Rufanlage

772,571. Inductive signalling. NAAMLOOZE VENNOOTSCHAP NEDERLANDSCHE INDUSTRIE RADIO ASTIKELEN (NIRA). May 23, 1955 [May 26, 1954], No. 14771/55. Class 40(5) A calling system comprises a closed circuit 5 running through one or more buildings, e.g. a hospital, supplied with high frequency energy from a transmitter 4 which may be modulated by one of several low frequencies generated by a multivibrator or other modulator 2 selected by a switch 3 or keyboard, and so that persons carrying portable receivers each responsive to the HF energy and comprising an electromagnetic vibrator mechanically resonant to one of the call tones may be selectively called. The receivers may have vertically arranged ferrite or iron rod aerials and single grounded emitter amplifying and detecting transistors with parallel resonant collector circuits forming the energizing winding of a vibrator 12-15 having apermanent magnet 12 and mechanically tuned resilient blade 13. The latter may carry a spring blade 14 striking against a sound board 15 or the blade 13 may close contacts operating a buzzer or loudspeaker. The high frequencies may extend from 20 Kc/c 50 Kc/s and low frequencies from 50-300 c/s with relative frequency difference of 2¢ per cent. Further call channels may then b eproduced by using the same low frequency range and a different high frequency. Further, an oscillator 1 of frequency about 1 c/s comprising a valve whose resistance forms part of the multivibrator 2 may wobble the frequency of the latter, e.g., from 49 to 51 c/s, to allow for temperature or other variations in the receiver vibrator frequency, and the frequency sweep may extend beyond the response of each vibrator to produce an intermittent calling tone. The transmitter may be of Class C type. The receivers may by carried by clips to pockets and may be permanently energized without excessive battery consumption.