On the pages of popular publications, including the magazine "Radio", was published a lot of descriptions of simple designs of electronic calls. As a rule, they were based on a tone generator (the generator of sound frequency), which was manipulirovaniya special devices when pressing the bell button. In the result created a alternation of tones or imitation of any sounds. To unfortunately, the tone generators in such constructions were built according to the schemes symmetrical or asymmetrical multivibrators or RC oscillators, the shape fluctuations which differed from sinusoidal, and hence the naturalness of the sound call left much to be desired.
The proposed melodious call (see figure) as a tone generator applied LC-generator of sinusoidal oscillations, performed on the transistor VT4. The oscillator frequency is selected equal to about 1500 Hz, but if desired, it can to change the selection of capacitors C5 and C6.
The signal generator is fed to a power amplifier in which the transistor works VT5 loaded on the dynamic head of BA1 through a matching transformer T2.
Transistors VT2, VT3 assembled symmetrical multivibrator which modulates the signal generator amplitude, creating a melodic sound of the call.
Eats a call from the network through a step-down transformer T1, doorbell button SB1. From the secondary winding of the transformer AC voltage is applied to a diode bridge made on the diodes VD1-VD4. Capacitor C1 smoothes pulsations the rectified voltage. The bridge is connected a voltage regulator, in which operate the Zener diode VD5 and the transistor VT1.
Is indicated on the diagram of the transistor KTA you can apply or any CTB from the series KT807, KT815, CT. In place VT2-VT4 permissible to install any transistors series MP39 - MP42, and the place VT5 - MP42B with the transmission coefficient the base current of at least 40. The diodes of the bridge is any small that can withstand current more than 50 mA. Resistors - MLT-0,25 or MLT-0,125, capacitors - any type a voltage less than 10 V.
Step-down transformer - any small voltage secondary the winding 5...10 V with a load current of 200 mA. Output transformer from small transistor radio or single-program subscription speaker. Dynamic head - the power of 1.2 W and impedance 4-8 Ohm.
Coil L1 is hand made. Her wound wire sew 0,18 (2000 turns) the frame inner diameter of 8 mm and a length of 35 mm, glued with cardboard. At the edges of the frame fastened cheeks with a diameter of 20 mm, and insert inside terminal with diameter of 8 mm and a length of 35 mm ferrite NN (this can be cut from a rod magnetic antenna transistor radio).
Details of the call are conveniently placed inside the enclosure of the subscriber loudspeaker, using its dynamic head and matching transformer. But it is possible, of course, other options.
The establishment of the call is to control the voltage between points A and B (4,5...5,5 In) and selection of capacitors C5, C6 (if necessary) to produce a pleasant sound. In addition, I recommend the selection of the resistor R10, to achieve the greatest the ringer volume with the transistor VT5.
Author: I. Detistov, Almetyevsk city, Tatarstan