IES65388B2 - A ringing voltage trigger circuit - Google Patents

Abstract

A trigger circuit for applying a voltage to a ring supply pin (2) of a speech chip (3) for activating a ringer (11) of a system phone in response to a ringing signal being applied to a telephone line (4) from a PBX. The trigger circuit (1) comprises first and second bridge rectifiers (8) and (9). The ring supply pin (2) is fed through a transistor (T1) from the second rectifier circuit (9). A smoothing capacitor (C4) biases the base of the transistor (T1) through a potential divider (R1,R2) for switching on the transistor (T1) on a ringing signal appearing on the telephone line (4). A voltage control circuit (12) provided between the transistor (T1) and the pin (2) which comprises a smoothing capacitor (C3) and a zener diode (ZD1) regulate and maintain the voltage being applied to the pin (2) at a level sufficient to hold the pin 2 on for so long as a ringing signal is being applied to the telephone line (4).

Description

A ringing voltage trigger circuit The present invention relates to a trigger circuit, and in particular, to a trigger circuit for connecting to an input ring supply pin of a speech chip of a system phone, whereby the trigger circuit is responsive to a ringing voltage on a telephone line from a PBX for applying a voltage to the ring supply pin of the speech chip for activating the speech chip to deliver a ringing voltage signal to a ringer of the system phone.

In general, such circuits require relatively high power components for coupling the trigger circuit to the two wires of a telephone line which carry the ringing voltage signal. Additionally, the components of the trigger circuit for applying the ringing voltage signal to the ring supply pin of the speech chip are also, in general, relatively high power components. This, thus leads to significant expense, as well as complex circuitry, and additionally, the components, in general are unsuitable for assembly using surface mount techniques .

There is therefore a need for a trigger circuit which overcomes these problems .

The present invention is directed towards providing such a trigger circuit.

According to the invention there is provided a trigger circuit for applying a continuous DC voltage to a ring supply pin of a speech chip of a telephone in response to an alternating ringing voltage signal on a telephone line supplying the telephone, the trigger circuit comprising a first rectifying means for rectifying the ringing signal, a DC blocking coupling means for coupling the first rectifying means to a pair of wires of the telephone line carrying the ringing signal, a switch means for switching a DC voltage to the ring supply pin in response to a ringing signal on the telephone line, a first smoothing capacitor for receiving a DC supply from the first rectifying means for switching the switch means for applying the DC voltage to the ring supply pin on a ringing signal being on the telephone line, and a voltage control circuit comprising a second smoothing capacitor cooperating with the feed from the switch means to the ring supply pin for maintaining the continuous DC voltage at a level sufficient to maintain the ring supply pin on for as long as the ringing signal remains on the telephone line.

Preferably, the switch means comprises a transistor, the base of which is connected to the DC supply from the first rectifying means, the supply on the base of the transistor being smoothed by the first smoothing capacitor. Ideally, a potential divider is provided across the first smoothing capacitor for reducing the voltage of the DC supply applied to the base of the transistor.

In one aspect of the invention the DC voltage which is switched by the switch means to the ring supply pin of the speech chip is derived from the ringing signal on the telephone line. Preferably, the DC voltage which is switched by the switch means to the ring supply pin is derived from the telephone line through a second rectifying means. Preferably, the second rectifying means is provided by a bridge rectifier comprising diodes .

Ideally, the voltage control circuit comprises a limit means for limiting the voltage applied to the ring supply pin. Preferably, the limit means comprising a zener diode connected in parallel with the second smoothing capacitor.

In one aspect of the invention the first rectifying means is provided by a bridge rectifier comprising diodes.

The invention will be more clearly understood from the following description of a preferred embodiment thereof which is given by way of example only, with reference to the accompany drawing which illustrates a circuit diagram of a ringing voltage trigger circuit according to the invention.

Referring to the drawing there is illustrated a trigger circuit according to the invention indicated generally by the reference numeral 1 for applying a substantially constant continuous DC voltage to a ring supply pin 2 of a speech chip 3 in a system telephone (not shown) in response to a ringing signal being present on a telephone line 4 feeding the system phone from a private branch exchange (PBX) (not shown). The speech chip 3 comprises an output ringer pin 10 which delivers a signal to a tone ringer 11 of the system phone for the duration of the ring supply pin 2 being on, in other words, while the voltage on the ring supply pin 2 exceeds approximately 20 volts, thereby, causing the tone ringer 11 to ring. The telephone line 4 comprises a pair of wires 5 and 6 to which the ringing signal is applied by the PBX. In this embodiment of the invention the ringing signal is a square wave alternating signal which alternates between -70 volts and +70 volts. A first rectifying means for rectifying the ringing voltage is provided by a first bridge rectifier 8 which comprises diodes DI, D2, D3 and D4, and is coupled to the two wires 5 and 6 by a DC blocking coupling means provided by a pair of capacitors Cl and C2. A second rectifying means provided by a second bridge rectifier 9 also for rectifying the ringing voltage is connected directly to the wires 5 and 6 of the telephone line 4 and comprises diodes D5 to D8. The DC voltage supply from the second bridge rectifier 9 is applied to the ring supply pin 2 of the speech chip through a switch means, namely, a transistor Tl which is responsive as will be described below to a ringing signal appearing on the telephone line 4. A voltage control circuit 12 maintains a continuous voltage on the ring supply pin 2 of sufficient value to keep the output ringer pin 10 active while the transistor Tl is conducting. This is described in more detail below.

