This circuit from Motorola is from 1968, appearing elsewhere on this site. The transistors admit equivalent and the relay is of the sensitive type for low voltage, according to the power supply.
We found this circuit at Modern Electronics in January 1991. The magazine no longer exists, but...
The integrated circuit used in this filter is not very common today, but the reader can use...
Found in Linear Technology documentation from the 90s, this circuit uses an operational amplifier...