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LM3914 Dimming Led's

C

croft

Jan 1, 1970
0
Hi,

I'm fairly new to this, so i'm sorry for any newby questions!

Ok my problem is:

i would like to set up to 10 LED's to come on and off, but i do not
want them to flash. i would like them to dim on and off.


I have found the chip LM3914 which can do this when it is in dot
mode....

Ummm i think i need to connect the leds to the chip and feed upto a 5v
source to it.
is this all i need? or do i need another source to get the chip to
switch the LED's?

any help would be wonderful! or a circuit diagram even better!!!!!!

Thanks in advance
mike
 
J

John Tserkezis

Jan 1, 1970
0
croft said:
i would like to set up to 10 LED's to come on and off, but i do not
want them to flash. i would like them to dim on and off.

You mean each led in the sequence starts off dimly, then gradually to full
brightness before the next led in the sequence? That's what many el-cheapo LED
VU meters do (by consequence of their el-cheapo design).

They use various value resistors to drive transistors which control (around
about) at which voltage each LED lights up. As a consequence, they do not
light up instantly to full brightness as each switching point is approached.
(which is what you want).
I have found the chip LM3914 which can do this when it is in dot
mode....

It's relatively expensive, and will not dim the leds in that manner. Each
LED lights correctly to full brightness at each step.
Ummm i think i need to connect the leds to the chip and feed upto a 5v
source to it.

Yeah, if you use an IC, but the el-cheapo method would be more flexible.
is this all i need? or do i need another source to get the chip to
switch the LED's?

A better option would be a PIC, you can use PWM to emulate the switching
brightness at each point, and would resolve any wide tolerance issues for each
unit you build.
 
A

Al Yeager

Jan 1, 1970
0
You need to super impose (sic) a small amplitude ramp or triangle
waveshape into either the reference or input signal by means of a summing
circuit. This is a process refered as dithering. There was an app note on
the national site but it may be gone by now. When the input is at the low
end of the 1-of-10 range the LED glows dimly. As the input amplitude gets
larger then the LED will glow more brighter before switching over to the
next LED. I did this for a thermometer circuit a few years back. e-mail me
a reminder and I'll fish up the schematic and possible the NS app note if
I still have the CD from national.

hth

al
 
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