# newbie lm3915 question

Discussion in 'The Projects Forum' started by patbb, Sep 16, 2012.

1. ### patbb Thread Starter New Member

Aug 2, 2012
1
0
hello,

link to datasheet for the lm3915

But my knowledge isn't good enough yet, i can't understand most of the datasheet.

- first question:
They say the Vled (which is the positive for the leds) must be less than the supply voltage (the positive for the lm3915).

When i build the circuit, should i use a voltage divider to reduce the voltage of the LED's positive (Vled)?

- second question:
Audio guru (a user of these forums who seem to know EVERYTHING!!) always suggest to add a resistor to limit the current for 15mA+ circuits.
- where should i put this resistor? In series with the Vled?
- How should i choose it's value?
example

- my plan is to have 2 leds in series @20mA per output, should i add a resistor per output to help dissipate the heat? if yes how should i choose the value of the resistor?
12V (input) - (3.2V + 3.2V) = 5.6V to kill with the resistor?
5.6V/0.02MA = 280 Ω?

- Third question:
I want to be able to adjust the thing so that the whole tower will light up even if the volume isn't insanely high. (precision reading isn't a concern, just want to make it pretty

My idea is to reduce the range of the lm3915 to 6V, (most vu meter are setup to 10V range), but add a voltage divider to the signal with a potentiometer so that if the volume do get very high, i can reduce the voltage of the signal and still have the vue meter light up from the bottom to the top. (instead of having all of the LED lit permanently without moving when the sound gets too high).
Is this a good idea?

is this whats happening here? (this is not my circuit, just a circuit i've found searching the web)

- Bonus question:
What software is used here?

Thank you so much if you take the time to help me out even just a bit.

-p

Last edited: Sep 16, 2012
2. ### absf Senior Member

Dec 29, 2010
1,493
374
The software used is proteus.

Allen

3. ### akis02 Member

Apr 30, 2011
57
3
I think VLED can be anything. In my circuit it is the same as the V+ on the chip. Just make sure it is below 18V DC.

The LM3914 will consume 4.5mA on "idle" and then you add the LEDs. I think 20mA per LED is an overkill, 5mA is quite bright (depends on the LED of course), and 10mA per LED is certainly plenty. In my circuit I use 5mA per LED. The LED brightness is set by one resistor only, and in your circuit it is R3. The LED current is (12.5 / R3), in your case 10.5 mA.

R2+R4, as it is shown with pin 6 connected to pin 7, forms the high reference which in your case is about 3.5 Volts, whereas the low reference is 0 since pin 4 is grounded.

So this LM3914 will light up 10 LEDs from 0.2V to 3.5 Volts DC.

I am not sure what will happen when you connect AC to it...

Ideally to connect a sound source to it, you'd want to rectify it first using diode(s) and capacitor before you attach it to pin 5.