This project uses a simple 2-transistor relaxation oscillator to drive a passive buzzer.
Four switches allow the oscillation frequency to be changed by adding capacitors to the circuit. The more capacitors are connected in series, the higher the frequency becomes and the higher the sound produced by the buzzer.
How does it work ?
The oscillator is built around two transistors and a network of resistors and capacitors.
The two transistors provide positive feedback, allowing the circuit to repeatedly switch between two states.
When one transistor conducts, the voltage in the circuit changes and charges the capacitors. As the capacitor voltage changes, it eventually causes the transistors to switch state.
The capacitor then begins to discharge in the opposite direction, causing another transition.
This process repeats continuously, producing an oscillating electrical signal.
The oscillating signal is applied to the passive buzzer, which converts the electrical variations into sound.
The interesting part of this circuit is the capacitor network.
The four pushbutton switches determine how many capacitors are connected in series.
SW1 connects the first capacitor.
SW2 connects the first two capacitors.
SW3 connects the first three capacitors.
SW4 connects all four capacitors.
This gives four different capacitance values and therefore four different oscillation frequencies.
As more capacitors are connected in series, the equivalent capacitance decreases. The oscillator can therefore switch states more quickly, increasing the oscillation frequency.
Electronics component Fun Kit (with breadboard & power supply module)
General purpose transistors - BC547 (NPN) & BC557 (PNP) pack
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