This circuit is from an American 80's documentation. The circuit indicates when the voltage of a 9 V battery drops below 7 V. The transistors can be BC548.
This circuit monitors the humidity of a pot or plant bed, providing indication on two LEDs. The...
This “zero crossing detector” was found in a manual of operational amplifiers from the 1980s....
This narrowband data receiver is from the Philips Databook of 1990. The circuit is powered by 5 V...