Abstract: A DC power supply comprising a pre-regulator having an input connected to a DC supply, a control terminal connected to a reference voltage and an output connected to an input of a three terminal integrated circuit (IC) voltage regulator. The IC regulator has an output terminal for connection to a load and an adjustment terminal connected to a bias voltage, a zener diode being connected between the control terminal and the output of the IC regulator and between the input and output of the IC regulator. The zener diode has a breakdown voltage less than a maximum permissible input-output voltage differential for the IC regulator whereby an excessive load demand tending to increase the input voltage to the IC regulator causes the zener diode to conduct and reduce the voltage on the control terminal of the pre-regulator which thereupon decreases the voltage to the input of the IC regulator.

Abstract: Disclosed is an electronic hybrid circuit which enables telephone conferences to be set up. The known systems have various limitations. For example, in one type of system, more than one person cannot be heard simultaneously. Known systems also tend to be fairly limited in the number of telephones which may be connected up in a single conference. Otherwise, signal degradation takes place due to worsening signal to noise ratio and impedance matching problems. The present invention provides an electronic hybrid circuit which enables a large number of telephones to be connected in a conference set up and which has a number of advantages, including the fact that more than one person may talk at one time. The disclosed hybrid circuit comprises a local input and output and a system input and output. The local input is connected to an analog switch and to a comparator which provides a control voltage to turn on the analog switch if a signal on the local input exceeds a predetermined minimum.

Abstract: Disclosed is a switching arrangement operable by a tone of a predetermined frequency. An automatic gain control amplifier has an input connectable to a source of signals and an output connected to an input of a bandpass filter which rejects all frequencies except those of a narrow band centered on the predetermined frequency. The bandpass filter is 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 the predetermined frequency. The output of the phase locked loop is connected to a timer which only produces a pulse at its output upon receipt of the second level of voltage for a predetermined time. A pulse from the timer is fed to a flip-flop which changes state and operates a relay.