We found this circuit in an old Popular Electronics. The circuit is experimental, driving an LED from signals applied to the LDRs. The power supply does not have to be symmetrical.
Today we have more compact solutions. This is from a Radio Electronics magazine from October 1990....
This filter was found in Texas Instruments documentation from the 1980s. The circuit uses a common...
16 commercial test circuit for tubes and transistors appeared in Radio Electronics magazine in...