Patent GB-1372754-A (GB)
Title: Tone ringer circuits
IPC: H04M/
Filed: 1972-01-18
Abstract: 1372754 Substation circuits WESTERN ELECTRIC CO Inc 18 Jan 1972 [18 Jan 1971] 2303/72 Heading H4K  A ringing tone generator in a sub-set is activated by ringing current on the line via circuitry including a voltage threshold circuit 210, a voltage amplitude limiting circuit 211 and a rectifying circuit 212 so as to prevent actuation of the ringer by noise or dialling signals. A continuous, intermittent, amplitude modulated or frequency modulated tone may be produced and the tone generator may be powered by rectifying the line ringing current. In operation ringing signal arriving at 101, 102 activates a voltage amplitude threshold circuit 210 including back to back Zener diodes D1, D2, D3, D4, assuming the received signal is above the threshold and the threshold circuit produces an output signal or wire 202 which is amplitude limited by circuit 211 comprising two pairs of transistors in Darlington configuration. The amplitude limited signal is then used to charge a capacitor C3 in charging circuit 212 over ¥ of a cycle to a voltage approaching twice the maximum voltage on wire 202, the voltage to which it charges being governed by the energy of the signal on wire 202. The charging of capacitor C3 above a threshold level enables a power supply 106 via gate 213 so as to provide D.C. power, derived by rectifying the ringing signals on wire 203, to a tone generator 104 which produces a tone signal at electroacoustical transducer 105. The power is supplied on wire 204 i.e. across capacitor C4, and is in the form of a ripple voltage which is connected to the astable multivibrator via a transistor T9 turned on by gate 213.  In order that the multivibrator may have a constant frequency output, amplitude modulated due to the "ripple" on wire 215, transistors T9 and T10 are arranged to turn on and off so as to maintain wire 215 at a voltage which is directly proportional to the emitter-base turnon voltage of transistor T10. Transistors T9, T10 also provide the additional feature of assuring that transistors T5, T6 "snap" on in the presence of valid signals. If the power supply capacitor C4 has become charged due to transients and a valid ringing signal has not been detected, gate 213 is closed and transistor T9 is held off preventing power from reaching the multivibrator circuit. A transistor T15 is also held off to mute the transducer 105. To produce tones other than an amplitude modulated constant frequency tone additional circuitry is connected to points 1, 2, 3. In Fig. 4A (not shown) points 1 and 2 are coupled by a pair of back to back diodes and the common point of the diodes is connected via a switch to a D.C. source of negative potential. The switch may be situated at a second telephone in an intercom system, the closing of the switch causing the tone generator at the first telephone to emit a constant amplitude frequency tone which can be distinguished from the amplitude modulated tone caused by exchange ringing current. Fig. 4B (not shown) shows the modification necessary to produce an intermittent constant frequency tone for use in an intercom system as above described. A square wave generator is connected between the two diodes and is arranged to switch the supply to terminal 1 on and off, thereby to produce the tone. In a further embodiment (Fig. 4C, not shown) the square wave generator is connected to point 3 and a continuous two-tone signal is produced. Instead of the switch being manually operated as in an intercom system, it may be arranged to be operated automatically on receipt of exchange ringing current by the telephone. The extra power supply and extra circuitry are then all housed in the single set.