This optical theremin was found in a 90s Hands-On Electronics. The basic frequency is given by C1 and the output is made in a speaker through a small output transformer.
This circuit was found in an English publication in 1986, but it can still be implemented by...
This small transistor amplifier was found in an ancient American documentation. Transistors can be replaced...
In the figure we have a simple way to obtain (almost) sinusoidal signals from an integrated...