This circuit was found in a 90’s EDN. The components used are still common and the sensors can have their quantity increased. The supply voltage is according to the relay.
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...