Telephone privacy device with conference and indicator features

摘要

A simple and inexpensive telephone exclusion or privacy device. A conference feature is and am indicator light showing when the line is in use are also provided. The basic exclusion or privacy circuit includes only eleven components, thereby providing a very low cost device. The circuit includes a diode bridge with a Silicon Controlled Rectifier (SCR) connected across the midpoint of the bridge circuit. A resistor in series with the SCR limits the amount of current which the SCR can draw from the telephone line. A Zener diode in series with a resistor is connected between the anode and the gate of the SCR and a resistor in series with a capacitor is connected from the gate of the SCR to the second side of the bridge circuit i.e. to the point where the cathode of the SCR is connected. The breakdown voltage of the Zener diode is chosen so that at normal out of use line voltage (nominally 48 volts) the SCR will be placed into a conducting state and thus when someone picks up the handset they can access a line which includes the device. However, if a line is in use and someone picks up another handset which includes one of these devices, the line voltage will only be twelve volts and the SCR will not be turned on, thereby presenting the second handset with an open circuit.