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Old 10th January 2015, 08:09 AM   #1
lukutis is offline lukutis  Lithuania
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Default lm 3915 vu meter problem.

Hello everyone. So i started learning about electronics slowly, my physics knowledge is really bad but i hope i can fix that.
This is my second electronics project so i decided to do something cool... http://www.electro-tech-online.com/imgcache/8472-14.gif
I end up doing this, i have no idea how it works and i barely connected everything to my breadboard.
I use 9V power source
3,5V , 0,02A 10leds
10k potenciometer
and 1k(didint have 1,2k) resistor exactly as shown in the scheme.
everything seemed to work fine but when i play music only 5 or 6 leds are glowing and the last 4 are not doing anything ;/ Whats should be wrong?
I tryed to read lm3915 description but there is so much information my head almost blew up :/ there is no way i can understand something with brains like mine, so i hope u got some simple explanation how to fix that
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Old 10th January 2015, 08:29 AM   #2
Mooly is offline Mooly  United Kingdom
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Welcome to diyAudio

It sounds like you haven't enough signal to drive the device fully. Does altering the preset resistor in your diagram not give enough gain ?

For this to work best you really need another stage in front of this to condition the signal and detect and "hold on" to peaks so that the display becomes much slower and more readable. Applying AC (your audio) means the display is trying to respond to quickly and will be dim and poor in use.

Have you got the data sheet ? That shows a couple of very simple rectifiers, one using a transistor and one an opamp.
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Old 10th January 2015, 08:35 AM   #3
lukutis is offline lukutis  Lithuania
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So what do u mean exactly.I posted the data sheet of mine. I played with pot but that didint help.. Setting it to max only 2-3 led glow and setting to 0 , 5 -6. One more interesting fact is that when i connect to resistor to pin 6 isntead of pin 7 all leds starts glowing to music but not good.. Some are bright and some are not bright at all.
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Old 10th January 2015, 08:44 AM   #4
Mooly is offline Mooly  United Kingdom
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Type LM3915 into the blank search box here to get the manufacturers data sheet,
Datasheet catalog for integrated circuits, diodes, triacs, and other semiconductors, view

Your circuit is responding to an audio signal that varies really quickly. For the LED's to be readable and usable you need to track that audio signal and convert it to a more clowly changing one.

Imagine the red signal is the audio. You need to rectify and "hold" that signal to get a more usable input signal for the LM3915. The black line shows what that signal might look like and that would drive the LED's more slowly and accurately.
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Old 10th January 2015, 08:48 AM   #5
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Look at figures 17 and 18 in the Texas Instruments data sheet as they show two simple rectifiers.
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Old 10th January 2015, 09:04 AM   #6
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Ok. should i start with that i have no idea what is rectifier? Yeah i tryed to read those manufactures of the microscheme but this is way too hard for me to understand ... Im purely noob in electronics.
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Old 10th January 2015, 09:23 AM   #7
lukutis is offline lukutis  Lithuania
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what u mean exactly.. Im not sure what is rectifier and i cant read those manufacturer information ;/ this is wayy too hard for me.. so much information.
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Old 10th January 2015, 10:27 AM   #8
Mooly is offline Mooly  United Kingdom
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You need a grasp of the basics. A rectifier converts AC (thats your audio) into "DC". The simplest device for that is a diode but there are a couple of reasons why we would not just use one "as is".

Look at the diagrams of what the diode does,
Rectifier - Wikipedia, the free encyclopedia

The problem with a simple diode is that there is a 0.7 volts "dead zone" where nothing happens... so we use other parts (the opamp) to overcome that and make a precision rectifier.

For your usage the simple one transistor example would be a good starting point because it needs no additional power supply, just the supply the LM3915 is running on. The transistor can be any general purpose PNP device.
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Old 10th January 2015, 02:07 PM   #9
lukutis is offline lukutis  Lithuania
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Can someone explain me how do i calculate what current flows thorugh each of my led in this circuit? I cant understand the manufacturers
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Old 10th January 2015, 05:18 PM   #10
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You need to look at the data sheet.

A feature not completely illustrated by the block diagram is the LED brightness control. The current drawn out of the reference voltage pin (pin 7) determines LED current. Approximately 10 times this current will be drawn through each lighted LED, and this current will be relatively constant despite supply voltage and temperature changes.

So you need to measure the voltage on pin 7 and calculate the current (ohms law) taking the resistance as R1 + the preset pot value as set. Say you measured 3 volts and the preset was 4k. The current would be 3/(1200+4000) which is 0.57milliamps. To increase the current and keep everything else the same you alter the ratio of the resistors. Say you made R1 equal 120 ohm and the preset 400 ohm. Now you have 3/(120+400) which is 5.7ma. Ten times as much.
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