Spectacular Spectra

Every electronics enthusiast dreams of making a hi-fi deck at least once in his life time. While he dreams, he plans, “the deck set will have excellent sound quality and wattage. It will be equipped with a 10-band graphic equaliser. Each band of the graphic equaliser will be accompanied by a IQ-level spectrum analyser. And when lights in the ten bands of the spectrum analyser will dance with the beats of drum and throbs of cymbals, what a great display will it be! «But when becomes back to reality and faces the great expenses involved, he has to omit a lot of features. He comes to realise that the deck he can afford to make has so few attractions that it is hardly worth making. He abandons the idea after realising that his dream calls for a loaded wallet which unfortunately he doesn’t have.

But why sigh? Here is a project to encourage those enthusiasts who would like to realise their dreams. I’ll deal with the spectrum part of it. Yes, introducing to you, a 10-band, 10-level, spectacular super spectrum analyser at a down-to-earth cost!

One IC 3914 is necessary for a 10-level spectrum analyser for one input channel. So a 10-band level spectrum analyser conventionally built requires ten such ICS. But LM39I4 is expensive, and ten such ICs will certainly be outside our limited budget.


The solution for making such an eye-catching project lies in persistence of vision, i.e. if a person secs something, its image remains in his brain for about 0.1 second. For instance, if a bulb blinks at a rate of more than ten times a second, it appears as if the bulb is continuously lit.

The visual effects in movies and television etc are based on this principle.

Each column in this 10-column spectrum analyser will show the level of a particular frequency. The display shall show only one such column at a time. We shall display the columns one after another, and after displaying the tenth column we shall show the first column. But we finish this entire process in less than 1/10 second, so that each column is shown more than ten times a second. It will thus appear as if all the columns are being lit at a time.


The display matrix is the easiest pan. Here all we have to do is to solder 100 small LEDs in a 1 Ox 10 matrix. The PCB should be made as shown and the LEDs should be soldered on it such that all the anodes go to the columns and all cathodes go to the rows. You can, alternatively, buy a 10×10 readymade LED matrix.

Test the matrix by giving positive supply to the columns and negative to the rows. If you give positive to the nth column and negative to the mth row, the mth LED in the nth column should be lit. Check all the LEDs in this way because any mistake here will be hard to correct later. Faults may be caused by putting the LEDs in the wrong way. bad soldering and of course by bad LEDs. Now you have a 10×10 matrix which may be useful in many other electronic projects as well which you’ 11 probably make in future.

The main circuitry may be divided into two pans: the row drive and the column drive. The column drive gives ten output lines for the ten columns of the matrix and the row drive gives ten outputs for the ten rows. The row drive will give negative signal and the column drive will give positive signal for lighting the LEDs in the matrix.

I^et’s make the row drive first with IC3914, two resistors and a capacitor. LM3914 is used here just as in a one-band spectrum analyser. The IC has ten op-amps, each of which has two differential inputs and one output. One of the inputs is non-inverting while the other is an inverting input. The voltage at the output line goes high when voltage at the non-inverting input is higher than that at the inverting input.

Now we have ten op-amps with their inverting inputs getting 0, 0.5, 1….4.5 volts, respectively through a chain of ten resistors in scries. All the non-inverting inputs arc shorted and are getting the input signal. So, when the input signal is 0 volt, none of the op-amps will have a high output, and so none of the outputs will be active.

Lei’s imagine that the input voltage is growing higher. When the input voltage crosses 0.5 volt limit, both the first and second op-amps will be active. The reason is, the non-inverting input of the second op-amp is getting more than 0.5 volt whereas the inverting input is getting 0.5 volt, which is less than the voltage in the non-inverting input. Thus, when the input signal crosses 1,1.5,2, 2.5 …… 4.5 volts respectively, the 3rd.

4th, 5th…..10th op-amps become active. Thus, 10 op-amps put in this fashion can show the input voltage limit, just as a mercury thermometer shows the temperature.

Since the op-amp compares the voltage between its two input pins, it is called a comparator. Hut there is a difference between an actual comparator chain and the chain in LM3914. In actual comparators, the voltages in the outputs go high when they are active. But because of a different circuitry, the outputs of the LM3914 go low when they are active. So the outputs of the LM3914 are called ‘active low’ and the others ‘active high’ outputs.

The LM3914 has an extra feature. In an actual op-amp chain, the first op-am p stays active when the second one is lit. But in the LM3914, there is a pin called mode selector. Putting this pin high (connected to positive) you select the bar mode and get a display as in an actual op-amp chain. But if you ground this pin, you select the dot mode where the previous op-amp goes inactive when the next one is lit. So in dot mode, there is only one output active at a time. We’ll use the bar mode in our circuit. You can try the dot mode too, but I think you won’t take it.

