Stereo Spectrograph

Most stereo systems have some kind of music level indicator, The indicator may be as sim pie as a set of LEDs, ordinary low-voltage lamps or V-U (volume-unit) meters. These were later replaced by bar-graph type LED indicators. Though attractive, they provide only one-dimensional ‘movement’ of LEDs with the music. Now this system has also become very common.

Presented here is a spectacular system to replace the outdated music level indicators. This system is capable of providing a smart, two-dimensional ‘dancing’ effect on a square matrix of 100 LEDs.

In this system, out of 100 LEDs arranged in a square matrix, only one LED glows at a time and it keeps dancing within the so-formed LED array along with the music being played or the stereo system. The movement or dancing of the LED depends basically on the ‘stereo’ effect of the music Hence, the system may be called s stereo spectrograph.


This system ’s working depends not only on the type of music being played but also on the ‘stereo’ effects of the music. Greater the stereo effect, hater the movement of the dancing LED. As you know, a stereo signal is recorded or played on two independent channels which arc purposely isolated from each other. Therefore, at every instance the signal strengths of the right and left channels are different from each other. As shown in the schematic block diagram (Fig. 1).

The audio signals from the left channel are made to force the glowing LED to move along the vertical columns and the signals from the right channel provide the movement of the LED along the horizontal rows. Hence, we see ‘movement’ of the glowing LED in a two-dimensional square area where t he vertical and horizontal movements are isolated from each other* just like audio signals in both the channels.

The audio signals from the left channel are first amplified by a high-gain voltage amplifier that is build around an op-amp. The gain of this amplifier can be controlled by controlling the negative feedback. Hereafter the amplified signal is rectified using diodes. llien it is fed to a 10-level comparator circuit using LM339. The output of these are XORed (using CD4030/ 4070). So it works like a digital peak-detector for 10 levels.

The output of an XOR gate is high only if one input is high and the other is low. The output is low only if both the inputs are either high or low. Hence, we get high voltage at only one row that corresponds to the peak level of I he audio signal in the left channel. An emitter-follower using npn transistor (UC547H) is connected to each row.

Hiis is required only to boost up sufficient current required for an LED. If these transistors are not used then the XOR gates will get damaged due to excessive current taken by an LED as compared to the source current capability of the XOR gates. In a similar fashion, the signal is amplified, compared and XORed from the right channel. Then using invertor logic, one column is made low and all others are kept high. Again we use voltage follower (pnp transistors) to drive the columns of the LED matrix.

Hence, only one row will have high voltage and only one column will have low voltage at a time. Therefore, only one LED that is connected between these rows and columns will glow. And it will keep on dancing with the stereo effects of the music being played.

If you play a mono record or cassette then the movement of the dancing LED will be equal in both the horizontal and vertical directions or the movement will be inclined at 45° typically.

The dual potentiometer, which is used in the feedback path of the op-amps, acts as a sensitivity control potentiometer for the system. You should adjust it properly to get a full wide area movement of the LCD.

As shown in the circuit diagram (Fig.2), the system is driven by the preamplifier of the stereo system. Therefore. the visual effects must be made independent of the volume level of the power amplifiers used in your stereo systems. As this circuit requires a number of chips, the pin diagrams of the used chip arc shown in Fig.3 (a, b, c and d).

You can construct this spectograph as per your convenience. The number of LEDs can be reduced or increased as required, but it must form a square array. As it requires very less power, you can directly take the supply lines (12V) from your stereo system. This stereo spectograph will surely add a new dimension to the visual effects of your stereo system.

Readers’ Comments:

Stereo Spectrograph project published in EFY March’92 issue is good. Please convey my thanks to the author for this very innovative idea.

The circuit uses too many ICs, transistors and resistors, however. The entire idea could have been realised by using three ICs (one LM1458 and two LM3914), 10 transistors and 20 resistors, besides a few other components.

The new circuit can be constructed even on a general-purpose PCB and consumes very little power.



The author, Mr Dinesh Kumar Raheja, replies:

I’m thankful to Mr Shankar for his suggestion. His approach is very nice. His circuit can be further improved by connecting all the ten outputs of IC3 to +Vcc (12V) through pull-up resistors of 10K (1/4W) each. This is required as the outputs of LM3914/3915 are ‘current-regulated open collectors’. However, IC2 does not require these pull-up resistors because the ‘open collectors’ of IC2 are connected through LED matrix and transistors network.

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