This circuit memorizes through an LED indicator if your phone was used during your absence. The circuit was found in a 90's EDN. Transistors are commonly used.
This circuit is suitable for SSB modulation and was obtained from an amateur radio documentation from...
This circuit was obtained in a 90’s Popular Electronics. The operators admit equivalents such as...
This 7400 TTL circuit has a memory, keeping the LED on when a pulse is detected. The circuit must be...