JP2848084B2 - Off-hook detection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an off-hook detecting device for reliably detecting the presence or absence of an off-hook in a branch circuit using a photocoupler.

[0002]

2. Description of the Related Art In a branch circuit using a photocoupler,
An off-hook detection circuit is provided to detect the presence or absence of off-hook of the branch phone. However, if the photocoupler itself has characteristic variations, the presence or absence of the off-hook cannot be detected with high accuracy. For this reason, conventionally, a photocoupler that falls within a characteristic region where there is no problem in use is selected to constitute a branch circuit.

[0003]

However, in such a conventional branch circuit, the above-mentioned operation of selecting the photocoupler is difficult, and a large error occurs in the detection result of the presence or absence of the off-hook. There was a practical problem that it could not be performed.

The present invention has been made in view of the above-described problems, and provides an off-hook detecting device capable of detecting the presence or absence of an off-hook of a branch phone with high accuracy, irrespective of variations in characteristics of a photocoupler. The purpose is to do.

[0005]

According to the present invention, there is provided an off-hook detecting device including a transistor having a base connected to a collector of a phototransistor in a photocoupler and inputting a signal corresponding to the presence or absence of an off-hook to the off-hook detecting circuit. A series circuit of a resistor and a diode for absorbing a fluctuation in the collector potential of the phototransistor is connected between a DC power supply connected to the collector of the transistor and the collector of the phototransistor.

[0006]

The series circuit of the resistor and the diode according to the present invention compensates for the variation of the current flowing through the phototransistor in accordance with the magnitude of the current due to the variation in the characteristics of the photocoupler. Is reliably taken in, so that the off-hook detection by the off-hook detection circuit can be detected with high accuracy irrespective of the variation.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, reference numeral 1 denotes a telephone lead-in, to which a transformer 2 for connecting a telephone is connected, and a circuit on a branch phone side is a light emitting element 4 of a photocoupler 3 for circuit insulation.
Connected through. Reference numeral 5 denotes a phototransistor serving as a light receiving element of the photocoupler 3, whose emitter is grounded, and whose collector is connected via a resistor 7 to the base of a transistor 6 for detecting off-hook.

The transistor 6 has a collector connected to the DC power supply, and an emitter connected to an input-side voltage divider (voltage detector) in an off-hook detector 8 connected to a microprocessor or the like.

Reference numeral 9 denotes a series circuit of a resistor 10 and two diodes 11 and 12, which are connected between the power supply and the collector of the phototransistor 5. Acts so as to absorb the change in the collector potential.

Next, the operation will be described. Now, when an off-hook signal of the branch phone is input through the drop line 1, the off-hook signal is photoelectrically converted by the photocoupler 3. Therefore, the phototransistor 5 is turned off by the input of the off-hook signal, and the base potential of the transistor 6, which has been biased via the resistor 10, the series circuit 9 of the diodes 11, 12 and the resistor 7, increases.

Therefore, the emitter potential of the transistor 6 rises to a high level. The increased emitter potential is input to the input-side voltage dividing circuit of the off-hook detection circuit 8, which detects that the voltage has changed, and detects off-hook of the branch phone. On the other hand, when the off-hook signal is not input, it is detected that no off-hook signal is input because a current constantly flows through the phototransistor 5 and no voltage change occurs in the input side circuit portion of the off-hook detection circuit 8. Is done.

As described above, the characteristics of the photocoupler 3 usually vary, and if this variation is large, the off-hook detection by the photocoupler 3 may be erroneous. For this reason, a photocoupler 3 having such a degree of variation that does not hinder practical use is usually selected and used. For example, as shown in FIG. 2, the relationship between the current (for example, 20 mA to 100 mA) flowing through the phototransistor 5 and its collector potential (for example, 5 V) is changed from a dotted line P to a dotted line Q.
The photocoupler 3 is used in a region sandwiched between the two. Therefore, the detection accuracy of the off-hook varies within this region.

On the other hand, according to the present invention, when the current flowing through the phototransistor 5 in the series circuit 9 of the resistor 10 and the diodes 11, 12 is small, the diodes 11, 1
2, the current is compensated. On the other hand, when the current is large, the resistor 10 prevents the photocoupler 3 from saturating due to an unnecessarily large current flowing through the photocoupler 3. This allows
The base potential of the transistor 6 is stabilized. For this reason,
Off-hook detection accuracy is improved. Therefore, even if the characteristics of the phototransistor 3 vary within the region shown in FIG. 2, the voltage and current characteristics of the phototransistor 5 can be changed by the solid line R as shown in FIG.
Always exist on the stability curve shown by.

In the above embodiment, since two diodes are provided in series, appropriate characteristics as shown by the solid line R in FIG. 2 can be obtained. However, according to experiments, when one diode is used, The characteristic curve R becomes a curve approaching the broken line Q by increasing its slope. From this, it can be said that one diode may be provided to obtain the effect peculiar to the present invention, but it is desirable to provide two diodes.

[0015]

As described above, according to the present invention, a transistor is connected to the collector / base of a phototransistor in a photocoupler and inputs a signal corresponding to the presence or absence of an off-hook to the off-hook detection circuit. Since a series circuit of a resistor and a diode for absorbing a change in the collector potential of the phototransistor is connected between the DC power supply to which the collector is connected and the collector of the phototransistor, the characteristics of the phototransistor vary. However, an effect is obtained that off-hook detection can be performed reliably and with high reliability.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an off-hook detection device according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a current flowing through a phototransistor according to the present invention and a collector potential thereof, in comparison with the related art.

[Description of Signs] 1 Drop-in line 3 Photocoupler 5 Phototransistor 6 Transistor 8 Off-hook detection circuit 9 Series circuit 10 Resistance 11 and 12 Diode

Claims (1)

(57) [Claims] 1. A photocoupler connected to an incoming line of a telephone for detecting a voltage change of the incoming line due to off-hook of a branch phone, and an off-hook detecting circuit for judging an output level of the photocoupler to detect the presence or absence of the off-hook. In a branch phone off-hook detecting device comprising: a transistor having a base connected to a collector of a phototransistor in the photocoupler, and a signal according to the presence or absence of an off-hook input to the off-hook detection circuit; and a collector of the transistor. An off-hook detection device, comprising: a series circuit of a resistor and a diode connected between a DC power supply connected thereto and a collector of the phototransistor to absorb fluctuations in the collector potential of the phototransistor.