CMOS Schmitt Trigger IC Makes VCO - By changing the supply voltage fed to a classic 4584 Schmitt trigger type oscillator, the oscillator frequency can be changed over a range of 50:1. A 74HCU04 inverter is used at the output of the 4584 to maintain a constant TTL logic level signal.
Miniature Audio Oscillator - The Miniosc, low distortion battery powered audio oscillator
Minimal audio oscillators - In order to generate a single note you may try these simple circuits. With only three components you may implement some basic buzzers. You need a telephone earpiece for the first circuit. Any old telephone set has got one of those magnetic earpiece that is right for our purposes. Add an extra capacitor and a transistor and you have your buzzer. Frequency of operation is about 1800 Hz and the capacitor must be changed if you wish to have a different frequency.
Simple Audio Oscillator - For use with the millivoltmeter, for testing amps and speakers
Variable Frequency Audio Oscillator - Variable Frequency Audio Oscillator circuit