2N2646 is a general purpose unijunction transistor. It is ideal to use in oscillators, timer circuits, sensor circuits, triggering SCRs etc.
In this circuit it is used as a tone generator. The transistor Q2 is used to amplify the output sound of the circuit to drive an 8 ohms speaker. The three variable resistors of 100K are used to generator different types of tones. Increasing or decreasing the value of the 100nF capacitor can change the frequency of the oscillator. The operating voltage of the circuit is 9V to 12V DC.
The figure below shows a project of a multi tone generator circuit. The circuit can produce different tones and can be used with any project where a tone generator circuit is required. The circuit is very simple and using minimum parts, the heart of the circuit is a 2N2646 transistor.