Audible signaling device for informing when a telephone line is available or in use

A device is provided for connection to a shared telephone line for detecting and audibly indicating the on-hook and off-hook status of the line. The device includes sensing and changing circuitry connected to the shared telephone line for detecting a voltage step occurring during status change transitions between on-hook and off-hook states and correspondingly changing the device during these status changes transitions so that the device is prohibited from drawing power from the shared telephone line when the shared telephone line in not in use. A piezo speaker produces audible on-hook and off-hook sights corresponding to on-hook and off-hook states of the telephone line.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a telephone equipment. More particularly, 
the invention pertains to a signaling device for determining whether a 
telephone line is in an on-hook or off-hook condition. 
2. Description of the Prior Art 
It is common in both business and residential settings to connect more than 
one telephone or device to a common telephone line. The indication of the 
shared telephone line being in an in-use or available state has 
traditionally been accomplished by repeatedly picking up the telephone and 
listening for a dial tone to indicate that the line was available. This 
type of arrangement is both tedious for the user waiting for the line to 
become available and creates annoyance to the user currently utilizing the 
line. 
Various prior art devices and circuits are known in the art for detection 
of on-hook and off-hook conditions of a telephone line. However, these 
devices suffer in a number of regards. For example, many of such devices 
utilize complex or costly circuitry that is often directed for use with 
large telephone or switching systems. While others, require batteries or 
external power to operate, and rely on visual indicators to indicate line 
status which leads to both increased maintenance or installation and the 
repeated checking of the visual indicators by a user. 
A more desirable device is one which would be simple and inexpensive and 
provide an improved manner of signaling that would not require repeated 
checking by a user. An additional desirable feature is one which would not 
require batteries or external power, but at the same would comply with the 
direct current requirements of the Federal Communications Commission. 
As will be described in greater detail hereinafter, the device and circuit 
of the present invention differs from those previously proposed and 
employs a number of novel features that render it highly advantageous over 
the prior art. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a device for 
immediately notifying a user of the availability of a line without the 
user having to repeatedly check a visual indicator. 
Another object of this invention is to provide a device requiring no 
external power or batteries while at the same time complying with the 
direct current requirements of the Federal Communications Commission. 
Still another object of this invention is to provide a device that is 
simple, inexpensive, and easy to install. 
To achieve the foregoing and other objectives, and in accordance with the 
purposes of the present invention a device is provided for connection on a 
shared telephone line for detecting and audibly indicating the on-hook and 
off-hook status of the shared telephone line. The device includes sensing 
and charging means connected to the shared telephone line for detecting a 
voltage step occurring during status change transitions between on-hook 
and off-hook states and correspondingly charging the device during these 
status changes transitions so that the device is prohibited from drawing 
power from the shared telephone line when the shared telephone line in not 
in use. Output means are connected to the sensing and charging means for 
producing audible on-hook and off-hook signals corresponding to on-hook 
and off-hook states of the telephone line. 
In accordance with an aspect of the invention, the output means includes 
means for producing an oscillating signal connected to a piezo speaker.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawing, a telephone line signaling circuit and device 
10 is illustrated schematically in FIG. 1. The device 10 is shown in a 
typical application where the device is a stand-alone device easily 
plugged in-line with a telephone 20 in a modular system. 
The device 10 has modular jack 12 for connection to a common or shared 
telephone line 14 with extension telephone line 16 and wall jack 15. 
Modular jack 18 provides interconnection of the device 10 to the telephone 
20 with extension telephone line 22. The modular jacks 12, 18 connect the 
device 10 to the tip 24 and ring 26 lines of the telephone line 14. Outer 
lines of the modular jacks 12, 18 can provide a path for a second 
telephone or device, but are not intended as part of the invention. 
The circuitry of the device 10 in a preferred embodiment includes a first 
capacitor C1 for sensing voltage changes between on-hook and off-hook 
states of the telephone line 14. A first resistor R1 is connected in 
series to the first capacitor C1. The first capacitor C1 is preferable a 
metalized polyester capacitor at 1 uf and 250 vdc. A second resistor R2 is 
connected in series to the first resistor R1 with a first diode D1 
connected in parallel to the second resistor R2. 
