This circuit was found in documentation from the 1970s. However, it can be implemented with modern transistors such as 2N2222 for Q1 and BF494 for Q2. The crystal depends on the desired frequency.
The frequency of this circuit is approximately 1 Hz. Whenever the output is at low level, the LED...
This circuit can be used in a digital measuring or shield interface. The circuit is from old...
In the figure we show how to implement a TTL photo-trigger with the 7413. This circuit can be used...