This circuit rejects frequencies around 1 kHz and can be adjusted using C1. Equivalent operational amplifiers can be used, and the power supply must be symmetrical.
A simple version of VCO CMOS using 4049 inverters is shown in the figure. This circuit can be...
This simple SCR relaxation oscillator was indicated by a 90s Popular Electronics. The frequency is...
This circuit can be used as an answering password system in a store, office, or other location....