Using an optocoupler in a telephone ringer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Using an optocoupler in a telephone ringer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Using an optocoupler to transmit the ringing signal to a separate ringer prevents telephone line congestion, avoids misunderstandings with the telephone company, and maintains the line receiving the signal. By means of an optocoupler, other devices can also be switched on, such as a telephone message recorder. The ringer draws 10 mA of ringing current and almost no load on the line on DC voice signals.

In this circuit, the optocoupler transmits the call signal to the rest of the ringer, isolating it from the telephone line. The output current of the optocoupler drives a 555 type timer, which is connected in a self-oscillating multivibrator circuit. The audio frequency of the multivibrator is amplified and fed to a remote loudspeaker, which produces a sound signal when a call signal appears on the telephone network.

(click to enlarge)

As shown in the figure, a call signal with an amplitude of 100 V and a frequency of 20 Hz has an on time of 2 s and an off time of 4 s. This signal is applied to the light emitting diode of the optocoupler through a 1uF capacitor with a reactance of 10kΩ at 20Hz, which limits the LED current to 10mA. The switching frequency of 40 Hz in loudspeaker sound is more pleasant to hear than 20 Hz, so the ringing frequency is doubled by a full-wave bridge rectifier.

The output signal of the optocoupler with a frequency of 40 Hz is fed to the reset input of a multivibrator assembled on an integral timer of the 555 type. The natural frequency of the multivibrator is set to 440 Hz. This frequency, corresponding to the call signal in the handset, is the most convenient for the subscriber. The frequency of the multivibrator is controlled by a 250 kΩ potentiometer. The duty cycle of the multivibrator is set by a 1 kΩ resistor and is theoretically 50%, but in practice it is 35% due to modulation by the switching signal.

The output signal of the timer can be fed without additional amplification to a small loudspeaker through a series current-limiting capacitor. However, in some cases, this power may be insufficient. Switching amplifiers must be used to amplify the square wave output of the 555 timer. To obtain maximum efficiency and output power at currents less than 50 mA, the integrated timer must be used not as a source, but as a sink of current, so the switching amplifier is assembled on pnp transistors.

With the component parameters shown in the diagram, its circuit output power is 5,5W, which is close to the theoretical limit for a single 8Ω speaker, 12V Vcc, and 35% duty cycle. By using an amplifier with a higher gain or a loudspeaker with a lower impedance, a higher output power can be obtained. In this case, you can use several loudspeakers connected in parallel-series, with separate volume control by means of an L-shaped resistive circuit.

In idle mode, the circuit consumes 12 mW of power from the 120 V supply. This power can be reduced to zero by using a dual optocoupler. The second isolated and synchronous output is used to control the static switch on the triac, which turns on the power supply.

Even though the opto-coupler circuit puts little load on the line, you still need to notify the telephone company that you are installing a ringer.

Author: Kreingel Jr.; Publication: N. Bolshakov, rf.atnn.ru

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