This humidity sensor indicates the different levels of conduction of the sensors through an LED bar. The circuit is from Popular Electronics and must be powered by cells or battery.
This circuit provides a reference voltage for calibration. He uses a diode in a small thermal...
This circuit for model making was found in an English documentation. The SCR can be the TIC106, and...
The operational amplifier used in this circuit is no longer very common, but the configuration can be adapted...