JPH0256872B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a public telephone set which prevents incoming calls from being disabled even if the handset is lifted immediately upon arrival of a bell signal for an incoming call.

FIG. 1 shows a schematic configuration of a conventional public telephone.

In the figure, L1 and L2 are call line terminals connected to the exchange, and 1 is contact a when the handset is hooked up.
2 is a loop control circuit that determines charge input and controls the formation of a call loop to enable a call, and 3 is a call circuit. be. A bell 4, which rings in response to a bell signal coming from the exchange for incoming calls, is connected between the contact a of the switch 1 and the line terminal L2 via a DC blocking capacitor 5. A bell signal detection circuit 6 is connected to both ends of the bell 4 for detecting a bell signal and outputting a detection signal when the handset is raised. Connected to the bell signal detection circuit 6 is a control circuit 7 which receives a detection signal from the bell signal detection circuit 6 and outputs a control signal to the loop control circuit 2 for forming a communication loop.

The operation of the above configuration will be explained.

Normally, a DC voltage from an exchange is applied between call line terminals L1 and L2. When the handset is hooked up, the hook switch 1 is connected to the a side. In this state, direct current is blocked by the capacitor 5.

To make a normal call, first raise the handset. Then, the control switch 1 is switched to the b side. Next, insert money. Then, the loop control circuit 2 forms a call loop. After dialing, if the called party answers, call circuit 3
It becomes possible to make a telephone call.

In the case of an incoming call, an AC signal (bell signal) for ringing the bell 4 arrives from the exchange side (as shown in FIG. 2). This AC signal passes through the capacitor 5 and reaches the bell 4 and the bell signal detection circuit 6. The bell 4 thus rings, and the bell signal detection circuit 6 detects the arrival of the bell signal and outputs a detection signal to the control circuit 7 when the handset is raised. For this reason, the loop control circuit 2 enables the formation of a communication loop by means of a control signal from the control circuit 7. When I heard bell 4 ring, I raised the handset and switched to switch 1.
switches to side b, and bell 4 stops ringing. Then, the exchange detects this lifting of the handset and stops transmitting the bell signal, making it possible to make a call.

Furthermore, as shown in Figure 2, the bell signal is inactive.
It will arrive after T _r . For this reason, the bell signal detection circuit 6 is configured to memorize the previous arrival during the rest period so that a detection signal is output even if the handset is put up during the rest period T _r .

Therefore, the bell signal detection circuit 6 detects the ringing of the bell 4 due to the arrival of the bell signal after the arrival of the bell signal.
It is designed so that it can be detected only after a predetermined delay time t _d (several 100 ms).

The reason for this is as follows.

When the handset is hung up at the end of a call, etc., when the switch 1 is switched from the B side to the A side, the capacitor 5 is charged by the DC voltage applied between the line terminals L1 and L2. For this reason,
A charging current flows instantaneously (approximately 90ms) to bell 4, and bell 4 rings once (so-called tingling). Therefore, if the bell signal detection circuit 6 is designed to detect the bell signal immediately after the arrival of the bell signal, the bell signal detection circuit 6 will operate even when the handset is hooked up, since it will also operate with this charging current when the handset is hooked up. This will result in a misjudgment that a bell signal has arrived. Therefore, even if a bell signal does not arrive, by moving the hook on the handset, a call can be made without inserting money in the same way as when making an incoming call. Therefore, in order to eliminate this problem, the system is designed to detect the arrival of the bell signal after a predetermined delay time _td , which is longer than the time during which the charging current flows due to the handset hook.

However, in this case, if the handset is raised immediately after the bell signal arrives and the bell 4 starts ringing (before the delay time t _d of the bell signal detection circuit 6), the bell signal detection circuit No detection signal is output from 6, and the arrival of the bell signal also stops when the handset is raised. For this reason, the control circuit 7 does not operate and no loop formation is performed, resulting in the inconvenience that incoming calls cannot be made.

The object of the present invention is to provide a public telephone set that overcomes the above-mentioned drawbacks.

In order to achieve this object, the gist of the present invention is to prevent the bell from ringing during the delay time of the bell signal detection circuit even if a bell signal arrives.

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 3 is a block diagram showing the configuration of a public telephone according to an embodiment of the present invention.

