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 works only with carbon microphones, due its low impedance. The speaker should be good or...
The integrated circuit used in this regulator is not very common today. The circuit is from old...
This is a typical point scale or moving bar circuit with the LM3915. We found it in old American...