This circuit indicates when a rain is acidic by monitoring its pH. The circuit is from a 90s RE Experimenter Handbook. Some components used can be difficult to obtain today.
This circuit is from an old documentation. However, it can be designed based on modern transistors...
The figure shows how to use the LM3914 bargraph with expanded scale. The circuit has two...
These reset circuits when the power is turned on were found in an 80's EDN. The circuit also...