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LM3915 dot/bar display driver

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Newbie level 5
Apr 16, 2011
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Here is the datasheet:

**broken link removed**

i'm having problems with circuit on page 2.

I'm using only one led (calibrated with external equipment i only need to know when it's clipping). Configured it as bar mode with pin 9 connect to +

LED is drawning very few current (0,040 mA), voltage across it is ok 1,3v

Powering the circuit with 15v and tried different powers for Vled (5 to 15v) as R1 and R2, i tried various resistors

Also following this circuit:

**broken link removed**

What's the problem?
Thank you

Can you determine whether the lowest output pin (#1) ever goes low in response to your signal? Guess you know that you must supply current to a pin to find this out. The outputs do not source current.

Are you using the logarithmic version (3915) because you're applying an AC (audio) signal? It would be good to experiment first by applying a DC voltage.

Sweep up and down to discover what input level you need to apply in order to get something to happen at the output pins.

The goal is to get familiar with the chip operation, and gain a basis of comparison for applying your intended signal.

It would help if you attach an led to each output pin. Even if it's just to troubleshoot at this point. It would let you see if the output pins really are activating all the way up to the pin where you've got your one led attached.


And very important: Are you using V_LED higher than 7V? If so then the datasheet says you need to install a dropping resistor to limit heat dissipation.


Since you say it's to detect clipping, then I guess it's for audio power measurement.

My databook by National Semiconductor Corporation has a circuit showing a value of 390 ohms for R1, and 2700 for R2. (Whereas the first diagram link in your post shows 1240 for R1 and 8060 for R2.)

Don't know how much difference it makes. You'll probably want to use a potentiometer instead of specific odd resistor values.
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Everything is clear... but sorry... what is the problem?

Sorry, I see it now... you mean the LED bar is moving fine but the LED current is very low... 40uA.
I assume it is the case for all 10 LEDs when on.

Just to be sure let us do some calculation while referring to figure 1 of the “LED Audio VU Meter”:

Vref (at pin 6 & 7) = 1.25*(1+R4/R3) +R4*0.08
Vref = 1.25*(1+2.2/1.2) +2.2*0.08 = 3.72 V

This means that we get the LED full scale when the audio peak reaches 3.72 V at pin 5.

Now let us calculate the LED current:
I_led = 12.5/R3 + Vref/2.2
I_led = 12.5/1.2 + 3.72/2.2 = 12.1 mA (a reasonable one though it is less than 20mA)

The audio full scale to the circuit:
V_audio = Vref*{1+[R1+(VR1-x)]/x}
where 0 < x < VR1 , that is 0 < x < 50K
V_audio = 3.72*{1+[10+(50-x)]/x}
x=0 ==> V_audio = infinity
x=25K ==> V_audio = 8.9V
x=50K ==> V_audio = 4.5V

For the LED voltage, I prefer you use the lowest one you have from a voltage supply.
For 12mA, the drop at the open collector output will need just about 1V to start acting as a constant current source. The LED forward voltage can be assumed about 2V if red and 3.2V if green.
So the minimum V_led that works fine is:
For red color, V_led = 1 + 2 = 3V
For green color, V_led = 1 + 3.2 = 4.2V
Let us assume that we have 5V to supply both colors then:
Vout = 5 - 2 = 3V for red, IC pin dissipation = V * I = 3 * 12.1 = 36.3 mW (relatively low)
Vout = 5 - 3.2 = 1.8V for green, IC pin dissipation = V * I = 1.8 * 12.1 = 22 mW (in this case it is even lower)


Could you measure the various pin voltages excluding the outputs since they are surely close to V_led I guess (minus the LED forward voltage which is about 1.5 for low currents).
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Problem is that LED should be drawning more current, it makes a very weak light.

Thank you BradtheRad for the support.

Yes it's going to be for audio but not for power signals, it's for line level signals.

What you mean exactly by: ''Can you determine whether the lowest output pin (#1) ever goes low in response to your signal? Guess you know that you must supply current to a pin to find this out. The outputs do not source current''?

I'm using a function generator for the signal, 5v rms @ 1khz cause that's the threshold where my mixer meter starts to go red.
I know there are transistors at the output that could also drive logic and Vled acts as Vcc.

I tried different situations, starting with the potentiometer and different ratios of R1 and R2. Sweeping the signal up and down.

Excepts for current drawn at the output pins every other measurement is correct, as datasheet says reference current is for example 1.20mA, 10 times less the the expected output current.

I tried different Vled voltages, higher and lower than 7 volts, heat dissipation is not a problem till now cause led is barely lighting up!

Yes, also tried to connect all the LEDs array but current is so low that they are not powering.

Where am i wrong?

I will try those values R1 and R2

Yes calculations are right even if mine were not as accurate as yours.

My first intent was just to test on breadboard that it would work.

Ok, will measure all the pin's voltages except outputs and will let you know.

Thank you for your support!

What you mean exactly by: ''Can you determine whether the lowest output pin (#1) ever goes low in response to your signal? Guess you know that you must supply current to a pin to find this out. The outputs do not source current''?

It's a quirk of how certain IC's are designed. We expect to see positive output voltage when the output turns on. So we habitually touch the red meter lead on the pin, and we touch the black meter lead to ground.

However this will not work.

So are you measuring voltage at output pins by connecting one meter to lead to a pin, and the other meter lead to V+?

Because the output pins do not ever produce a positive voltage. So you won't get a reading that way.

I get the idea you've installed one or more LED's at the output pins. And that they're connected to V+ in the proper manner through an inline resistor.

One more thing to try: Vary the supply voltage. This may bring levels at one pin or another within a range which makes an output LED light up.

Also you must ensure the input is not being diverted through a low resistance somewhere. This could reduce its strength down to where it has no effect at the input of the 3915.

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