cold cathode VU meter

T

theshadow27

Gawd
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Sep 7, 2004
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alright [h]ard... im trying to take a LM3916 logoritmic LED vu meter chip. and hook it up to 4 or 5 cold cathode inverters. im thinking i can gang together 1+2, 3+4, etc, and use a transistor of some sort to switch the 12v. also, i want it to be most responsive to base, so i need an inductor of some sort.

specificly i want to build this myself, so:

-any thoughts on how many coils the inductor would be
-any thoughts on what type of transistor i could use (im thinking MOSFET?)
-ganging the outputs on the 3916 together.. could that be bad?
-instead of ganging, could i make the first one do 8v and the second do 12v for a smoother effect? i know inverters dont like low voltage, but i know they work at 10v and even 8v
-anything i need to know before i throw this on the output of an amp

EDIT: the same chip, but a diffrent project, im having 2 of these drive 2 rows of LEDs, but i want to use BI-Color LEDs. sence the chip sinks the LEDs, i figure that if the LEDs are red, orange, or yellow, they will alwase sink the same current, and alwase be the same brightness. but heres where it gets complicated: i want the LEDs to be yellow untill the 3rd LED lights up, then they change to orrange (all of them), then when it hits the 6th one, they all turn red. i would just throw a transistor on the 3rd and 6th pin, but i dont want to throw off the current draw for thos LEDs. so do i need to get 2 more (top and bottom) 3916 chips? or should i get a quad comparitor and hook that up?
 
>> "i want it to be most responsive to base, so i need an inductor of some sort. "
You mean bass, correct - not base? An inductor would reduce higher frequencies, but I'm not sure exactly what value you're going for. There are more advanced types of filters, but I'm not the one to ask. Google has some info though.

>> "-any thoughts on what type of transistor i could use (im thinking MOSFET?)"
You can use practically anything that meets the current requirement, which is probably about 200ma. I personally would use a mosfet, because you're only interested in turning it on and off, nothing in between. Just make sure that the lm3916 puts out enough voltage to fully turn on the gate of the mosfet, otherwise you will need a seperate driver stage, which in it's simplest form would simply be a small transistor such as 2n3904.

>> "ganging the outputs on the 3916 together.. could that be bad?"
I can't see any problems that would be caused by that, but if you wanted to be safe you could put a diode on each output. It's probably unncecesary though.

>> "-instead of ganging, could i make the first one do 8v and the second do 12v for a smoother effect? i know inverters dont like low voltage, but i know they work at 10v and even 8v"
You mean if you connect output 1 and 2 together, when output 1 goes on it outputs 8v to the inverter, and if output 2 goes on it outputs the full 12? It would be pretty simple to do, but cold cathodes are on or off, not in between. So if 8v is enough to turn it on, then it should be on all the way. If you want to try it though, then use a voltage regulator such as the adjustable lm317 or a dedicated 8v regulator.

>>"-anything i need to know before i throw this on the output of an amp"
Yeah, I'd use a potentiometer, something like 10-100k on the input. Just set it real low the first time then crank it up till it gets where you want.
 
each inverter needs to sink 650 ma MAX

if i went with 4 of them (not 5) could i use a 339 (4x 741 op amps), with the +on the ref resistor array and the - going to the input, (so on = +) then use a DS3658 chip to drive the inverters?

i guess i could set each intervel whatever i wanted...
 
okay the output buffer is on its way -- would anyone like a tut or should i not even bother taking pictures?
 
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