help regarding audio spectrum analyzer using active filters and lm3915

[japanjot]

hello frnz, i am working on 10 channel audio spectrum analyzer, now i am stuck with lm3915
i made 10 active filters now i want that output of every filter should be fed into the input of lm3915 ic....
the problems i am facing is with lm3915 i~~>

1):the leds are blinking very dimm
2);even if i put the ic in dot mode it works in only bar mode
3);if i put the ic in bar modes the leds get little bright but not up to the mark and not what i expect

After these problems are solved i would realy like to know :
1) shal i use multiplexing in this? if yes then plz guide me
2)if multi plexing is used will there be loss/compromise with the brightness?
3)or shal i use 10xlm3915 ie one ic each for every filter


the schematics for the lm 3915 i use are:
View attachment 66824

the schematics for the 10 filters i made are:
View attachment 66825

the list of components used in filters is here:
View attachment 66826

 

[audioguru]

R1 for each LM3915 is 1.2k and always has 1.25V across it. Therefore the current in R1 and in R2 is (1.25V/1.2k=) 1.04mA.
If R2 is set to 10k then it has a voltage of (10k x 1.04mA=) 10.4V.

Pin 6 of the LM3915 is the top of the voltage divider. The PIN 6 voltage is (1.25V + 10.4V=) 11.65V. This is the peak input voltage that lights all 10 LEDs.

The datasheet shows a graph of the current from pin 7 (called the Reference Load Current) that produces a current in each LED. For the pin 7 current of 1.04mA the LED currrent is only 10mA. If R1 and R2 have half the value you used then the LED current will be almost 20mA which will appear very bright.

The datasheet shows a few peak detectors but does not say that short duration lights cannot be seen. Anything less than 30ms appears dimmed. The peak detector stretches each short duration pulse to a minimum duration of 30ms so they are bright.

I didn't look at how narrow the filters are. If they are too narrow then they pass only a few frequencies which will make their outputs appear to be dimmed since each one will be turned off most of the time.

The problem of the dot mode looking like a bar mode could be caused because your circuit is missing a very important supply bypass capacitor. The datasheet shows a 2.2uF tantalum or a 10uF electrolytic capacitor.

 

[ronsimpson]

In 2007 we had a long talk about a audio spectrum analyzer using active filters and lm3915. I posted drawings and PCB layout. search for "Peak meter for certain frecuencies" back in2007.

Search for spectrum analyzer. Remember to search back in time beyond one year.

 

[audioguru]

If you use a single LM3915 and multiplex it with all 10 frequency bands then the LEDs will appear extremely dimmed because each one is turned off 9/10th of the total time. Their current must be 10 times higher for them to appear bright but the high current might burn out the LEDs.

 

[japanjot]

if i attach a pnp transister to the each output of lm3915 and then multiplex it ?
and shal i use peak detector? i want to run in proper dot mode and i want each led to be bright.
plz guide me how to multiplex without loss of brightness

 

[japanjot]

i was searching on net and saw some people rectify the signal using two diodes, shal i also do it for better result? and plz tell me what is the need of rectification of audio signal in this circuit.

 

[audioguru]

if i attach a pnp transister to the each output of lm3915 and then multiplex it?

Yes, but the LED current must be 10 times the current of a single LED which might burn out the LEDs. Look at the maximum allowed peak current rating for your LEDs.

shall I use peak detector? I want to run in proper dot mode and I want each led to be bright.

Yes a peak detector is important for good brightness.

Since I have 10 plz guide me how to multiplex without loss of brightness

Simply calculate LED current that is 10 times single LED current.
You must design a multiplexing sequencing circuit, maybe with a CD4017 IC and an oscillator.

I was searching on net and saw some people rectify the signal using two diodes, shall I also do it for better result?

the LM3915 has a logarithmic sensitivity so that its outputs copy our hearing's logarithmic sensitivity to loudness. Then since your full-scale input voltage is 11.65V, the input voltage required to light the lowest level LED is only 0.37v which is less than the 0.7V to forward-bias a rectifier diode.

The datasheet for the LM3915 has a simple peak detector/rectifier using one PNP transistor. It works well with a maximum input voltage of 10V or more.

plz tell me what is the need of rectification of audio signal in this circuit.

The diode prevents the transistor from quickly discharging the timing capacitor. The 1M resistor discharges the timing capacitor after 30ms so a short duration input is stretched long enough for our vision to see it brightly.

 

[japanjot]

can i get working schematics in which peak detector is used with lm 3915?
and how would i make the current 10 times?

 

[ronsimpson]

Peak meter for certain frequencies.

 

[canadaelk]

Here is a design (30 years old). I am just finishing a pcb design based on it, just for fun:

 

[ronsimpson]

Here is a simple version of that.
I started on this last year but making a living got in the way. I made 10 PCBs with this IC and a PIC + LDC. I have not started on the PIC software.

You can get the IC from SPARKFUN.COM.