A first smoothing capacitor C4 which is connected across the DC supply from the first bridge rectifier 8 biases the base of the transistor Tl for switching on the transistor Tl during the presence of a ringing signal on the telephone line 4. A potential divider circuit comprising resistors Rl and R2 apply a divided voltage across the capacitor C4 to the base of the transistor Tl. The values of the potential divider resistors Rl and R2 are chosen to avoid any danger of the transistor T1 being switched on by signals on the telephone line 4 other than a ringing signal, for example, the values of the resistors R1 and R2 are chosen to avoid any danger of a speech signal on the telephone line 4 switching on the transistor Tl. The value of the first smoothing capacitor C4 is chosen so that the transistor Tl is held on for so long as a ringing signal is on the telephone line 4.

The voltage control circuit comprises a second smoothing capacitor C3 and a zener diode ZD1. The value of the second smoothing capacitor C3 is chosen so that on being charged through the transistor Tl, the capacitor C3 maintains a voltage on the ring supply pin 2 which at all times for so long as the transistor Tl is conducting is at a voltage sufficient to maintain the ring supply pin 2 on. In this case, as mentioned above, the capacitor C3 maintains the voltage on the ring supply pin 2 at a value just over 20 volts. The zener diode ZD1 limits the value of the voltage across the capacitor C3 to prevent the voltage on the capacitor C3 exceeding an excessive voltage for avoiding damage to the speech chip 3.

A diode D9 connected between the transistor Tl and the voltage control circuit 12 prevents damage to the transistor Tl as a result of discharge from the second smoothing capacitor C3. A resistor R3 is located between the transistor T1 and the diode D9 for current limiting purposes. A resistor R4 between the negative terminals of the first and second bridge rectifiers 8 and 9, respectively, and the ring supply pin 2 of the speech chip 3 provides a discharge path for the smoothing capacitors C3.

In this embodiment of the invention the values of the capacitors C3 and C4 are lOnF and lOOnF, respectively. The value of the resistors R1 and R2 are IK and 220K, respectively. The values of the resistors R3 and R4 are 3K3 and 2M2, respectively. The values of the capacitors Cl and C2 can be significantly lower than would be normal in a conventional telephone circuit, and in this case, the values of the capacitors Cl and C2, are each lOOnF.

In use, on a ringing signal being applied to the two wires 5 and 6 of the telephone line 4, the voltage across the first smoothing capacitor C4 rises until the voltage between the resistors Rl and R2 which is applied to the base of the transistor T1 is such as to switch on the transistor Tl. The second smoothing capacitor C3 of the voltage control circuit 12 is charged from the rectified voltage supply from the second bridge rectifier 9. On the voltage across the second smoothing capacitor C3 exceeding approximately 20 volts, the speech chip 3 is activated to output a ringing signal on the ringer pin 10 to the ringer 11 of the system phone. The zener diode ZD1 limits the voltage being applied to the ring supply pin 2 to 30 volts or less, thereby avoiding damage to the speech chip 3. The second smoothing capacitor C3 remains charged for so long as the ringing signal remains on the telephone line 4, thereby maintaining the voltage between just over 20 volts and 30 volts on the ring supply pin 2 which continues to operate the speech chip 3 for generating a ringing voltage signal for delivery to the ringer 11. On the ringing signal being removed from the wires 5 and 6, the voltage across the first smoothing capacitor 4 drops, and in turn, the voltage applied to the base of a transistor T1 drops below a level sufficient to maintain the transistor T1 switched on, and thus, the transistor T1 is switched off. At this stage the second smoothing capacitor C3 discharges below 20 volts, thereby removing the signal from the ring supply pin 2. Once the voltage on the ring supply pin 2 drops below 20 volts, the speech chip 3 no longer transmits a ringing signal to the ringer 11 thereby terminating ringing.

The advantages of the invention are many. By virtue of the fact that relatively low value capacitors Cl and C2 can be used for coupling the first rectifier to the wires of the telephone line the circuit can be manufactured from significantly lower cost components. Such components are suitable for automatic insertion, and are relatively low power components, thus facilitating the manufacture of the circuit using surface mount techniques. This is also true of the other components of the trigger circuit.

Because the coupling capacitors Cl and C2 are of low 10 value, the trigger circuit is significantly more resistive than normal, and because of this the circuit draws less peak current from the square wave form, and this in turn leads to a relatively undistorted wave form and a clearer ringing signal.

The invention is not limited to the embodiment hereinbefore described which may be varied in construction and detail.

Claims (5)

1. A trigger circuit for applying a continuous DC voltage to a ring supply pin of a speech chip of a telephone in response to an alternating ringing voltage signal on a telephone line supplying the telephone, the trigger circuit comprising a first rectifying means for rectifying the ringing signal, a DC blocking coupling means for coupling the first rectifying means to a pair of wires of the telephone line carrying the ringing signal, a switch means for switching a DC voltage to the ring supply pin in response to a ringing signal on the telephone line, a first smoothing capacitor for receiving a DC supply from the first rectifying means for switching the switch means for applying the DC voltage to the ring supply pin on a ringing signal being on the telephone line, and a voltage control circuit comprising a second smoothing capacitor cooperating with the feed from the switch means to the ring supply pin for maintaining the continuous DC voltage at a level sufficient to maintain the ring supply pin on for as long as the ringing signal remains on the telephone line.

2. A trigger circuit as claimed in Claim 1 in which the switch means comprises a transistor, the base of which is connected to the DC supply from the first rectifying means, the supply on the base of the transistor being smoothed by the first smoothing capacitor.

3. A trigger circuit as claimed in Claim 1 or 2 in which the DC voltage which is switched by the switch means to the ring supply pin of the speech chip is derived from the ringing signal on the telephone line through a second rectifying means.

4. A trigger circuit as claimed in any preceding claim in which the voltage control circuit comprises a limit means for limiting the voltage applied to the ring supply pin.

5. A trigger circuit substantially as described herein with reference to and as illustrated in the accompanying drawing.