Melodic call. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Calls and audio simulators
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On the pages of popular publications, including the magazine "Radio", many descriptions of simple designs of electronic calls were published. As a rule, their basis was a tone generator (an audio frequency generator), which was manipulated by special devices when the bell button was pressed. As a result, an alternation of tones or an imitation of any sounds was created. Unfortunately, the tone generators in such designs were built according to the circuits of symmetrical or asymmetrical multivibrators or RC generators, the oscillation shape of which differed from the sinusoidal one, which means that the naturalness of the ringing sound left much to be desired.
In the proposed melodic ring (see figure), an LC generator of sinusoidal oscillations, made on a VT4 transistor, is used as a tone generator. The generator frequency is chosen to be approximately 1500 Hz, but if desired, it can be changed by selecting capacitors C5 and C6.
The generator signal is fed to a power amplifier in which the transistor VT5 operates, loaded on the dynamic head BA1 through a matching transformer T2.
A symmetrical multivibrator is assembled on transistors VT2, VT3, which modulates the generator signal in amplitude, creating a melodious ringing sound.
The call is powered from the network through a step-down transformer T1 and a bell button SB1. From the secondary winding of the transformer, alternating voltage is supplied to the diode bridge, made on diodes VD1-VD4. Capacitor C1 smooths out the ripple of the rectified voltage. A voltage regulator is connected to the bridge, in which the Zener diode VD5 and the transistor VT1 work.
Instead of the KT801A transistor indicated on the diagram, you can use KT801B or any of the KT807, KT815, KT817 series. In place of VT2-VT4 it is permissible to install any transistors of the MP39-MP42 series, and in place of VT5 - MP42B with a base current transfer coefficient of at least 40. Bridge diodes - any small-sized ones that can withstand a current of more than 50 mA. Resistors - MLT-0,25 or MLT-0,125, capacitors - any type for a voltage of at least 10 V.
Step-down transformer - any small-sized one with a voltage on the secondary winding of 5 ... 10 V at a load current of at least 200 mA. The output transformer is from a small-sized transistor radio receiver or a single-program subscriber loudspeaker. Dynamic head - with a power of 1-2 W and a resistance of 4-8 ohms.
Coil L1 - homemade. It is wound with PEV 0,18 wire (2000 turns) on a frame with an inner diameter of 8 mm and a length of 35 mm, glued from thick cardboard. Cheeks with a diameter of 20 mm are attached to the frame along the edges, and a rod with a diameter of 8 and a length of 35 mm from 600NN ferrite is inserted inside (this can be a segment from the rod of the magnetic antenna of a transistor receiver).
It is convenient to place the details of the call inside the housing of the subscriber loudspeaker using its dynamic head and matching transformer. But other options are possible, of course.
Establishing a call comes down to controlling the voltage between points A and B (4,5 ... 5,5 V) and selecting capacitors C5, C6 (if necessary) to get a pleasant sound. In addition, I recommend the selection of the R10 resistor in order to achieve the highest ringing volume with this VT5 transistor.
Author: I.Detistov, Almetyevsk, Tatarstan
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