Patent GB-915364-A (GB)
Title: Improvements in and relating to alternating current signal detector circuits
Inventor: GOLDING JOHN LYNN
IPC: H04Q1/442
Filed: 1960-01-27
Abstract: 915,364. Automatic exchange systems. TELEPHONE MANUFACTURING CO. Ltd. Jan. 26, 1961 [Jan. 27, 1960; Oct. 25, 1960], Nos. 2910/60 and 36531/60. Class 40 (4). In a system having provision for answering calls to absent subscribers, ringing current on the absent subscriber's line is detected by the unbalance produced in an A.C. bridge connected across the line wires, caused by the difference in amplitude of the ringing voltages appearing on the outgoing and incoming terminals of the subscriber's line circuit during ringing. Thus in Fig. 1, ringing current is applied via ring trip relay 2 to the outgoing terminal B of a line wire extending to a subset 3, 4 and is returned via the other line wire to incoming terminal A, resistor 5 and the ringing return battery 6, the amplitudes of the ringing voltages appearing at terminals B, A being shown in Fig. 4. When a subscriber is dialling (Fig. 2, not shown) or when a call is being metered (Fig. 2, not shown) or when a call is being metered (Fig. 3, not shown), voltages of equal amplitude but out-of or in-phase respectively appear on the line wires. It is stated that this method of ringing-current detection, which is to be described, has the advantages of drawing negligible power from the ringing battery, being insensitive to transients and not responding to dialling or metering impulses. In Fig. 7, fullwave rectifier bridges MR1, MR2 are connected between earth and terminals B, A through capacitors C1, C2 respectively, the diagonals of the bridges each being connected to respective windings of a differential relay D so that in the event of voltages of unequal amplitude appearing at points B, A, i.e. ringing current, relay D will operate and a warning light, for example, will be lit at an answering switchboard. However, as this is an essentially low impedance bridge, speech currents may be shunted by it and also operating current for the relay has to be derived from the ringing source. In Fig. 8, the differential relay D draws its operating current from an auxiliary battery when the voltages at A and B differ in amplitude and result in the relatively greater conduction of one of the transistors T1, T2. In this case the network C1, R1, R3, R4, R2, C2 is of relatively high impedance. The application of unequal amplitude voltages to terminals A and B (Fig. 9) results in capacitor C5 receiving a negative charge on its " non- earth " side so that transistor T3, which is normally cut-off, conducts and relay D operates. In this case the current drawn from the ringing source is stated to be a fraction of a milliamp, but this may be decreased still further by using a thermionic valve (Fig. 10, not shown) in place of the transistor. In the drawing of the second Provisional Specification an " input " circuit similar to that shown in Fig. 9 is used but in this case the " non-earth " end of capacitor C5 is connected through rectifier MR8 to the base of a transistor T1, and rectifiers MR5 and MR6 are reversed. Transistor T1 is biased so as to be normally conducting, transistors T2 and T3, however, are normally cut-off. Upon the application of ringing current, capacitor C5 is charged so as to apply a positive-going signal to the base of transistor T1. This leads to the cutting-off of T1, the conduction of T2 and hence the conduction of transistor T3 with the consequent lighting of warning lamp LP1. When T3 conducts, the battery end of resistor R19 goes negative relative to earth and hence a relay connected in common to rectifier MR9 and others similar, will operate so as to light a "Call Pilot " supervisory lamp.