Improvements in or relating to circuit arrangements including lowfrequency relaxation oscillators

711,535. Automatic exchange equipment. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Oct. 10, 1951 [Oct. 12, 1950], No. 24900/50. Class 40 (4) [Also in Group XL (c)] Fig. 5 shows a tone generator for use in an automatic telephone exchange. Valve VH oscillates at a frequency of 400 cycles per second by virtue of the coupling via coils CO and its output is amplified in stage VG to give a continuous 400 cycles per second. An NU. tone at lead 10 and a second output at lead 11 is interrupted by contacts A3 of relay A, which is controlled by a relaxation oscillator, Fig. 1 (not shown) (see Group XL (c)), to give a busy tone of 400 cycles per second interrupted for two seconds in every three. The 400 cycles per second is modulated at 16.6 cycles per second by earth applied to the right hand winding of coil RT via contacts D1 of relay D, which is controlled by a multivibrator, Fig. 3 (not shown) (see Group XL (c)), and is interrupted at contacts C2 of relay C to provide interrupted ring tone at lead 12. Relay C is controlled by a multivibrator Fig. 2 (not shown) (see Group XL (c)). Transformer TRA is supplied on its primary side with earth alternately from contacts D1 and E1, which is controlled by relay E in a multivibrator current, Fig. 3 (not shown) (see Group XL (c)), so that dial tone having a peaky waveform and a frequency of 33.3 cycles per second is fed to lead 13. Continuous ringing current 16.6 cycles per second is supplied at lead 14 and interrupted ringing via contacts C3 at lead 15. The thermionic valves used in the generator have directly heated low voltage cathodes which are connected in series with a ballast resistor BR across the 50 volts supply (see Group XL (c)).