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 is from a Japanese publication in 1967. The circuit uses a Geiger-Muller valve and...
Motorola's LM13600 integrated circuit is not very common today. The input voltage is 10 mV. We...
This circuit is from a 70s linear integrated manual. The integrated circuit used is uncommon today...