance in the circuit and sound is heard in the sound-emitting device. By pressing another key, with a different resistor, you will create sound vibrations with a different tone. When two or more buttons are pressed, the resistors are connected in parallel, another resistance is created and the sound changes. Combining these clicks in a certain sequence, you can create primitive melodies - this is funny.
For flexible settings, I recommend connecting a variable resistor by rotating its shaft to achieve the desired sound tone, then measure its resistance with an ohmmeter, not twisting anything, and replace the nearest available resistor with the nearest nominal value. A capacitor, if you find it, you can turn on the tuning, but some may have problems measuring its capacity - not all multimeters are capable.
Particular attention is paid to the keys. Standard clock buttons are too hard, you have to use a relatively significant force to close their internal contacts. I recommend using them only with a certain lever, similar to a piano key. I found buttons that require extremely little effort to press and also have a long cylinder for pressing.
By short listening to the output signal with a change in the angle of rotation of the rotor of the variable resistor, good sound frequencies for each key were selected in my opinion. Below is a table of frequency and resistance of a resistor suitable for this purpose.
If you wish, you can easily calculate the ratings of the radio components for the frequency you are interested in, in those. The documentation indicates the maximum operating frequency of the timer 200 kHz. The human ear hears oscillations with a frequency of 20 Hz - 20 kilohertz, so the possibilities for this electronic component even more than we need. Briefly show how it is calculated. The first resistor was selected at 4.7 kOhm - 4700 Ohm. From the main formula, taken from technical documentation 555, the resistance R2 is easily derived for given R1, C1 and the selected frequency itself.
The entire board, thanks to the surface mount components, is extremely small. Any NPN transistor can be BC847, its KBE location is standard, the same as all bipolar transistors in the SOT-23 package. Power 5-18 V, but it works even from one lithium-ion cell.
It is also possible to insert such a scheme into an old non-working children's melody synthesizer. The fifth output of the “Control” microcircuit is best thrown to minus through the output capacitor with a capacity of about 100 nF.
When a low-impedance speaker is connected, the transistor noticeably heats up, this can be prevented and necessary by increasing the value of its base resistor or by turning on the high-impedance speaker from an old telephone. In my copy, it turned out that the buttons with resistors were located on one board, and the chip on the second: connected them with tinned tin plates. It is better to fix the buttons not only with contacts using solder contacts, but also fill this case with hot-melt adhesive or epoxy when the ratings for the desired sound are already precisely selected.
