ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Musical call on microcircuits of the UMS series. Encyclopedia of radio electronics and electrical engineering Encyclopedia of radio electronics and electrical engineering / Calls and audio simulators Now no one is surprised if, when you press the bell button located at the front door of the apartment, instead of the usual "tr ... r" or "ding-dong", a fragment of a popular piece of music or the voices of animals and birds is heard. On the shelves of stores selling consumer electronics, there are many options for domestic and foreign musical calls, which are often even cheaper than conventional electromechanical ones. Most domestic calls are built on the basis of microcircuits of the UMS-7 or UMS-8 series, included in an almost typical scheme. In amateur radio literature, the shortcomings of a typical circuit have been repeatedly described (a sharp sound caused by the pulsed nature of the output signal, when the start button is briefly pressed, the first melody does not sound completely, etc.) and improved versions of the switching circuit were proposed (L.1, L. 2). Figure 1 in the text shows a diagram of another variant of such a call.
The difference from the standard one is that the sound has become calmer and softer, and when you briefly press the "CALL" button, the device plays the entire piece of music. The sharpness of the sound of the bell, included according to the typical circuit, is caused by the fact that the dynamic head, included in the collector circuit of the output transistor switch, receives unipolar rectangular current pulses. Moreover, such a signal is rich in high-frequency harmonics, which, entering into resonance with the speaker coil and its mechanical system, as well as acoustic design, give the musical fragment an uncharacteristic coloring. In addition, the current flowing through the voice coil of the speaker contains a constant component, which shifts the cone and reduces the volume of the sound. In the intervals between different sections of the musical fragment, loud and sharp clicks appear, caused by the difference in this constant component. In addition, the operation of the transistor in the key pulse mode for a low-resistance load leads to the fact that the resistance of the transistor in saturation mode turns out to be much higher than that of the voice coil of the dynamic head. Therefore, a significant part of the energy is spent on heating the transistor, and not on the buildup of the diffuser. These shortcomings can be eliminated if the speaker is connected to the output of the transistor stage through a matching transformer having a high-resistance primary winding and a low-resistance secondary. In addition, by including a capacitor in parallel with the primary winding, we get an oscillatory circuit tuned to the average frequency of musical fragments. The presence of a transformer matches the low-resistance speaker coil to the relatively high-resistance output of the key, and the presence of a resonant circuit smoothes the square-wave pulses, making them closer to sinusoidal and suppresses unnecessary high-frequency harmonics. Since the quality factor of the circuit is not high, all the notes included in the jukebox are played. The presence of resonance in the circuit leads to the fact that the voltage on the primary winding of the transformer is slightly higher than the supply voltage of the microcircuit, and this leads to an increase in sound volume. The second defect of the typical scheme is that when the "CALL" button is pressed for a short time, the melody does not sound to the end. The fact is that the sound time in this case is determined not by the duration of the musical fragment, but by the capacitance of the capacitor blocking the start button. In the circuit shown in Figure 1, from the inverse output of the microcircuit (pin 14), the detector receives pulses through C1 to VD1 and VD2, so there will be a unit at the 13th output of the microcircuit all the time while the musical fragment is playing. The musical bell is powered by a transformerless power source on a D8 rectifier and a parametric stabilizer, consisting of a chain of VD4-VD7 diodes, on which 2-2,5V falls together and a damping reactance of capacitor C4. Capacitor C2 smooths out the ripple of the received direct current. As a basis for the call, the broadcasting subscriber loudspeaker "Etude" is used. It has a plastic case, speaker and transformer. All this is used in the design, except for the volume control, which is excluded. Most of the details of the bell are located on a small-sized printed circuit board, the drawing and wiring diagram of which is given in the text.
The board is made of getinaks with one-sided foil. You can use other foil insulating material used for printed circuit boards. The board does not contain buttons S1 and S2 (SK1 is displayed on the front door) and a transformer. The microcircuit can be UMS-8 or UMS-7, additional numbers (for example UMS-8-08) indicate the musical repertoire of the microcircuit. Buttons S1 and S2 are located on the body of the bell, with the help of the S1 button you can select a melody for further playback, and with the S2 button you can stop playback. Capacitors C3 and C4 are each made up of two 0,33 uF capacitors connected in parallel, they are marked on the wiring diagram as C3' C3" and C4' C4". The KTs405A rectifier bridge can be replaced with a bridge assembled from KD105V or KD209V, D226 diodes. In the absence of a ready-made base, you can use a transformer from the output stage of an old transistor receiver and any 0,1-3 W speaker. Establishing a properly assembled device from serviceable parts consists in selecting the value of the capacitor C1 in such a way that when you press SK1, a complete one-time playback of a musical fragment occurs. If the capacity of C1 is too large, the machine can play the melody several times in a row. If necessary, you can fine-tune the C3 capacitance so that the tone and volume of the sound are optimal. Literature
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