Light indicator of phone calls. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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Sometimes there is a need for a light indication of incoming phone calls. The indicator may be required in situations where, for some reason, the ringing device of the telephone set is turned off or its volume is turned down; hearing-impaired people use the telephone; The unit is installed in a noisy environment such as a car garage, etc.
From other similar devices previously published in various literature, the proposed design is distinguished by the fact that several 220 V incandescent lamps can be used for indication. The applied circuitry solutions make it possible to avoid failure of subscriber telephones, communication lines and equipment of automatic telephone exchanges in the event of an accident.
The diagram of the device is shown in the figure.
(click to enlarge)
To isolate the input part of the device from the power unit, thyristor optocouplers U1 and U2 are used, which provide reliable isolation from the mains voltage. When a ringing signal is received, the alternating voltage through the current-limiting resistors R1, R2 and capacitors C1, C2 is supplied for rectification to the diode bridge VD1. The rectified voltage is filtered by capacitor C3. As a load, the LED HL1 and the emitting diodes of the optocouplers U1 and U1 are connected in series to the output of the diode bridge VD2.
When a ringing signal is received, the thyristor parts of the optocouplers open, a current begins to flow through the diode bridge VD2 and the control electrodes of the thyristors VS1, VS2, sufficient for the thyristors to open, which will be accompanied by the ignition of the EL1 lamp.
The ratings are selected in such a way that when a ringing signal is given, the lamp glows, slightly flickering, almost at full intensity, and when dialing a number, short flashes follow at about half the power. This mode does not create much inconvenience when using the telephone. If you need to completely eliminate the flickering of the lamp during dialing, you can install an additional switch in the telephone set, which would turn off the light signaling device from the telephone line when the handset is picked up. Capacitor C3 prevents accidental flashing of the lamp during short bursts on the telephone line.
Resistors can be used MLT, C2-23. Capacitors C1, C2 - K73-11, K73-17, C3 - K50-35. Small-sized diode bridges KTs422G can be replaced with KTs407A or rectifier diodes of the KD209, KD258 series (with indexes B-D), KD105 (B-G). Thyristor optocouplers AOU103V can be replaced with ZOU103 with indices A-B or AOU103B. The ZOU103G optocoupler, according to reference data, can withstand an output reverse voltage of up to 400 V. When it is used, two optocouplers can be replaced with one, in which case the voltage equalizing resistors R3 and R4 are no longer needed.
The device can use thyristors of the KU202 series with indices K, L, M, and with an incandescent lamp power of not more than 400 W - KU201L. It is not required to install thyristors on heat sinks.
Fuse FU1 is selected for a current approximately twice as much as the lamps consume. For example, if one 200 W lamp is connected as a load, then the fuse must be rated at 2 A.
Correctly assembled from serviceable parts, the call indicator starts working immediately.
No more than one indicator should be connected to the telephone line. If necessary, you can connect several incandescent lamps to the device by placing them in different rooms.
The applied optocouplers have an isolation voltage of 500 V. For more reliable protection against accidental breakdown, it is desirable to use optocouplers with an isolation voltage of 2,5 kV or more.
Author: A.Butov, village of Kurba, Yaroslavl region
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