Using an operational amplifier, this circuit from a 90s Popular Electronics, is characterized by its high sensitivity. R5 can be added to control the gain.
We found this circuit that makes use of CMOS technology in a 1988 publication. The circuit has...
This circuit was found in a 1967 EEE. The transistors are from the time and can be tested...
We found this circuit in a 1991 Hobbyst Electronics. The circuit makes use of an unusual...