The present invention pertains to electronic circuitry and particularly to a specialized circuit useful in telephone communication.
It is well-known in the telephone communication arts to provide at a telephone sub-station a suitable dial or other device for interrupting a DC path on a repetitive basis so as to transmit dial pulses that affect switching systems at a central station. It happens that in certain types of telephone systems it is required that the bell, buzzer, or other audible means used to indicate an incoming call, be continuously in circuit with the dial means or other instrumentality that is connected across the lines.
The difficulty or problem that arises in a telephone subset context when the aforesaid conditions apply is that so-called "dial tap" is encountered, that is to say, the buzzer, bell or other audible means for signaling an incoming call necessarily operates in response to the dial pulses placed on the line as the dial means is being rotated in effecting its purposes. In other words, an annoying sound is heard by the user of the telephone instrument when he is trying to dial out on the line.
Accordingly, it is a primary object of the present invention to overcome the problem or difficulty of "dial tap" as it is encountered in certain telephone systems.
Another object is to accomplish the successful overcoming of the aforestated problem in a very simple, low-cost way.
A further object is to provide a simple electronic circuit in a package that can be interposed between the conventional telephone lines and the buzzer device.
The above and other objects are achieved and implemented by a primary feature of the present invention according to which an electronic circuit that operates as a discriminating means is provided for insertion between the so-called "line" terminals of a telephone instrument and the "device" terminals, which are the terminals directly connected to a buzzer, bell or other audible device. This discriminating means functions so as to block the passage of the repetitive dial pulses and prevent them from affecting the buzzer device, while permitting conventional or regular ringing current of appropriate frequency to pass to the buzzer device thereby to actuate such device and to make audible to the user the fact that there is an incoming call.
The discriminating means operates by reason of a suitable time delay arrangement which is effective, in the case of the extended time period of the ringing signal, to actuate a latching device, such as a silicon controlled rectifier or similar device. On the other hand, since the dial pulse signals that would ordinarily cause the annoying dial tap problem are of short duration, that is, on the order of less than 100 milliseconds, these will not be effective to cause actuation of the latching device and hence the path for transmission of such dial pulses will be completely blocked. Moreover, means are provided that prevent build-up of the succession or series of dial pulses from affecting the latching device and causing spurious operation.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.