Telephone interface circuit

A telephone interface circuit comprising a speech circuit, a control circuit turning on upon receiving a supply of a forward direction trigger current so as to connect first and second telephone lines and the speech circuit, and going off upon receiving a supply of a reverse direction trigger current so as to disconnect the first and second telephone lines and the speech circuit, a capacitor charging and discharging charge supplied from the first telephone line, and a hook switch supplying charge accumulated at the capacitor when off-hook to the control circuit as a forward direction trigger current, and supplying charge discharged from the capacitor when on-hook to the control circuit as a reverse direction trigger current.

BACKGROUND

The present invention relates to a telephone interface circuit, and particularly relates to improvement of complete cutting off of a telephone line at the time of an internal call.

FIG. 1shows a circuit configuration for a telephone interface circuit10of the related art. The telephone interface circuit10controls interfacing between a speech circuit30and telephone lines L1, L2. The telephone interface circuit10is mainly comprised of a varistor element V1, diode bridge20, hook switch40, and transistors Q1, Q2.

The transistor Q1is a PNP transistor, and the transistor Q2is an NPN transistor. A resistor R1is connected across emitter terminal E1and base terminal B1of transistor Q1. The emitter terminal E1of transistor Q1is connected to a cathode of diode D1, and collector terminal C1is connected to the speech circuit30. A resistor R10is connected across the base terminal B1of transistor Q1and a collector terminal C2of transistor Q2. The emitter terminal E2of transistor Q2branches into two, with one branch connected to the speech circuit30and the other branch connected to the anode of diode D4. Base terminal B2of transistor Q2is connected to a microcomputer (not shown) via resistor R4. The microcomputer (not shown) controls the potential of terminal HC at the time of an on-hook operation, off-hook operation, or dial pulse transmission operation etc. Further, the base terminal B2of transistor Q2is connected to terminal1of hook switch40via resistor R21.

Varistor element V1absorbs overvoltages (for example, high voltages in the order of 270V or more) across the telephone lines L1, L2.

The diode bridge20regulates the signal flowing through the telephone lines L1, L2for supply to the speech circuit30. The diode bridge20is configured from four diodes D1to D4.

Hook switch40connects terminal1and terminal3at the time of off-hook. As a result, a forward direction voltage is applied across the base terminal B2and emitter terminal E2of the transistor Q2via resistor R21and the transistor Q2therefore turns on. As a result, the base potential of transistor Q1rises, and the transistor Q1also turns on. At the time of making a call, the base potential of transistor Q2is controlled via terminal HC using control of a microcomputer (not shown) so as to correspond to a dial input. As a result, the transistor Q1sends a dial pulse signal.

Hook switch40connects terminal1and terminal2at the time of on-hook. In doing so, the transistor Q1disconnects the speech circuit30and the telephone lines L1, L2.

SUMMARY

It is therefore necessary for this type of telephone interface circuit10to supply power from the telephone lines L1, L2to the speech circuit30.

However, at the time of internal calls where the telephone lines L1, L2are disconnected for use, a resistor R21in the order of a hundreds of ohms remains between the telephone lines L1, L2and the telephone lines L1and L2therefore cannot be completely disconnected.

The present invention therefore is advantageous in resolving the problem of providing a telephone interface circuit with a straightforward configuration that is capable of completely disconnecting telephone lines.

In order to resolve the aforementioned problems, a telephone interface circuit of the present invention comprises a speech circuit, a control circuit turning on upon receiving a supply of a forward direction trigger current so as to connect first and second telephone lines and the speech circuit, and going off upon receiving a supply of a reverse direction trigger current so as to disconnect the first and second telephone lines and the speech circuit, a capacitor charging and discharging charge supplied from the first telephone line, and a hook switch supplying charge accumulated at the capacitor when off-hook to the control circuit as a forward direction trigger current, and supplying charge discharged from the capacitor when on-hook to the control circuit as a reverse direction trigger current.

DETAILED DESCRIPTION

The following is a description with reference to the drawings of an embodiment of the present invention.

FIG. 2shows a circuit diagram of a telephone interface circuit70of this embodiment. Elements with the same numerals as numerals shown inFIG. 1are the same elements and detailed description thereof is therefore omitted.

A resistor R22, resistor R21and capacitor C20are connected in series across terminal1of hook switch40and base terminal B2of transistor Q2. The capacitor C20has a capacitance in the order of 0.01 μF to 1 μF. Terminal2of hook switch40is connected to earth. Terminal3of hook switch40is connected to emitter terminal E1of the transistor Q1. The potential of terminal2of the hook switch40is taken to be earth potential.

