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 can function as a light alarm by changing the sound when light falls on the sensor....
In this circuit the photo-transistor signal is amplified by a Darlington step and then applied to a...
This is a traditional complementary-output transistor amplifier sourcing 2 to 4 watts to a 4 ohm...