Patent GB-1218717-A (GB)
Title: Sonic switch
Inventor: KOLM ERIC ARTHUR
IPC: G08C23/02
Filed: 1969-05-15
Abstract: 1,218,717. Controlling and actuating by sounds. SONUS CORP. 15 May, 1969, No. 24940/69. Heading G4F. A sound-operated receiver 10 for the control of a domestic apparatus 94 by a remotely situated sound source 14 of simple construction, e.g. a dog whistle, the receiver being made insensitive to acoustic signals throughout at least all but the upper portion of the audible range and operating an impulse-type switching relay 12 for controlling the apparatus only when a predetermined number, e.g. two, of sound signals are emitted in succession by the whistle, comprises an acoustic transducer 16 having its output connected to a frequency selective amplifier 18. The amplifier includes three common-emitter transistor stages 24 having load resistors 26, and a battery power supply 28. The first stage of the amplifier 18 also includes a frequency selective negative feedback network comprising a capacitor 34 and an inductor 36 connected in parallel between a biasing resistor 30 and collector C to form a resonant circuit giving a "peak" response of the amplifier at the upper end of the audible range, i.e., at 13À5 kc/s. The rapid drop-off in gain at lower frequencies renders the receiver immune from interference signals in the form of normal voice frequencies. The amplifier output is connected to a counting circuit 22 via a rectifier 20 comprising components 40, 42, 44, 46, 48. The counting circuit 22 includes two monostable multivibrators 50, 52 and a coincidence circuit 54 providing a signal when there is coincidence of the unstable states of the multivibrators. The multivibrators are similar except that multivibrator 52 has a longer time constant. Each multivibrator includes two transistors 56, 58 having a coupling capacitor 60 and a coupling resistor 62. The collector 58c of multivibrator 50 is coupled to base 58b of multivibrator 52 by a capacitor 74 followed by a shunt diode 78 and series diode 76. The coincidence circuit 54 comprises transistors 80, 82, the collector-emitter current paths being connected in series with the coil 12c of the relay 12. When both transistors 80, 82 are conducting current flows from the battery 28 through the coil 12c to open or close the contacts 12a, 12b. Base 80b receives the signal from collector 58c of multivibrator 50 and base 82b receives the corresponding signal from multivibrator 52. Normally, the multivibrators 50, 52 are in their stable states with the transistors 58 conducting and the transistors 56 turned off; also, the transistors 80, 82 are non-conducting. A signal from the dog whistle 14 is transmitted by the amplifier 18 and rectifier 20 to the base 56b of multivibrator 50 to trigger the latter, causing the transistor to conduct and cut off transistor 58. Thus the collector 58c of multivibrator 50 rises to the potential of battery 28 and base 80b becomes conducting. Diode 76 prevents these positive-going signals from reaching base 58b of multivibrator 52. After a time interval, e.g. 0À2 sees., after cessation of the incoming sound signal, multivibrator 50 returns to its stable state and base 80b becomes nonconducting; also, the resulting negative-going signal at the collector 58c is passed by capacitor 74 and diode 76 to trigger multivibrator 52. Thus base 82b becomes conducting. Multivibrator 52 returns to its stable state after cessation of the sound signal after a time interval of, e.g., 0À5 sec.. However, if a second sound signal is received from the source 14 before this interval has elapsed multivibrator 50 is again triggered to its unstable state. Thus both bases 80b, 82b are conducting and the relay 12 is operated into its other position by the current impulse, the relay remaining in this position until a further impulse is received. Undesired operation of the relay 12 by two successive sound signals from the whistle 14 is minimized since the time interval between the two signals must fall between lower and upper limits. Thus there is no relay operation if the interval is less than 0À2 sec. or greater than 0À7 sec. since in both these conditions the base 82b is non-conducting. Unwanted recycling of he receiver 10 due to the latter responding to the noise emitted by the relay 12 itself, when operating, is prevented by capacitor 104, 106 connected between the base 56b of multivibrator 52 and the junction of coil 12c and transistor 80. When transistors 80, 82 are both conducting the base 56b of multivibrator 52 is at ground potential. Thus a stable state is imposed on multivibrator 52 so that even if the relay 12 emits a sound which is passed by the amplifier 18 the counter 22 does not respond. Further, transistor 82 is turned off but the base 56b of multivibrator 52 remains at ground potential until capacitors 104, 106 are charged through resistor 108. This charging time may be varied by the resistor 108 to obtain the optimum time duration for desensitization of the counter 22 to prevent recycling.