 

[japanjot]

hey thnx ron, u made so easy for me. iam gonna try your circuit and will post here results too
thans alot

 

[japanjot]

hello AUDIOGURU sir i found one of your circuits ~~>
this circuit is working absolutly fine with proper brightness
the problem is that the first led never blinks in dot mode when we lower the volume, the 2nd led remains on low volume,but when i raise the volume leds from 3-10 works totally good.
in bar mode led 1 nd 2 remain on...
i even checked wires,connections replaced leds but not success

 

[ronsimpson]

Look at this video on the subject. https://vimeo.com/13982035

Found another video. This one uses two ICs to get 14 bands.
https://www.youtube.com/watch?v=EpLLqgySCD4

 

[audioguru]

hello AUDIOGURU sir i found one of your circuits ~~>View attachment 66857
this circuit is working absolutly fine with proper brightness
the problem is that the first led never blinks in dot mode when we lower the volume, the 2nd led remains on low volume,but when i raise the volume leds from 3-10 works totally good.
in bar mode led 1 nd 2 remain on...
i even checked wires,connections replaced leds but not success

The circuit has pin 8 of the LM3915 at 0V so pin 6 and pin 7 are at +1.25V which will be the peak input voltage to light the 10th LED.
The first LED lights when the peak input is +0.052V and the second LED lights when the peak input is +0.0578V.

There is nothing in my circuit to turn on the first and second LEDs when there is no signal. If you built the circuit on a breadboard then the opamp might be oscillating or the wiring picks up mains hum.

I used an MC33171 opamp that has inputs that work at 0V and an output that goes down very close to 0V in this circuit that has a single polarity supply. You probably used an ordinary opamp that needs a dual polarity supply in this circuit.
Which opamp did you use?

 

[japanjot]

The circuit has pin 8 of the LM3915 at 0V so pin 6 and pin 7 are at +1.25V which will be the peak input voltage to light the 10th LED.
The first LED lights when the peak input is +0.052V and the second LED lights when the peak input is +0.0578V.

There is nothing in my circuit to turn on the first and second LEDs when there is no signal. If you built the circuit on a breadboard then the opamp might be oscillating or the wiring picks up mains hum.

I used an MC33171 opamp that has inputs that work at 0V and an output that goes down very close to 0V in this circuit that has a single polarity supply. You probably used an ordinary opamp that needs a dual polarity supply in this circuit.
Which opamp did you use?

sir i also used opamp which works on single polarity , i used AP 358.......but i think i am using breadboard thats y there must be noise...what alternate op amp would you suggest as MC33171 is not available here nearby..
and one more question, when i use in dot mode the leds goes up and down at very fast speed, is there any way which could decrease the response time or delay in microseconds?
and i used tl074 for filters, is that a good op amp for audio filter circuits, what alternative would you suggest?

sorry but one more question , how can i cascade two of lm3915 so that i can increase the range ??any circuit u have would be helpful to me..

 

[ccurtis]

I wonder if anyone has tried using a single switched-capacitor filter and changing the clock frequency (center frequency) of the filter as the display is multiplexed from one band to another band, thus reducing the hardware required for multiple filters as well.

 

[japanjot]

I wonder if anyone has tried using a single switched-capacitor filter and changing the clock frequency (center frequency) of the filter as the display is multiplexed from one band to another band, thus reducing the hardware required for multiple filters as well.

hello frnd, what are single switched-capacitor filters ???? u mean passive filters??? or they are different?

 

[ronsimpson]

I wonder if anyone has tried using a single switched-capacitor filter and changing the clock frequency (center frequency) of the filter as the display is multiplexed from one band to another band, thus reducing the hardware required for multiple filters as well.

In Circuit Cellar magazine a while back, there was a design using a DSP. In software they made a single filter and tuned it from one frequency to another. A single filter that tunes for 20 to 20khz is common. The problem is that if the filter is looking at 50hz it is not looking at 1khz and may miss information. With 7 to 15 separate filters, running all the time, no information will be lost.

 

[audioguru]

i used AP 358.......but i think i am using breadboard thats y there must be noise...what alternate op amp would you suggest as MC33171 is not available here nearby?

An LM358 has trouble above 2kHz. The MC33171 opamp I use is also low power and works from a single supply but works well up to 35kHz so it covers the entire audio band of frequencies.

when i use in dot mode the leds goes up and down at very fast speed, is there any way which could decrease the response time or delay in microseconds?

Mine has a fast attack and a fairly slow decay. Increase the value of the 0.1uf filter capacitor to make both slower.

i used tl074 for filters, is that a good op amp for audio filter circuits, what alternative would you suggest?

The TL074 is a good audio opamp if its is biased properly or if it uses a dual polarity supply.

how can i cascade two of lm3915 so that i can increase the range? any circuit u have would be helpful to me.

The circuit is on page 9 of the datasheet. I made it about 33 years ago with 20 LEDs.