In the figure, L1 and L2 are line terminals, 1 is a switch, 2 is a loop control circuit, 3 is a communication circuit, 4 is a bell, 5 is a capacitor, 6 is a bell signal detection circuit,
Reference numeral 7 denotes a control circuit, which is similar to the conventional example shown in FIG. Bell signal detection circuit 6 and line terminal L
2 and one end of the bell 4, _{there is} a delay time t d of the bell signal detection circuit 6 from the arrival of the bell signal.
A bell ringing prevention circuit 10 is connected for preventing the ringing of the bell 4 for a larger preset time T.

FIG. 4 shows a specific circuit configuration of the bell ringing prevention circuit 10.

In the figure, 11 is a rectifier that rectifies the AC bell signal, 12 is an FET whose drain is connected to one end of the resistor R1 and the gate is connected to the collector of the transistor 13, and 13 is a capacitor 16 charged by the bell signal. This is a transistor that becomes conductive when the voltage exceeds a certain voltage. 14 has an anode connected to the other end of resistor R1, and a gate connected to FET12.
15 is a Zener diode, 16 is a capacitor that is charged by the bell signal through resistor R1, FET 12, resistor R2, and diode 17. . 17 is a diode, and R1 to R5 are resistors. resistor R3,
R4 forms a discharging circuit for the capacitor 16, and as will be described later, the rest period T _r (see Figure 2) of the bell signal, in which the charge charged in the capacitor 16 by the bell signal is slowly discharged and intermittent, is a predetermined period. A time constant is set so that the voltage does not drop below the voltage. The diode 17 is connected to the capacitor 16 by resistors R2 and R5.
This is to prevent discharge from occurring through the The resistance value of the resistor R1 is set to a large value such that when the bell signal passes through the resistor 1, the current flowing through the bell 4 is lower than the current impressed by the bell.

Next, the operation of the above embodiment will be explained.

Since the case of a normal telephone call is the same as the conventional example shown in FIG. 1, the explanation will be omitted, and the case of an incoming call will be explained.

When a bell signal for an incoming call arrives from the exchange side, it passes through the capacitor 5 and arrives at the series circuit of the bell 4 and the bell ringing prevention circuit 10.
The signal arrives at the bell signal detection circuit 6.

The bell signal detection circuit 6 detects the bell signal after a predetermined delay time _td from the arrival of the bell signal.

At the time the bell signal arrives, the capacitor 16 of the bell ringing prevention circuit 10 shown in FIG. 4 has not yet been charged. Therefore, the base current of transistor 13 is not supplied, so transistor 13 is cut off. Since the transistor 13 is cut off, the gate and source of the FET 12 are at the same potential, and the FET 12 is in a conductive state. The thyristor 14 is therefore cut off since it is not supplied with gate current.

In this way, FET 12 conducts and thyristor 14
is cut off, the incoming bell signal (alternating current) is rectified by the rectifier 11, and the following is established: Rectifier 11 (+) - Resistor R1 - FET 12 - - Resistor R2 - Diode 17 - Capacitor 16 - | | | | ｜ ｜ -------- Zener diode 15 ------- - The capacitor 16 is gradually charged through the rectifier 11 (-) path. Since the resistance value of resistor R1 is large, the alternating current flowing through bell 4 is less than the current of the bell, and during this time,
Bell 4 does not ring.

After a time T which is longer than the delay time _td of the bell signal detection circuit 6, the voltage of the capacitor 16 reaches a predetermined voltage or higher. Therefore, the base current exceeds a predetermined value and transistor 13 becomes conductive. transistor 13
Due to the conduction of FET 12, the gate of FET 12 becomes a negative voltage with respect to the source, and FET 12 is cut off. FET
When 12 is cut off, a gate current is supplied to thyristor 14, and thyristor 14 is turned on. In this way, FET 12 is cut off and thyristor 14 is turned on, so the bell signal is transmitted to thyristor 1.
4. Flows through the Zener diode 15.

While the bell signal continues, the potential of the capacitor 16 is held by the Zener voltage of the Zener diode 15, so the transistor 13 remains conductive, so the FET 12 is cut off and the thyristor 14 is turned on. maintained.
Furthermore, as shown in Fig. 2, the bell signal repeats intermittent cycles until the handset is raised, but during the rest period T _r of the bell signal, the voltage stored in the capacitor 16 is passed through the resistors R3 and R4. Discharged. However, since the time constant is set so that the slow discharge takes place in a time slightly longer than this rest period, the transistor 13 remains conductive even during the rest period of the bell signal. Therefore, FET12 is cut off, and thyristor 1
4 remains turned on.