The collector terminal C1of the transistor Q1branches, with one branch connected to the speech circuit30and the other branch being connected to the positive feedback circuit50. The positive feedback circuit50has a resistor R20and a speed-up capacitor C22connected in parallel. The transistors Q1and Q2and the positive feedback circuit50function as a thyristor or a Schmitt trigger. Here, an example is shown of a case where the transistors Q1, Q2and the positive feedback circuit50function as a thyristor60. Namely, base terminal B2of the transistor Q2functions as a gate terminal G of the thyristor60, emitter terminal E2of the transistor Q2functions as a cathode terminal K of the thyristor60, and emitter terminal E1of the transistor Q1functions as an anode terminal A of the thyristor60.

Base terminal B2of the transistor Q2branches into three, with one branch being connected to the positive feedback circuit50, another branch being connected to earth via a Zener diode D23(in the order of 3V), and the remaining branch being connected to a microcomputer (not shown) via an RC circuit (a circuit containing a resistor R5, a resistor R4, and a capacitor C21).

Emitter terminal E2of the transistor Q2is connected to earth via a level shifting diode D20.

Next, the operation of the telephone interface circuit70is described.

At the time of off-hook, the hook switch40connects terminal1and terminal3. In doing so, a positive trigger voltage (transient direct current potential) is applied across base terminal B2and emitter terminal E2of the transistor Q2via the capacitor C20. This positive trigger voltage (referred to as a “forward direction trigger” for convenience) acts as a forward direction voltage across the base terminal B2and emitter terminal E2so as to cause the transistor Q2to go on. In doing so, the thyristor60that is in a forward blocking state causing the gate to go on so that a current begins to flow between the anode terminal A and the cathode terminal K. At this time, rise in the collector potential at transistor Q1causes the base potential of the transistor Q2to rise via the positive feedback circuit50and the on state of the thyristor60therefore continues. In particular, the positive feedback circuit50has a speed-up capacitor C22. This is therefore effective in causing operation so as to make the turn on time of the transistor Q2short and the thyristor60stable.

At this time, as a result of control exerted by a microcomputer (not shown), even if an internal call operation takes place, the direct current resistance of the capacitor C20is infinite in theory, and the telephone lines L1, L2can therefore be completely disconnected.

On the other hand, at the time of on-hook, the hook switch40connects terminal1and terminal2. In doing so, charge accumulated at the capacitor C20flows as a trigger current (referred to as a “reverse direction trigger” for convenience) so as to cause a reverse bias across the base terminal B2and emitter terminal E2of the transistor Q2. In doing so, the transistor Q2goes from being turned on to being turned off, and the thyristor60goes off.

Further, the Zener diode D23is for reliably discharging charge accumulated at the capacitor C20when on-hook. An anode of the Zener diode D23is connected to a positive plate of the capacitor20, and a cathode of the Zener diode D23is connected to a negative plate of the capacitor C20via the resistor R21.

Further, the level shifting diode D20is made to perform an on-hook operation, off-hook operation, or dial pulse transmission operation etc. by a microcomputer (not shown) and is provided so that the transistor Q2does not operate in an erroneous manner when the base potential of the transistor Q2is controlled.

However, the level shifting diode D20is not essential.

In the above description, an example of the thyristor60is shown as a control circuit for controlling opening and closing of connection between the telephone lines L1, L2and the speech circuit30but this control circuit is not limited to being the thyristor60. For example, application of a control circuit (for example, a Schmitt trigger) having a function for receiving supply of a forward direction trigger current, going to a turned on state, and connecting the telephone lines L1, L2and the speech circuit on the one hand, while on the other hand, receiving supply of a reverse direction trigger current, going to a turned off state, and disconnecting the telephone lines L1, L2and the speech circuit30as a control circuit for controlling the opening and closing of connection of the telephone lines L1, L2and the speech circuit30is also possible.

The telephone interface circuit70has a configuration supplying charge accumulated in a capacitor C20when off-hook to the thyristor60as a forward direction trigger current so as to cause the thyristor to go on and connect the telephone lines L1, L2and the speech circuit30on the one hand, and supplying charge discharged from the capacitor C20when off hook to the thyristor60as a reverse direction trigger current so as to turn off the thyristor60and disconnect the telephone lines L1, L2and the speech circuit30. It is therefore possible to make the direct current resistance of the telephone lines L1, L2theoretically infinite when off hook, and it is possible to control the thyristor60to be on or off using the forward direction trigger current and the reverse direction trigger current.