A bridge rectifier circuit 28 for rectifying voltage includes second diode 
D2, third diode D3, fourth diode D4, and fifth diode D5. The bridge 
rectifier circuit 28 is connected to the second resistor R2 and first 
diode D1 and includes first output line 30 and second output line 32. A 
zener diode D6 for regulating voltage is connected in parallel between the 
first output line 30 and the second output line 32. A second capacitor C2 
is connected in parallel to the zener diode D6. A piezo speaker 34 
produces auditory signals corresponding to on-hook and off-hook states of 
the telephone line 14. A piezo speaker 34 is preferably a piezo transducer 
at 90 DB, 3.75 KHZ, and 5 vpp. 
In operation, when a remote telephone goes on-hook or off-hook, there will 
be a step in voltage across the line from approximately 48 volts down to 
approximately 8 volts. This large step in voltage will cause a charging or 
discharging current to go through the first capacitor C1. In one polarity, 
the charging and discharging current is required to go through first 
resister R1, and in the opposite polarity through the first resister R1 
and the second resister R2. In the opposite polarity, first diode D1 will 
be conducting and a higher current will be available to only the first 
resister R1. By adjusting the values of resisters R1 and R2, one can 
adjust the relative magnitude and sound of the two status portable 
signals. 
The polarity is then converted by the bridge rectifier circuit 28 to a 
forward voltage that is limited to a safe magnitude by the zener diode D6, 
which is typically a value of 5 or 6 volts. Second capacitor C2 maintains 
a stable voltage for the integrated circuit chip U1. U1 is a hex inverting 
schmitt trigger integrated circuit chip of conventional design, such as 
the 74HCT14, with hysteresis so that whenever voltage is applied to power 
the chip, the output from PIN 36 will go through a third resister R3 and 
charge a third capacitor C3 and reverse the input causing the output to 
reverse and then discharge the third capacitor C3. In other words, it 
creates an oscillating signal. That oscillating signal is inverted by the 
second stage which outputs at PIN 38. The piezo speaker 34 is connected 
across PIN 36 and PIN 38 so that it will get the benefit of a positive 
push/pull electrical signal allowing for a doubling the voltage so that 
the piezo speaker 34 will create an adequate amount of sound from the 
small amount of energy that is available from the charging and discharging 
of the first capacitor C1. The other stages of the device have their 
inputs grounded for safety but have no function in the design. 
In use, the device will produce two audibly distinct status signals. To 
test if the telephone line 14 is in use initially and with a minimal 
disturbance, one can briefly pick up and hang up the telephone 20. If the 
user does not hear an "in use" signal or tone followed by an "available" 
signal, then the telephone line 14 is in use. An "available" signal will 
be produced when the line 14 is free again, as well as producing an "in 
use" signal should the telephone line 14 be again in use. 
It should be understood that the device may be used as a small stand alone 
device plugged into an existing system or may be formed integral with the 
telephone 20 by simply connecting the tip 24 and ring 26 lines of the 
device to the corresponding lines of the telephone for connection to the 
tip and ring lines of the telephone line 14. Since the present device 10 
will produce an electronic ringing sound in response to incoming ring 
signals, the device 10 could be used to replace the existing ringer 
circuitry of the telephone 20. 
It is important to note this device 10 only takes power from the telephone 
line 14 during the transitions, or in other words during the voltage steps 
up or down because of the telephone going on-hook or off-hook. Hence, 
power is obtained without taking direct current in order to comply with 
the direct current requirements of the Federal Communications Commission. 
Although the invention has been described by reference to some embodiments 
it is not intended that the novel device be limited thereby, but that 
modifications thereof are intended to be included as falling within the 
broad scope and spirit of the foregoing disclosure, the following claims 
and the appended drawing.