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 control for 12 V drills was found in a 90s Elektor. The components are current, and Q1 can be...
Found in an old English publication, this circuit applies to the 240 V network, but can be changed to...
This circuit was found in a 90s Popular Electronics. With it, using a 4066, it is possible to...