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 “brainwave” amplifier was published in Radio Electronics magazine in August 1964. The...
This common 555 circuit was found in documentation from the 90s. The circuit uses two 555s and the...
A frequency range determined by the network of resistors and capacitors with negative feedback and input...