As shown in the circuit diagram, the ten outputs of the LM3914 are the row drive outputs. The input signal goes to the input pin after being filtered by a capacitor and a resistor. And another resistor is used which determines the LED brightness.

The column drive has a timer, a counter and some switches. Lei’s start with the timer which gives out electronic pulses.

The capacitor between pins 1 and 2 of the 555 determines its pulse rale and the two resistors at pins 6, 7 and 8 determine the proportion of the time the output remains high and the time it remains low.

Counter IC4017 counts the number of square wave pulses passing through its input. The 4017 has ten outputs. Before any pulse comes, its first output stays high. When the first pulse passes by, the second output goes high. Thus when the nineth pulse passes by, the tenth output goes high. And when the tenth pulse passes, the counter returns to its first output and restarts the process.

In a 4017, only one output stays high at a time. Therefore ii is called a mutually exclusive output counter. Besides, its outputs arc active high.

The outputs are used to put some switches on and off. Here, we are using two kinds of switches—analogue and SCRs (silicon controlled rectifiers). Both of them have a pin called gate. The switches arc put on when the gate voltage goes high. The analogue switches are like mechanical switches, but the SCRs actually are rectifiers that are enabled to pass current only when the gate voltage is high.

We’ll use 10 SCRs (D400) and 10 analogue switches. The analogue switches come in IC packages, such as IC 4066. Each of these ICs has four switches. The gates of t he ten SCRs and the ten analogue switches are connected to the ten outputs of the 4017. Here ten resistors of 1k and ten rectifiers are used for safety.

The positive ends of the rectifiers (in the SCRs) are connected to the 5-volt power supply and the ten negative ends arc taken as the ten columns drive outputs. So when the outputs of the 4017 go high, the gate voltage of the SCRs go high and the SCRs are enabled. So positive current can pass through the rectifiers and go to the columns. This is necessary as the currents obtained from the outputs of the 4017 arc insufficient for so many LEDs.

Now each of the ten analogue switches has two points. One of the points of each switch is shorted together and goes to the LM3914 input. Then remain the ten other points for the ten analogue switches. These points get the input signal from the graphic equaliser. A band equaliser gives us ten outputs (five from the right channel and five from the left channel). The outputs are to be taken from the output of the op-amps (741, TL084. LM338 etc) if the equaliser is made using op-amps. But if that is not the case, and you have only two channels (for stereo input), you can still divide them by a simple capacitor network.

Now its about time we got the simple algorithm of this complex circuit. The timer 555 is set such that it oscillates at a rate of more than 100 Hz (more than hundred pulses per second). Its output is fed to counter 4017 and after every ten pulses its outputs are repeated. When no pulse has passed, the first output is high. Then after 10 pulses it goes high again. Thus the first output is repeated after every 10 pulses. This is true for all the outputs. So the outputs will go high at a rate of more than 100/10, i.e. ten times a second.

Now, when the first output of the

4017 is high, the first SCR is put on and the first column gets positive current. So the analysis of the LM3914 is shown in the first column. That is because all of the columns are getting negative from the 4017, but only the first one gets positive from the SCR, At this time the first analogue switch is on and hence the LM3914 gels impul from the first input source. So the analysis of the first source is shown in the first column. When the second output goes high, the second column is enabled, the second analogue switch is on and the LM3914 gets input from the second source. So the analysis of the second source is shown in the second column. This continues up to column ten and then the whole procedure is repealed.

So the circuit is doing an analysis of the ten input sources one by one, and is displaying the analysis at the ten corresponding columns one by one. But the rate of doing this is so high that it appears to us that the analysis of the ten sources is being done and displayed simultaneously.

The circuit diagram and the PCB layout are sufficient for assembling the spectacular spectra display. After assembling it, replace the 1 mF capacitor at pins 1 and 2 of the IC555 with a 47mF capacitor. I am sure you’ll get the idea once you assemble the circuit and try it. Happy assembling and viewing!

Readers’ Comments:

With reference to the fascinating Spectra Analyser project published in EFY May’92 issue, I would like to make a few enquiries,

Can SCRs D401 be used in place of the ten D400 SCRs used i n the circuit?

The cost of the circuit has increased considerably due to the SCRs. As these have been used as switches in the ciruit, can analogue switches like CD4066 be employed here? Or can transistors in switching inode be used here?



I have found the circuit good but I see no use of diodes Dl through D20. Since the output of CD4017 remains high only, there is no need of safety.



The author, Mr Uttiya Chowdhury, replies:

Other SCRs (D401 etc) and transistors in switching mode (SL100,C1383 etc) can be used instead of D400 SCRs. Analogue switches like CD4066, however, can’t be used as their maximum current capacity is small.

Diodes D1-D10 can be done without, but if the SCRs go bad and connect positive supply to the output of 4017, the IC may get damaged.

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