# LM3915 "VU" Meter

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#### jwomack1547

##### New Member
I saw a video about a month ago, https://www.youtube.com/watch?v=HKWt4zf2EGM , and sort-of wanted to make one. I spent the past few weeks learning how to read basic schematics and buying assorted parts etc. I found this site by accident and saw all the information and help that was posted by members.

The schematics the poster used on the video was **broken link removed**

I was wondering if I could get help with starting something similar, but with knowledgeable input (I heard about using preamps and potentiometers, I just don't know what would be best).

I basically want to learn how to make a tower in the video that will connect to an mp3, ipod, or cellphone that pulses with the bass of the music and also will play the music through speakers on the tower itself. Any help will be greatly appreciated. Thanks.

#### alec_t

##### Well-Known Member
Welcome to the forum!
That's quite an ambitious project for someone starting out in electronics. I suggest you look at several schematics initially, to get a flavour of what's involved. Then perhaps you can post a link to your preferred one and we can go from there. Try googling for 'color organ' or 'light organ'.

#### audioguru

##### Well-Known Member
The datasheet for the LM3915 tells you what the resistors do and how to calculate them. There are many circuits in the datasheet.
R3 is 1.5k in your circuit and it sets the current in the LEDs. The current is calculated with Ohm's Law and the LED currents are shown on a graph in the datasheet as 8mA which is fairly dim.

If the power supply voltage is much higher than the voltage of an LED, the current is high, there are no resistors in series with the LEDs, the mode is BAR and all LEDs are continuously lighted then the LM3915 will get too hot.

The datasheet says you can add one power resistor feeding power to all LEDs to share the heat which I did in the LM3915 that I use as a VU meter that has a 9V supply, the LED currents are very high to be bright and they are red LEDs with a low 1.8V voltage. I used a 10 ohms/1W resistor.
For R3 in your circuit I use 390 ohms so my LED currents are all 26mA so they are very bright. Then my LM3915 and the 10 ohms resistor get hot but not too hot.

#### jwomack1547

##### New Member
I was thinking of maybe using a wall electrical socket with possibly a fuse and resistors for convenience. And from schematics, what would be the best location to start, the lm3915 chip?

#### davenn

##### Active Member
I was thinking of maybe using a wall electrical socket with possibly a fuse and resistors for convenience. And from schematics, what would be the best location to start, the lm3915 chip?

that sounds scarey and very dangerous !!

for heavens sake, use a proper power supply in the wall outlet, else you will be gaining a "Darwin Award"

Dave

#### audioguru

##### Well-Known Member
Stay away from the high voltage wall AC electricity until you understand that a power supply has a transformer that reduces the voltage.
The power supply has a rectifier and filter capacitor that converts the AC to DC.

Maybe power the circuit from a 9V battery. A little 9V battery will not last long but 9V can be made from six AA alkaline cells in series that will last much longer.

#### jwomack1547

##### New Member
What about those 12V plugs? I also have a 9V laying around also. Before powering and all that, I heard that I also need a lm385 to make an amp to amplify the output of the mp3 player. Is this correct or what would you recommend?

#### Mosaic

##### Well-Known Member
keep everything battery powered for now, until u understand the subject better

#### audioguru

##### Well-Known Member
The LM3915 might get too hot if it is powered from 12VDC.
An LM386 is a little power amplifier for driving a speaker. If you have an LM386 or any other power amplifier powering a speaker then its output can also drive the LM3915.
An opamp can be used to amplify the output of an MP3 player so that the lower LEDs light.

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