This old circuit, from the 80's, uses an integrated circuit that is uncommon today. The circuit is intended for loads controlled by triac and the resistors depend on the desired control.
This circuit was found in an American documentation of the 90s. Optical resources with the LDR as a...
Measuring capacitances and resistance this simple instrument was found in Radio Electronics...
Other operational amplifiers can be used in this conventional filter that rejects only one...