Here is the circuit diagram of a simple yet interesting mini organ based on UJT 2N 4891 (Q1). You can produce the desired sound by pressing each pushbuttons. You have also the option to set each push button to produce the desired tune you want.
The circuit is based on a UJT oscillator who’s frequency is determined by the Presets R6 to R13 (one at a time) and capacitor C2. The transistor Q2 drives the speaker to produce reasonable sound output. For each pushbutton press the corresponding preset will be added to the circuit, and the UJT oscillates in a frequency proportional to it. This frequency will be amplified by the transistor to produce the sound at the speakers. A 9V battery or 9V DC power supply can be used to power the circuit.
Circuit diagram with Parts list.
UJT Organ Circuit