Electronics > Projects, Designs, and Technical Stuff
LM3914 led driver chip
Tegra:
About a year ago I was informed that the DIP LM3914 was in "lifetime buy" state. Kind of like last call at the bar. Appears the SMD is still a going concern.
These are a rather pricey chip but have had good success with the "ebay versions". (by its very nature it is not a precision device)
However, as this source may also "dry up" have decided to update the design using an inexpensive 3 digit voltmeter module instead.
T
floobydust:
Replicating a LM3914 in software is very difficult.
The LED outputs are not hard on/off, they can go to partial brightness. The LM3914 also has dithering.
So you would need an MCU to PWM the LED's with some random jitter added, to preserve its resolution.
Example is a 3 LED analog bargraph verses MCU driving 3 LED's. The analog bargraph can give over 6 distinct levels, but the "digital" bargraph has only 3.
westfw:
--- Quote ---The LM3914 also has dithering.
--- End quote ---
It does? I guess I was misled by the "simplified internal schematic" in the TI datasheet; there are apparently some tricky things you can do with the MODE and REFERENCE inputs. But I don't see dithering, specifically?
--- Quote ---The analog bargraph can give over 6 distinct levels [on three LEDs]
--- End quote ---
You mean like this: https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_16/Sec_53/Pease_Lab_Notes/Dithered_Bar_Graph_Display.pdf ?
I guess. Takes additional chips. I'm pretty sure I can do the same thing in software, although getting the exact effect might be ... annoying.
T3sl4co1l:
The comparator gains aren't terrifically high and don't have hysteresis, so there is a small linear range between steps; or it may self-oscillate from coupling, or dither from noise.
A large resistor from each output, to the bottom of the resistor divider (and a small resistor from there to ground), implements hysteresis per step, if you're so inclined.
Tim
SiliconWizard:
Replicating it exactly will be obviously difficult, all the more that some of the "special" features mentioned are either undocumented, or pretty vaguely, and some seem to rely mostly on field experience with the chip?
Now for people willing to just use this as a bargraph driver, implementing it in a small MCU is relatively easy. Even if you have to do PWM for each LED output and implement some overlap in software. It's not rocket science either. Will it be exactly like a 3914? Nope. Will it matter? Only for those using it in an extremely specific way. Probably not for a typical vu-meter.
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