Figure selection with push buttons

691,237. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. July 23, 1951 [July 21, 1950], No. 17438/51. Class 40 (iv). A subscriber transmits the called number to a register by pressing push-buttons to emit corresponding frequencies which pass through appropriate filters F1-F10 in the register to fire one or more gas tubes, the higher frequencies being substantially harmonics of the lower frequencies and the low-frequency filters F5-F10 being associated with two tubes, one of which is also associated with the filter passing a harmonic of said low frequency. A subscriber initiates a call by depressing button K, Fig. 1, which is locked down by a spring 17 and reverses contacts 15, 16 to close the line circuit through the filament of tube E1. A line relay starts a line-finder Sa-Sc, test relay SR3 pulling up when the calling line is encountered. A register-finder Va-Vc selects a free register, whereafter SR1 pulls up to close circuit over the loop, the filament of E1, wipers Va, Vb, relay R7 and tube E21. Tubes E21 and G2 feed tube E1 with a constant current irrespective of the resistance in the line. Tube E1 is thus energized and emits an audio-frequency signal over transformer T12, the signal being shaped by trigger E23 and passed over transformer T2 and through filter F11, Fig. 3, to actuate relay R6 which pulls up before the slugged relay R7. The first digit is transmitted by depression of one of push-buttons P1-P10 which changes the frequency of tube E1, one of filters F1-F10 responding to fire one or more of gas-tubes G31-G34 and actuate the corresponding relays R11-R14 which lock up. Simultaneously, release of R6 energizes both opposed windings of R5 and the upper winding of R4. When the signal ceases, filter F11 re-operates R6 to release the upper winding of R5 which then pulls up under the influence of its lower winding to actuate R1, which locks up so that the second digit fires one or more of tubes G35-G38 and the associated relays R15-R18. Similarly, operation of R2 causes the third digit to be registered on tubes G39-G42 and relays R19- R22 after which R3 pulls up to extinguish the fired tubes and indicate over lead t5 to the register that three digits have been entered. The register then functions in known manner and is released by earth on lead t6 operating SR2, which restores SR1, dismisses the register and establishes connection between leads a, b and a1, b1.