Tone operated switching arrangement

A switching arrangement operable by a tone of a predetermined frequency comprising an automatic gain control amplifier having a signal input, a control input and an output, the signal input being connectable to a source of signals and the output being connected to an input of a bandpass filter which rejects all frequencies except those of a narrow band centered on said predetermined frequency, said output also being connected to said control input whereby the sensitivity of the automatic gain control amplifier is adjusted to be a maximum for the largest amplitude frequency appearing at said signal input, said bandpass filter being connected to an input of a phase locked loop which produces a change in voltage from a first level to a second level at its output only in response to a narrow band of frequencies centered on said predetermined frequency, the output of said phase locked loop being connected to a timer which only produces a pulse at its output upon receipt of said second level of voltage for a predetermined time, a pulse from said timer being fed to a flip-flop which changes state and operates a relay.