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 was found in a 1967 Raytheon manual. The transistors are from the time. Equivalent modern...
This circuit was published in a 1979 magazine, but it can still be easily assembled as the...
The circuit presented generates short pulses. This configuration was found in a publication from...