When the FET 12 is cut off and the thyristor 14 is turned on, the bell signal does not pass through the resistor 1, resulting in low impedance, and the alternating current flowing through the bell 4 is larger than the bell's moving current, causing the bell 4 to ring. Therefore, the bell 4 is prevented from ringing for the time T after the bell signal, and thereafter rings as the bell signal is interrupted.

In this way, the ringing of the bell 4 starts when the time T, which is longer than the delay time _td of the bell signal detection circuit 6, has elapsed after the arrival of the bell signal, so the handset is raised immediately after the ringing of the bell 4. However, the arrival of the bell signal is detected by the bell signal detection circuit 6, and the detection signal is output from the bell signal detection circuit 6 to the control circuit 7. The control signal from the control circuit 7 causes the loop 2 to stop the call. It allows the formation of a loop, thus allowing incoming calls.

As mentioned above, the diode 17 is for preventing the electric charge of the capacitor 16 from being discharged through the resistors R2 and R5 when the transistor 13 is kept in a conductive state. It is not necessary if the values of R2 and R5 are sufficiently large. Further, the Zener voltage of the Zener diode 15 is determined by the gate-source voltage for shutting off the FET 12, and may be 2 to 3V.

Also, when the bell 4 rings, the bell ringing prevention circuit 10
The voltage drop caused by this is very low, so the ringing of the bell will hardly be reduced. Conversely, the Zener voltage of the Zener diode 15 slightly increases the input voltage of the bell signal detection circuit 6, making it easier for the bell signal detection circuit 6 to detect the bell signal.

In addition, by hooking up the handset, the capacitor 5 is instantaneously charged due to the DC voltage applied between the line terminals L1 and L2. At this time, current momentarily flows through the bell 4 and the bell ringing prevention circuit 10. but,
As described above, since the FET 12 is conductive and the thyristor 14 is cut off, this current flows through the path of the resistor R1, the FET 12, the resistor R2, the diode 17, and the capacitor 16. Therefore, due to the large resistance value of the resistor R1, the current passing through the bell 4 is less than the bell's moving current, and the bell 4 does not tingle due to the instantaneous current.

It should be noted that the present invention is not limited to the configuration of the above embodiment, and it goes without saying that various modifications can be made to the configuration of each part. For example, transistors or other switching elements may be used instead of the FET 12, thyristor 14, etc. It is clear that it is good.

Since the public telephone of the present invention is configured as described above, (1) even if the handset is raised immediately after the bell 4 starts ringing, the inability of the bell signal detection circuit 6 to detect the arrival of a bell signal can be prevented. can.

(2) It is possible to prevent the bell 4 from ringing due to the handset being hung.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the configuration of a conventional public telephone, Fig. 2 is a waveform diagram showing a bell signal, Fig. 3 is a block diagram showing the configuration of an embodiment of the present invention, and Fig. 4 is a block diagram showing the configuration of a conventional public telephone.
FIG. 2 is a block diagram showing a specific configuration of the bell ringing prevention circuit 10. As shown in FIG. L1, L2...Line terminal, 1...Hook switch, 2...Loop control circuit, 3...Telephone circuit,
4... Bell, 5... Capacitor, 6... Bell signal detection circuit, 7... Control circuit, 10... Bell ringing prevention circuit, 11... Rectifier, 12... FET, 13
...Transistor, 14...Thyristor, 15...
... Zener diode, 16 ... Capacitor, 1
7...Diode, R1-R5...Resistor.

Claims (1)

[Claims] 1. A bell provided between telephone line terminals via a switch and ringing in response to a bell signal arriving from the telephone line, and a direct current blocking capacitor connected in series with the bell to prevent direct current from flowing through the bell; a bell signal detection circuit that detects the arrival of a bell signal at the bell and outputs a detection signal after a predetermined time has elapsed since the arrival of the bell signal; and a loop control means that receives the detection signal and forms a call loop. A public telephone set comprising: a bell ringing prevention circuit that is connected in series with the bell and prevents the ringing of the bell until a detection signal is output from the bell signal detection circuit.