Music box on solar cells. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Music box on solar cells. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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What power can subdue an angry beast? Well, the music, of course! And there is nothing more charming and soothing than the wonderful sounds of a melody pouring from a music box. For centuries, these melodies have fascinated men, women and children - without exception.

The music box in question is powered by solar cells.

Power from the sun

Power from the sun is not the only unique feature of this music box. Unlike its predecessors, this music box contains no moving parts and is entirely powered by silicon cells.

The solar-powered music boxes of the past were just an improvement on their old mechanical counterparts.

In them, instead of a traditional factory spring, an electric motor was used to rotate the musical drum, which in turn was powered by a solar battery.

In our version of this classic toy, the mechanical drive is replaced by a silicon microcircuit (chip). Inside this chip are stored all the notes and musical intervals needed to play the melody. When applied to the power circuit from solar cells, notes are extracted from the memory of the microcircuit and played by a loudspeaker.

Schematic diagram

Our toy uses the 7920 integrated circuit manufactured by Suwa Seikosha (the IC is available from Epson America, Inc., Torrance, CA). The microcircuit can play a melody from 64 notes stored in its memory. This is almost four times more than the number of notes in a conventional mechanical box.

The microcircuit also has an internal generator that extracts notes in the required sequence. The only external components of the box are an RC pitch control circuit and a final amplifier. The complete scheme of the electronic music box is shown in fig. 1.

Music box with solar cells
Ris.1

The pitch is set by capacitor C1 and resistor R1. These two elements determine the frequency of the internal generator, which determines not only the pitch of the sounds, but also the tempo of the melody. Unfortunately, pitch and tempo are intertwined in our box, and a change in one causes a change in the other. You can experiment with different keys and tempos by changing the value of resistor R1.

The output stage is a simple power amplifier with two transistors Q1 and Q2. The output signal of the microcircuit through the resistor R2 controls the base current of the transistor Q1. From the collector of this transistor, the amplified signal is fed to the base of transistor Q2, the collector current of which feeds the loudspeaker with a resistance of 8 ohms.

You may have been surprised by the unusual turn on of transistors Q1 and Q2 in the output stage. It is used because of the low supply voltage of the circuit. The fact is that the 7920 chip is designed for a supply voltage of 1,5 V, which can be easily obtained from solar cells. But for most transistor circuits, this voltage is usually unacceptable.

To achieve high gain, transistors can be connected in a Darlington circuit, as shown in Fig. 2. However, it is easy to see that in this case the emitter junctions of the two transistors are connected in series.

Music box with solar cells
Ris.2

It is known from the basics of radio electronics that the voltage drop across a forward-biased silicon diode, namely, the base-emitter junction in a transistor, is 0,7 V. The voltage drop across two emitter junctions will be already 1,4 V. In other words, A circuit of two such transistors would require a bias voltage of at least 1,4 V to operate.

With such a bias, the signal gain will be extremely small if the stage is powered from a 1,5 V source. This supply voltage is simply not enough. For the normal operation of our amplifying stage, it is necessary to provide power at least 2 V, and even better 3 V.

By connecting the transistors as shown in Fig. 1, we have completely resolved the base offset problem. By isolating the two emitter junctions from each other in this circuit, we use the collector current of transistor Q1 to operate. When such a stage is powered by a voltage source of 1,5 V, the required input signal swing can be more than 0,7 V.

The solar power source for this homemade product is one of the simplest among those described in this book. It includes three solar cells connected in series. This is not a ready-made solar battery, but simply any three elements.

But don't rush. Before you start soldering and connecting together the first elements that come to hand, you must first drop some facts related to the circuit. On average, the power consumed by a music box is very small. The average current consumption is about 30 mA. However, at the beginning of note playback, significant current surges are observed. At these moments, the current consumption sometimes reaches 90 mA.

This affects the operation of solar cells and creates an interesting musical effect.

Let us first consider the phenomenon itself, and then the effect to which it leads. A characteristic feature of all sounds reproduced by musical instruments is the change in amplitude during the sound. Pull back and then lower a guitar string or strike a piano key. Note the sharp attack of the sound that you will hear at first, then turning into a damped oscillation, which the musicians call a steady state. That's the whole point.

The first moment a note starts to sound, a very sharp peak of energy is created. Imitating real instruments, the 7920 generates this sound electronically. As a result, a short current surge occurs, which, entering the final stage, brings the transistor Q2 to a near-saturation state. This means that for a short period of time, all the current from the solar cells flows through the 8 ohm load resistor.

But during this short time, the operating point on the current-voltage characteristic of the solar cell shifts. As a result, it is not the amount of current generated by the solar cells that changes (since the current is self-limiting), but the output voltage. For a short time, the output of the solar generator turns out to be short-circuited, as it were, and its output voltage changes accordingly.

Well, now let me tell you that the clock generator in the 7920 chip is slightly sensitive to voltage changes. Therefore, a voltage drop at the beginning of note playback leads to a short-term decrease in the sound frequency.

To combat this phenomenon, the following measures have been taken. First, the size of the solar cells has been increased. Using elements of a obviously larger (than necessary) size, it is possible to achieve a decrease in the effect of sound modulation, however, voltage surges (although not so significant) will still remain. And yet, some troubles are caused by the inconstancy of the load of solar cells, the value of which varies practically from infinity (in the absence of sound) to some small value (when playing a note). In fact, the total voltage change is about 0,5 V or more in total.

Secondly, to smooth out these small fluctuations, you can install a capacitor C2 in parallel with the solar array. At that moment, when the circuit requires a large current, the capacitor C2, by discharging, provides most of it. During a pause between notes, capacitor C2 is charged by solar cells. When using these two circuit solutions, it is possible to more or less stabilize the supply voltage of the microcircuit.

Design

The music generator is placed on a printed circuit board, the size of which allows you to place the entire device in a small volume, for example, in a box from under earrings or cufflinks.
Figure PCB is shown in fig. 3, and the placement of elements on it - in Fig. 4.

Music box with solar cells
Ris.3

Music box with solar cells
Ris.4

During installation, please note that the capacitor C2 is located to save space not from above, but from the side of the printed conductors. The loudspeaker, which is an oversized 8 ohm speaker from a transistor receiver, should not be placed too close to the circuit board. The best imitation of the sound of a music box is obtained by using a dynamic head with a rigid cone made of plastic or metal.

Music Box

Now it's time to give the music box a proper look. Think about it, and you will definitely come up with different options. As a case, a jewelry box is also suitable, the sizes of which can be very diverse, as well as a coin box or even a terrarium. The possibilities here are only limited by your imagination. Keep in mind that our music box is practically eternal, so it can be used in many different ways than its imperfect mechanical predecessors.

Personally, I chose a piano-shaped hinged lid jewelry box. It seems to me that the shape of the piano corresponds very precisely to the purpose of the music box. Whichever case you choose, make sure it has an open top or a clear enough window in the lid to accommodate the solar cells.

Connect the solar cells in series and glue them to the inside of the lid. For this purpose, rubber glue is preferable, but any other transparent glue will work. If the solar panel is located on a moving part of the box (on a hinged lid), route the lead wires so that their movement is minimal. In this case, a flexible wire can be used.

The electronic part of the circuit can be placed in any suitable place. However, this remark does not apply to the loudspeaker. When placed inside a closed box, the diffuser must communicate with the outside, otherwise you will not hear anything. In this case, drill several through holes in the box body against the speaker.

To test your new toy, put it under a powerful lamp or expose it to the sun. And then the room will be filled with magical sounds of music.

Author: Byers T.

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