LM3915 based VU meter

Rez0

Member
2016-02-11 5:22 am
Hi all,

After owning a very cool car audio VU meter (D'amore VU-DIN), I have come to a point that will not allow me to continue using this product in my new vehicle.

As I still need to install a bass knob, volt meter and USB port in a single din space, I have decided to go DIY on the VU meter:

I want to monitor mono amp output ([email protected] ohm), I need only a single VU meter.
Plan is to mount 11 leds (yes, 11) in a 270 degree layout around the bass knob.

Led #1 will play two parts:
-Power on indicator,
First step of the scale - As soon as the first first on the LM lights up, led #1 will switch off, and the other 10 leds will light up in dot mode.
If we know the voltage threshold that is needed to light up the 1st led on the LM, we can use that to turn off Led #1 in the array.

Led #2-11 will represent 1-10 on the LM3915.

Additionally, I would need some sort of rectifier and/or variable voltage divider on the LM input, to step down 10-100 volts AC (mono amp output).

This does not need to be super accurate, as I will calibrate led 10 to light up as soon as soft clipping starts on the amplifier output.

Is anyone here able to assist with a simple yet effective design here?


Many thanks,
David
 
Last edited:
. . . . Led #2-11 will represent 1-10 on the LM3915. . . .
That IC has been around for a long time, and has been the basis for hundreds (if not thousands) of audio level indicators. Do a search for "LM3915" (or the similar chips, LM3914 and LM3916) and you should find many examples of how others solved the signal conditioning problems, as well as some additional features you may want to include in your project.

However . . . the LM3914 has been declared "obsolete" and production is scheduled to end soon. Get it while you can!

Does anybody know of another IC with comparable functionality?

Dale
 

Rez0

Member
2016-02-11 5:22 am
Hi guys,
Thanks for the replies.

I think the basics around the LM - I can handle.
I would need some help with Led #1 though.
So insead of making the led light up when a certain voltage is reached, I'd like to actually switch it off when that voltage is reached.
 
You can use op-amps (or dedicated comparators) as voltage comparators. It's extremely simple. You supply voltage (12v) to the op-amp. You set the reference voltage (with a potentiometer). When the voltage on the other pin of the op-amp (voltage taken from output of amp) goes above or below the reference voltage, the output of the op-amp toggles high or low. The output of the op-amp drives the LED.

Another option would be to use diodes from each output of the 3915 to disable the LED any time that the LEDs on the 3915 are lit.
 

Rez0

Member
2016-02-11 5:22 am
Done some reverse engineering:

Here is what I have for the Power On/Led#1:

D1-10 connect in series with the led output 1-10 from the LM3915.
Transistor collector will go to the same supply voltage as the LM.
LED cathode to ground.

How does this look so far?

I know R1 (300 ohms) should be limiting the current to the led, identical to the current that the LM will provide to leds 2-11, so its value may change.
 

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Rez0

Member
2016-02-11 5:22 am
That looks right. When no 3915 LEDs are lit, all outputs are pulled high and the NP transistor is driven to conduct. When any LED is on, the base/emitter voltage on the NPN transistor goes negative and the transistor switches off.

Perfect!
Now on to part 2 - the input stage of this thing.
Ive been able to gather so far a network of resistors and diodes used to step down the input voltage.
I'll post a diagram later on to try and figure out what is going on there, or maybe just start the variable input stage from scratch.


Thanks,
David
 

Rez0

Member
2016-02-11 5:22 am
To step down the input voltage a 10k 1w resistor in series with a 1k potentiometer would be all you need. The pot would be connected between the 10k resistor and ground. The wiper of the pot would go to the input of the IC. The output of the amp would go to the remaining terminal of the 10k resistor.

What input voltage range should I expect with this network?
 

Rez0

Member
2016-02-11 5:22 am
So I've finally gotten around to building this thing.
The 1st led works great with the NPN switch, but only with a 5 volt supply.
I'm using blue LEDs for leds 2-9 (1-8 on the LM) and amber/red on 10/11 (9 and 10 on the LM).
Here are the issues when all powered by 5v:
1. When amber/red are lit, the 1st switched led turns on but dim.
2. There is alway a small voltage on switched led - it can be seen lit in dark.

When on 12v i get the following:
1st led is lit no matter which other led is on.

here is what I suspect: there is not enough voltage drop after the led, so the NPN is not being pulled to ground.

How do I overcome this? Add 1k resistor on the led supply? This "should" drop the voltage enough, but I'll have to up the current on the LM?
 

Rez0

Member
2016-02-11 5:22 am
Also important to mention - i have a 10k resistor between pin #1 and v+
Reason is, at full scale the first led would leak current and light up slightly.
In the data sheet it is mentioned that the max current leak on pin 1 is around 120uA as opposed to 3-4uA on pins 10-18.

Regarding schematic - dont have a full one at hand, but the PNP is as illustrated a few posts back.

On the LM, using variable resistors between pins 7-8-ground - set to 1.2/2.2k between pin 7 and 8, and 2.2k between pin 8 and ground.
As mentioned above,10k resistor between pin 1 and V+.
2.2uF cap on pins 2/3.
Pin 6 connected to ground.

On pin 5 I have a 2 stage voltage devider with a half wave rectifier and a 0.5uF cap for slower led decay.
So first voltage divider is variable between 1.1:1 and 15:1 ratio. I then rectify the AC signal and smooth/delay it with the cap, then drop voltage down again at a 3:1 ratio. Additional 10v zener to limit voltage.
Input stage works great.
 

Rez0

Member
2016-02-11 5:22 am
Sorry, it is an NPN. I had a blank moment there.

So you suggest to put a resistor between each IC pin and LED. The dioides will then connect as usual at the LED - diode taps off between LED and resistor?

What is the difference in placing the reaistor on the IC side rather than the supply side?