The following construction allows a simple cable tester to determine whether the cable network is present in high-frequency signal from the source. It is a contactless version of the tester to detect the presence of a signal at distances up to 2m. It can be used to control all types of signal cables, such as divorce Ethernet,modem connections, digital TV, etc. The circuit lights an LED at the moment it accepts RF energy around the cable.

Source: Pandatron