LM3915 - More then 10 LED's

[sizzox]

For my project i'm building a huge VU Meter to go on the beer pong table. Seams like most people use the LM3915 to do this. Now I want to use about 48 leds per VU Meter Have 2 Rows of 24. Maybe it wil be easier to make 2 seperate Meters? Now to do this can I connect more LM3915's or should I just use one LM3915 and connect the LED's with transistors?

For reference skip to 2:25 this is what I want on the border.
YouTube - Best Beer Pong Table EVER (now taking orders for tables)

 

[jimlovell777]

It depends on what effect you want. Using one LM3915 would mean at least two LEDs would light per step. If you want each LED to be independently lit based on input you would need no less than 6 LM3915s.

 

[sizzox]

Lighting 2 LEDs per step is fine with me. I think if only 1 lit up per step it would be to big of a range and the top LEDs would never light up.

 

[audioguru]

The datasheet of the LM3915 shows how to cascade two of them for a 60dB range.
Classical music has such a wide range of levels but rock and pop music should use only a single LM3915 for a 30 dB range and use two or three leds in series on each output.

I would use microphones as inputs so people talking and screaming (in addition to background music) would be indicated.

 

[sizzox]

I think I would be better off using just 1 lm3915. With that figured out for my design I am gonna need to run 16 LEDs per output from the lm3915. How do you suggest doing that. Using transistors? The LED info is as follows Max forward current 20mA continuous, 5VDC Max reverse Voltage. 2.2VDC (typ) Forward voltage drop

 

[bountyhunter]

Lighting 2 LEDs per step is fine with me. I think if only 1 lit up per step it would be to big of a range and the top LEDs would never light up.

You set the minimum (lowest LED) and highest LED turn on level by adjusting the voltages applied to the LM3915 resistive divider.

 

[sizzox]

You set the minimum (lowest LED) and highest LED turn on level by adjusting the voltages applied to the LM3915 resistive divider.

Thanks for the info but I think it will be better with just 1 LM3915 and having 2 LEDs light up per step, per meter.

Now can someone help me figure out how to wire the leds up. Im going to have 8 separate meters on this table all running off the same LM3915 so there will be 16 leds running off each output on the LM3915. What is the best way to wire the LEDs up?

 

[sizzox]

Any help on how to wire?

Running 12V

LEDs - Max forward current 20mA continuous, 5VDC Max reverse Voltage. 2.2VDC (typ) Forward voltage drop

Want to run 16 LEDs per output on the LM3915. Yes thats correct 160 LEDs total.

 

[audioguru]

Any help on how to wire?

Running 12V

LEDs - Max forward current 20mA continuous, 5VDC Max reverse Voltage. 2.2VDC (typ) Forward voltage drop

Want to run 16 LEDs per output on the LM3915. Yes thats correct 160 LEDs total.

Each output of the LM3915 can drive a transistor that can drive 18mA into strings of series'd LEDs, each string needing its own current-limiting resistor.

 

[sizzox]

Each output of the LM3915 can drive a transistor that can drive 18mA into strings of series'd LEDs, each string needing its own current-limiting resistor.

So would I have to run 4 transistors for each string with 4 leds per string to equal 16 leds per output?

But with this set up it draws 80 mA

 

[audioguru]

You limited the supply to only 12V then you can have only 4 LEDs in series for each string. Then you need 4 strings to drive 16LEDs at 18mA each and the total current is 72mA.

 

[sizzox]

OK I gotcha. Thanks for all the help

 

[MrDEB]

what about using a TIP120

then have the 3915 drive it, have the LEDs 4 sets of 4 in series wired in parallel?
not sure of amout of loss etc, just thinking off top of head??
If I was building such a table, why not just use a PIC, then control using A/D with a mic inputting the analog signal.
might work??
then program several different patterns as well as the AD. using a rotary switch or ? to designate pattern desired.
just a thought???

 

[audioguru]

LEDs should never be connected in parallel unless their forward voltages are all matched.

 

[MrDEB]

good point

get out the DVM

 

[audioguru]

You would need to buy hundreds or thousands of LEDs to find some that have the same forward voltage. Then they can be connected in parallel and all will have the same current.

 

[sizzox]

So what do you recommend?

 

[audioguru]

Each output of the LM3915 has a regulated current of up to 30mA.
LEDs in one series string can be driven from each output. If more LEDs must be driven then they can be in a series string and in series with a current-limiting resistor and driven with a transistor.

 

[MrDEB]

not sure but?

connect 4 or 5 in series, then measure current and voltage then using resistors, match up the strings?
maybe even use pots?
might work??

 

[bountyhunter]

LEDs should never be connected in parallel unless their forward voltages are all matched.

Or unless you put a little bit of resistance in the tail of each one then tie them together above the balance resistors.

But, he is correct, without some balance resistance they will not be the same brightness or conduct the same current.