WO1996002103A1

WO1996002103A1 - A telephone interface circuit

Info

Publication number: WO1996002103A1
Application number: PCT/SG1995/000010
Authority: WO
Inventors: Ee Theow Lau
Original assignee: Ee Theow Lau
Priority date: 1994-07-11
Filing date: 1995-06-21
Publication date: 1996-01-25
Also published as: GB9413933D0

Abstract

A circuit comprising a handset off-hook detector, a ring detector and a line unlooped detector with a combined impedance up to the mega-ohm range, designed for parallel connection to a common telephone line with its high impedance.

Description

A TELEPHONE INTERFACE CIRCUIT

This invention relates to circuits used in telephone equipment to interface with a common telephone line. Many telephone equipment, such as a PABX system or a feature -phone, have circuitry to detect various events such as ring, line unlooped and handset off- hook. This interface circuit will usually send the detected signals to a microcontroller for some particular control action.

Until now, many circuits have used an opto-coupler as the detecting device. The common opto-coupler has a LED (Light Emitting Diode) and a photo transistor assembled in one package. When current is passed through the LED, the photo transistor will conduct, and a signal would thus be detected. Suitable resistors, capacitors or diodes are connected with the opto- couplers to meet the voltage requirement of the circuit. The opto-coupler device needs a large amount of current to operate. A relay or capacitor is usually used to cut off or block the DC voltage in order not to draw excessive current from the telephone line. Further, it is not practicable to connect many such interface circuits in parallel as too much current will be drawn from a telephone line.

According to the present invention there is provided a telephone interface circuit comprising a ring detector means, a line unlooped condition detector means and a phone handset off-hook detector means, with the said interface circuit means having a high impedance characteristic up to the mega-ohm range and is designed for parallel connections to a common telephone line with its high impedance, and where the said phone handset off-hook detector means has a double transistor detector means for working with any voltage polarity of a telephone line, and the high resistor means for blocking any noticeable voice signal to reach the handset.

The present invention does not use an opto-coupler as the detecting device and thus is not hampered by the limitation as mentioned above. It is essentially an interface circuit with a very high impedance. More than one of the preferred embodiment circuits can be connected to the same telephone line without drawing a substantially large amount of current from it. The circuit will detect and provide separate signals for events of ring, line unlooped, and handset off-hook. A bypass relay is provided for direct connection of a telephone device to the telephone line. The preferred embodiment of the invention is now described with reference to the accompanying drawing.

Figure 1 shows the complete interface circuit and its connection between the telephone line 21, 22 and the telephone device 12. In Figure 1 the micro-controller 11 is shown for illustration purposes. A handset off-hook detector consists of two transistors Ql and Q2 as shown in the drawing. They are connected in series with the associated components, namely resistors Rl, R2, R3, R4, diodes Dl, D2 and transistors Q4 and Q5. When the handset from the telephone device 12 is off its hook, current flowing through these components will cause either transistor Ql or Q2 to be turned on, depending on the voltage polarity of the telephone line 21 and 22. Resistors Rl and R4 are of substantially high values so that no noticeable voice signal can be heard at the handset. When either transistor Ql or Q2 is turned on, it will cause current to flow through resistor R10, and diode D3 to transistor Q3. Transistor Q3 will further amplify the signal current and send a detected off-hook signal 24 to the micro- controller 11.

Either transistor Q4 or Q5 will be switched on only when the handset is off-hook, otherwise it will remain in a switched off state. Together with diodes Dl, D2, D3 and D4, they will prevent current from the ring and line unlooped detectors from affecting the proper detection of the handset off-hook condition, and vice versa. A relay 10 is connected between telephone lines 21 and 23 so that, by means of the signal 27, the micro-controller can connect the telephone device 12 directly to the telephone line 21, 22.

Bridge rectifier 13 provides the proper voltage polarity for the line unlooped and ring detectors. Transistor Q6 is used for line unlooped detections. Zener diode Zl has a preferred voltage value of 28 volts. A voltage significantly above 28 volts across telephone lines 21, 22 will thus cause transistor Q6 to be switched on providing a line unlooped signal 26.

A transistor Q7 is used for ring detections. It has another Zener diode Z2, also of a preferred value of 28 volts, connected in series with the resistor R16 to the base of transistor Q7. The combined voltage value of zener diodes Zl and Z2 will cause transistor Q7 to detect any voltage significantly above 56 volts as a ringing condition and provides a ring signal 25.

Resistor R19 which is connected in series with the detector circuit, has a preferred value of one mega- ohm. With the other resistors R14, R16 and R18 in series connection, this will result in the circuit having a very high impedance to the telephone line. It will of course be realised that whilst the above has been given by way of illustrative example of this invention, all such and other modifications and variations hereto, as would be apparent to persons skilled in the art, are deemed to fall within the scope and ambit of this invention.

Claims

1. A telephone interface circuit comprising a ring detector means, a line unlooped condition detector means and a phone handset off-hook detector means, with the said interface circuit means having a high > impedance characteristic up to the mega-ohm range and is designed for parallel connections to a common telephone line with its high impedance, and where the said phone handset off-hook detector means has a double transistor detector means for working with any voltage polarity of a telephone line, and the high resistor means for blocking any noticeable voice signal to reach the handset.

2. A telephone interface circuit as claimed in claim 1, wherein an additional two transistor means and diodes means provide a voltage switch off means which will switch off current from the telephone line if the handset is not off-hook.

3. A telephone interface circuit as claimed in any preceding claim wherein a transistor amplifier means is provided to amplify any signal from the said double transistor detector means.

4. A telephone interface circuit as claimed in any preceding claim wherein a bypass means is provided to connect signal voltage from a telephone line to a telephone device.

5. A telephone interface circuit as claimed in any preceding claim, wherein a bridge rectifier means is used to provide proper voltage polarity for the detector circuit means comprising a ring detector means and a line unlooped detector means.

6. A telephone interface circuit as claimed in any preceding claim, wherein the ring detector means and the line unlooped detector means use Zener diode means and high value resistor means to detect the voltages on the telephone line as being in unlooped or ringing conditions.

7. A telephone interface circuit as claimed in any preceding claim, wherein the ring detector means and unlooped detector means use diode means as polarity guard to the ground.

8. A telephone interface circuit substantially as described herein with reference to Figure 1 of the accompanying drawings.