An oscillator is a circuit that is deliberately designed to generate signals from a steady voltage supply. Any oscillator can be thought of as an amplifier with positive feedback and some type of circuit to determine the frequency. If this frequency-determining circuit is a resonant circuit, the oscillator is a tuned oscillator and it will generate just one frequency component, or close. Other circuits can be used that will cause the oscillator to generate a square or pulse waveform. It might be worth noting, that oscillators can also be designed so that their oscillation frequency can be varied when a steady voltage input is varied, and this type of oscillator is called a voltage controlled oscillator.
Transducers are another important class of electronic devices.
A transducer converts one form of energy to another. For electronics purposes important transducers are those with an input or an output which is electrical, particularly if that input or output takes the form of a wave. An example of a transducer is a microphone, which converts sound waves into electrical signals. Other examples include various sensors, which change their output in response to changes in their environment. A temperature sensor will generate signals stimulated by temperature or temperature differences.