# accuracy test for LM3915 led bar graph driver

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#### sachinkp21587

##### Member level 2
I am using LM3915 for a application of mine i.e audio level detection.
I am able to run the ic well but my doubt is that the data sheet says that it acts as per 3dB/Step.

How i decode this is dt when we say 3dB change that means audio power has doubled i.e. in formula 10log(base10)(P2/P1), the P2 = 2*P1
Since, P(Power) = V(square)/R it implies that 10log(base10)(P2/P1) = 20log(base10)(V2/V1)

M i right till here?

if yes thn in proteus as well as practical how come my following readings relate to 3dB/step change?
1st led at .06V
2nd led at .08V
3rd led at .12V
4th led at .16V
5th led at .22V
6th led at .32V
7th led at .44V
8th led at .63V
9th led at .88V

in place of leds actually i have pulled up every pin with a 10k resistance and monitoring the pin by help of mulitmeter. when no audio the pin remains at 5v and becomes low between mV when some audio exists depending on audio level

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in proteus the 10K R11 is replaced with value between 100R to 1k for the same result else it doesnt work in protues but practically is working

#### godfreyl

when we say 3dB change that means audio power has doubled i.e. in formula 10log(base10)(P2/P1), the P2 = 2*P1
Since, P(Power) = V(square)/R it implies that 10log(base10)(P2/P1) = 20log(base10)(V2/V1)

M i right till here?
Yes, that's right.
So for a 3dB increase:
P2/P1 = 1.9953 (almost = 2)
V2/V1 = 1.4125 (almost = sqrt(2))

if yes thn in proteus as well as practical how come my following readings relate to 3dB/step change?
It looks about right. Here I've added the dB increments between each LED:

1st led at .06V
2.50dB
2nd led at .08V
3.52dB
3rd led at .12V
2.50dB
4th led at .16V
2.77dB
5th led at .22V
3.25dB
6th led at .32V
2.77dB
7th led at .44V
3.12dB
8th led at .63V
2.90dB
9th led at .88V

#### sachinkp21587

##### Member level 2
2.50dB is approximated as 3dB ? Is int it over approximation? i mean .50db error
added i would like to know that is there any way that i can only switch of or on the led in place of toggling the brightness

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also please let me know the difference between LM3916 and LM3915.

#### godfreyl

2.50dB is approximated as 3dB ? Is int it over approximation? i mean .50db error
The manufacturer doesn't claim great accuracy. The datasheet says:
minimum: 2dB
maximum: 4dB
typical: 3dB
so it's within spec.

LM3916 is similar, but the step sizes are different. Check the datasheet or Google for details.

#### sachinkp21587

##### Member level 2
added i would like to know that is there any way that i can only switch off or on the led in place of toggling the brightness
Please guide in this regard also

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