This circuit was found in an old Electronic Design. It makes use of sidacs and operational not very common today. The power supply must be symmetrical.
This circuit was obtained from an old American documentation, and practically any general-purpose transistor...
This circuit is intended for monitoring an environment. It fires a relay with a sound, but only if...
This circuit is from an old documentation, but perfectly current. In place of the battery, 2 or 4...