Truncated VU-Meter

I'm planning on building a VU-Meter/Spectrum Analyzer for a car stereo. The case it is to fit into is a standard DIN chassis(50mm tall & 180mm wide). That's the WINDOW, the actual case is a little bigger. There's room to stack 5 rows high, and 34 LEDs across using 3mm LEDs. Each "row" will be a bandwidth, with 17 LEDs left and right, going from treble up top to bass at the bottom. Each respective "channel" (left or right) will be driven by cascaded LM3915 ICs in bar mode.

From what I've read, you cannot have any unused leads for the LEDs on an LM39xx, they have to be in banks of 10. Yet I don't have enough room in the display window to show all 20. Nor do I want to have any unused LEDs illuminating the inside of the case, so I thought, what about infrared LEDs? Even if they're energized, the wavelength of light they produce is outside the visible spectrum of human sight.

Which brings me to the question: The IR LEDs are rated at 1.2-1.5 volts, whereas the colored LEDs are 2.2-3 volts. Can I mix and match? If so & they burn out, would the whole driver malfunction & not work at all? Should I use resistors on the IR bulbs to ensure their longevity?

(picture is of a previous failed attempt, but it shows the case & all the lights at maximum threshold & gives an idea of what I'm attempting to do)

How about using moulded LED strips? They are readily available is several colours.
10mm x 25mm with ten LEDs on 0.1" pitch is quite a common one, and it looks like five rows of four would fit the display window?

Red, yellow and green are fairly common; blue ones exist but are rarer.
Some examples on this page:

If you use individual LEDs, you can put anything in the "dummy" positions. The driver IC regulates the current so the exact LED voltage does not matter.

Super-Dave said:

From what I've read, you cannot have any unused leads for the LEDs on an LM39xx, they have to be in banks of 10.

Click to expand...

Not sure that's true.
But if it is, just connect a resistor of suitable value (one that drops about 2-3V at the operating current of the other LEDs) in place of each unused LED.

The different forward voltages of different coloured LEDs shouldn't be a problem as they are current-driven by the LM3915 pins.
Do you have the ICs? They're obsolescent.

alec_t said:

Do you have the ICs? They're obsolescent.

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Yes, ordered & received, got a tube of 10, & 20 LM3914s as well. They may be obsolete, but someone is making & selling them online, there are 10s of 1000s of them available thru different vendors.

Super-Dave said:

From what I've read, you cannot have any unused leads for the LEDs on an LM39xx

Click to expand...

Not true - thinking logically, what is the difference between an LED not lit and one that isn't there?

Besides, I've used these chips with odd numbers of LEDs without problems and there are plenty of old projects out there as well that do the same.

Somebody is making and selling fake obsolete LM3915 ICs online.
Each output of a real LM3915 has its own comparator that is completely separate from all the other comparator outputs so each output does not need an LED or a resistor.

17 LEDs will show a range of 51dB which is good enough in a car.

I have an LM3915 VU meter in my family room with 10 levels plus a switched gain of 10 times for very low levels. Its input is a microphone.

Ok, next question. Some of the different colors have different voltage ratings. Blues & greens are 3v, yellow, orange and red are 2v. Do I change the values of pin 6 of the drivers to compensate? Each band will have a single color so it's not mix & match per driver. I don't want to have any burnt out bulbs 6 months down the road from overvoltage. I know a 2v bulb will work with 3 volts going thru it, but it won't last long.

The resistor on pin 6 programs the LED current, not voltage.

As long as the defined current is within the permitted range for the LEDs and the supply voltage is adequate, the individual LED colour & forward voltage do not matter.

Super-Dave said:

From what I've read, you cannot have any unused leads for the LEDs on an LM39xx, they have to be in banks of 10.

Click to expand...

No, and yes.

There is no requirement that all outputs have a component attached. You are free to skip some, use every other one, etc.

"banks of ten" - yes and no. The part has 10 outputs, so cascading parts increments the number of outputs in increments of 10. But as above, there is no requirement to use every outputs. Cascade two parts, use the first 17 outputs, and ignore the last three. Or ignore the first three, and use the remaining 17.

The circuits on page 14 of the datasheet show how to do this.

ak

Our vision is most sensitive to green and modern green LEDs are much brighter than other color LEDs so you might need to reduce the current on the green LEDs.
Then blue is the next brightest color LED.
Pin 6 voltage has nothing to do with LED brightness. Pin 6 voltage sets the input voltage that lights the highest LED. The current out of pin 7 sets the current in all LED outputs but additional series resistors with the green and blue LEDs will reduce their currents and brightness.