LED Audio Spectrum Analyzer

ronsimpson said:

Attached is a IC that has 7 audio filters and does much of the work. Just a thought.

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audioguru said:

I have no clue where that EQ IC in post #2 is sold, Digikey has never had it.

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audioguru said:

Your opamp probably will not work because it is not powered and is not biased.
You did not read the datasheet for the LM3916 to see that its LED outputs have regulated current so the 500 ohm resistor is not needed, 500 ohms is much too high anyway. But I cannot read your writing for the resistors in series from pin 7 to ground, their total value sets the LED current.

You have a huge expensive 0.68uF capacitor and a 1k resistor on the input when you could use a small less expensive 0.068uF (68nF) capacitor and a 10k resistor, or a very small 6.8nF capacitor and a 100k resistor. They cut frequencies gradually below 235Hz, no bass. Then the opamp feeds a 1k resistor that overloads it and a 0.039uf capacitor that cut frequencies gradually above 4100Hz, no highs.

The rectifier is passive and does nothing at low levels, the datasheet shows active rectifiers using an opamp so they rectify all levels.
Since you did not read the datasheet then the LM3916 might melt when many LEDs are turned on and are bright because the 12V supply voltage is much too high.

Your calculations show a passive gradual highpass and a passive gradual lowpass that together make a very poor bandpass filter. You should make an active filter in an opamp bandpass filter circuit.
Your calculations show a huge expensive 0.51uF capacitor and a 100k resistor but your schematic shows 0.051uF, which is it?

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audioguru said:

Audio is AC that alternates positive and negative over and over. But the LM3916 is active only for the positive parts of the input signal so the LEDs would be turned off for each half-wave of the input signal and at low frequencies they would flicker and at higher frequencies the LEDs would appear dimmed because they are turned on only half the total time.

The rectifier passes only the positive parts of the signal and its filter capacitor holds the peak voltage long enough for our vision to see its brightness and to avoid flickering on and off. But the diode needs about 0.65V for it to work which is near the signal level required to light the lowest LEDs of the LM3916 so they might not light with low levels. An active rectifier using an opamp is shown in the datasheet and it rectifies very low levels perfectly.

Your "filter" is a simple passive RC highpass filter amplified by an opamp and it feeds a simple passive RC lowpass filter. It is not an active bandpass filter. It is very simple so its slopes are gradual and not sharp. Its highpass and lowpass cross and reduce frequencies that both are reducing. An active bandpass filter has the RC parts inside its negative feedback loop and some active filters even use positive feedback to make sharp slopes or a narrow bandpass. active filters can use multiple RC networks and they have "orders":
With one RC it is a first order very gradual sloped first order filter.
With two RCs it is a second order filter that has sharper slopes.
With three RCs it is a third order filter etc.

An active highpass filter feeds an active lowpass filter to make a wide bandpass with sharp slopes. An active bandpass filter using one opamp makes a narrow bandpass.

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ciruit said:

im thinking of only doing 3 filters for now( 20-250hz, 250-4000hz, 4000hz-20khz)

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AnalogKid said:

Since these are not "brickwall" filters (vertical attenuation slopes, zero overlap), I recommend designing them so that there are gaps between the three passbands. This will increase the amount of time that all three outputs look different.

ak

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audioguru said:

You show second order active filters with opamps that are not powered, are not biased properly and the grounds are missing. Why do you use resistor values as low as 1k which need HUGE capacitor values?? Most opamps cannot drive a load as low as 1k.
The peak detector shown in the datasheet is fairly complicated and needs a positive and negative power supply. My peak detector uses an IC that has inputs and outputs that go all the way down to ground so a negative supply is not needed and the circuit is simple:

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audioguru said:

The datasheet for the LM324 opamp and most other opamps shows that its voltage gain and output voltage swing are spec'd with a load of 2k ohms or more. There are a few opamps that can drive a load as low as 600 ohms.
Here is a short description of how the circuit should be designed:

The lowpass filter must be fed from a low impedance like the output of another opamp.
The input of the filters must be able to swing up and down but since you do not have an additional negative supply then the new added opamp's (+) input must be biased at half the supply voltage with 2 series resistors and a capacitor. Then its input and output can swing up and down with the signal. Since the lowpass filter opamp is DC-coupled on its input then it will also have its input and output swing up and down.

The highpass filter also must have its input and output swing up and down then the resistor to ground on its (+) input must be disconnected from ground and connected to the half-supply voltage resistors added for the lowpass filter. The resistor to ground on its (-) input must be disconnected from ground and connected to a capacitor to ground
The simple half-wave peak detector also must have its input swing up and down but must have an input that swings positive and negative. Then all the previous opamps need an added negative supply. Use my peak detector circuit instead since its input has a capacitor that blocks the DC from the highpass filter opamp.

Your half-wave peak detector has its transistor upside down but you should use my peak detector circuit instead.

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