This variable frequency circuit was found in a Popular Electronics from the 90s. The circuit outputs to a small speaker and can be powered by voltages as low as 3 V.
This simple circuit allows testing of general-purpose diodes and rectifiers and also general...
This circuit not only indicates when the battery is on but also when it is low. The circuit is...
This circuit shows how to add a 1st regulator to a source that already has a step for this...