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.
The figure shows how to implement an undervoltage detector using a Norton amplifier and a zener...
We found this circuit in a Philips documentation from 1965. The circuit uses a common LDR sensor...
This circuit also detects IR, providing LED indication. The circuit is used to indicate humidity in...