We found this circuit in a 1991 Hobbyst Electronics. The circuit makes use of an unusual transducer today. It is a piezoelectric buzzer that can eventually be replaced by an equivalent configuration.
This second-order filter was found in a 1978 publication and can use common operational amplifiers like...
This circuit was obtained in a 1973s documentation. The transistors may be BC558 for the PNP and...
The gain of this amplifier is controlled by light shining on an LDR. The circuit is from a Radio...