Patent GB-1411075-A (GB)
Title: Telephone systems
IPC: H04M9/00
Filed: 1971-06-24
Abstract: 1411075 Automatic exchange systems WINSTER PRODUCTS (HAZARDOUS AREAS) Ltd 25 Sept 1972 [24 June 1971] 29603/71 Heading H4K A telephone substation intended for use in a deflagrable atmosphere includes a speech amplifier powered by a local battery charged over the telephone line while the substation is idle. As shown the substation equipment shown in the lower half of the figure is coupled by a telephone line 101, 102 to an adaptation unit shown in the upper half of the figure and which converts between signals employed on the line 101, 102 and signals on an ordinary exchange line t3, t4 said adaptation unit being located outside the deflagrable atmosphere (e.g. at the head of a mine). The arrangement is mainly concerned with keeping the currents to and within the substation below low safety levels. The substation battery included in a circuit 107 is charged over the line 101, 102 from a battery (connected at t1, t2) in the adaptation unit while the substation is idle. The line 101, 102 is looped through the substation at all times except during the transmission of dial impulses, the battery loop being completed at all times except while the substation is off hook and while incoming ringing signals are being received. Incoming calls.-Incoming ringing signals on exchange line t3, t4 are detected by relay AC/4 which enables a ringing tone generator 105 and couples it to the line 101, 102 in place of the battery at t1, t2. A transistor detector in the charging circuit 107 responds to the loss of the supply voltage turning on via a transistor switch 113 an audio transistor amplifier coupling the line 101, 102 to a loud-speaker 114 thus ringing tones are broadcast by the loud-speaker which may be duplicated for security. The telephone is answered by lifting the receiver and operating a pressel switch S3. Lifting of the receiver operates a switch S4 which in turn via a reset delay circuit 110 enables a relay KR/2, the contacts KR2 of which break the battery supply loop so that it is not normally re-established during the call. Operation of the pressel switch enables a relay PR/3 which disconnects the loud-speaker amplifier and connects the handset microphone 111 to line. Reset delay circuit 110 includes a capacitor which slowly charges up while the pressel switch is not operated and if allowed to reach the threshold set by a diode releases the relay KR/2 to re-establish the battery charging current. Outgoing calls are initiated by lifting the receiver. Relay KR/2 operates contacts KR1 and KR2 which remove the short across impulsing contacts S1 and break the battery supply loop respectively. A detector circuit comprising transistors TR1 to TR4 responds to each loop break enabling a relay A/2 a contact A1 of which makes the loop t3, t4 to the exchange. A contact A2 enables a relay CD/3 which maintains itself for at least a time determined by a capacitor C4 and maintains the loop via contacts CD1 and CD2 during dialling. At the end of dialling the loop is maintained for speech via Al, L3, R5 and L4. Multi station and alarm arrangements.-As described three substations connected to the same line are provided. Each station includes a transistor tone generator 115 which when energized by a contact S5 transmits a continuous tone to line if another substation is required, and which when energized by a contact 56 transmits an intermittent alarm tone to line, an audio desk at the adaptation unit responding to the latter. If the modulator for the intermittent tone fails a timer causes the continuous tone to be transmitted.