Telephone-bell simulator

Alan G Hobbs 18telephone-bell simulator This circuit is intended for use in a small private telephone installation. The ringing tone sequence is 400 ms on, 200 ms off, 400 ms on, 2 s off. In the accompanying diagram, Ni and N2 form an oscillator that operates at a frequency of 5 Hz, which gives a period of 200 ms. The oscillator signal is fed to two decade scalers, which are connected in such a manner (by N3 and N4) that the input signal is divided by 15. The second input of N4 may be used to switch the divider on and off by logic levels. If this facility is not used, the two inputs of N4 should be inter- connected. Resistors R3 to R6 incl. form an OR gate that controls a relay via T1 and T2 which are connected in a darlington circuit. Outputs 5 to 9 of IC2 go high sequen- tially, so that the relay is energized for 400 ms (when 5 and 6 are high), then off for 200 ms (output 7 is not connec- ted), and then energized again for 400 ms (when 8 and 9 are high). After that, the relay is...