Thank you, it's very interesting  

I've made a test circuit like this one:

(Pi filter bridge?)

With a 1000µF polarized capacitor and a 0.1µF capacitor (right side) but for the inductance it's write:

The inductance value is not too critical, but generally as big as you can manage, a few tens of mill Henrys is normal.

It makes me... Thinking  :-X

Is a spare part from an ATX power supply (I'v an old one to use as parts) from the line +12V could be fine for my case?
I don't know the Henry value, but.. It's just before wires, associated with a 2200µF polarized capacitor (it looks like a small vertical cylinder).

Hi all  

I was asking myself to solve a little problem what appends in a small project.

In this project, I want to drive a heating element (resistor) by PWM with a Arduino. The resistor (6Ohms) is powered with 12V (security question) by a Darlington transistor (TIP120). The frequency of the PWM is very low (1Hz, but it have to be like that), and resistor' temperature is controlled by the PWM ratio/cycle.

All's working very fine, except the resistor makes strong voltage variations in all the 12V line when it's powered on/off, and there's other electronics stuff power by the same line. I recorded variations around 1V and I need <0.1V.

I learned it's usual in switch-mode, but what's the best way (simple/sort place to make something) to isolate the rest of the line and other electronics parts?

I found some texts speaking about inductance, inductance + capacitor (low pass), but it's not very clear for me, and I'm not an engineer..  :-[

Any idea or schematic will be appreciate,
thank!

Haa, ok... I told to myself there's something what I don't understood ^.^
Thank you bircoe -.^

o.O
So my schema is good?

Of course, it read Vout on the Arduino pin #1 and Vref in pin #2. It's a bit different from your original post where it was #0 & #1.

If I'm again wrong, is it possible for you to draw a simple schema of your setup? (I think it will be more easy ^.^)

ekaki: nailed it in one, you take analog readings off pin 1 and 2, pin 2 is the ground reference.

So I'v to disconnected the Arduino pin #2?
And just read pin #1?
Oops, no, we need a LM35'ground reference...
Hum.. But where is taken the LM35'ground reference?

I'm pretty interesting by this excellent solution with a LM35, but I'm not sure to understand exactly the configuration of the initial wiring (1st post), so could you say if it's like that:



(big IC top-right is the Arduino)

Wooh yes, well seen -.^
I could cut the link between the pin #11 of the PIC and the NPN, and link it directly to an I/O port in the Arduino; there's a resistor (R9) on board between the PIC and the NPN. I'll see that, it's a micro-surgery operation ^.^

Yes, it's why I ask in my first post if the most simply solution will be by using an auxiliary way to control the backlight. A way where I could have fine tune possibilities.

I will see.. Cause I will have to make a complete power system, use a I/O output on the Arduino, etc. It's a bit a pity to get a component like this pretty serLCD but to have to hack it at end.

However, thank you for your help Grumpy_Mike -.^

Woho... I just understand something..
In:
http://www.sparkfun.com/datasheets/LCD/SerLCD_V2_5.PDF
We can see:
"By sending the special command character 0x7C followed by a number 128-157, the backlight PWM value will be set."

But in fact, we don't know if it's frequency or duty cycle that will be changed...
In regard of results (flickering) at half values it's possible that the command is on frequency and not on duty cycle (?). I don't have scope to see the signal shape.

Well, it does not solve the problem... :-/

It's a SparkFun serial LCD 16x2:
http://www.sparkfun.com/commerce/product_info.php?products_id=9395
with a common HD44780 character LCD:
http://www.sparkfun.com/datasheets/LCD/HD44780.pdf
driven by a PIC16F688 (@10MHz) CMOS µcontroller on board:
http://ww1.microchip.com/downloads/en/DeviceDoc/41203B.pdf
http://www.sparkfun.com/datasheets/LCD/SerLCD-v2-schematic.pdf
http://www.sparkfun.com/datasheets/LCD/SerLCD_V2_5.PDF

It looks that backlight is driven through the pin #11 of the PIC, with an 1A NPN. But I'v no ideas about how force/change the PWM frequency generated by this PIC. :-/

I did say google it. Never mind I did it for you:-
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1152547089

Thank you to help me, but... I'm not sure to understand what you want to tel me.  :-/

In the datasheet of the LCD board I see how changing the duty cycle of the PWM to adjust backlight by sending a cmd to the PIC to change the PWM frequency (0x7C + 128<157). So I think the backlight PWM is driving by the PIC, not by the Arduino (?).  :-?
I imagine I have to send a cmd to the PIC to change the PWM frequency like in your example, but I don't see how.

Sorry for all my questions but I like to know what I do and not just copy some code parts ^.^

NO, see how PWM works:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html

Hum, yes, I had to say "voltage is always the same at high level", I spoke about filtering the signal, in our case.
Of course, in PWM, voltage is low or high; not always the same ^.^

No the PWM frequency is controlled by the arduino, you just need to alter some values stored in registers.

:-?
I did not understood that in the serLCD docs..
Could you tell me the process to alter those values?

Exact Grumpy_Mike, as the voltage is always the same with PWM, if I filtering it, it'll be impossible to change dimming of the LED... -.-

I thought to change the PWM frequency, but as it drive by the PIC on my serial LCD, it may be not easy. I'll see that -.^

I find the backlight very powerfull (very well to use by daylight), but in most of case what I'm making will be use by night (power supply for astro-photo applications) so it's a real need (we try to hide a maximum of lighting).
During tests I set the backlight at a very low value (130, BYTE), in regard of possible parameters; it's quite fine, but not too much ^.^

Else, I asked to find a way with the PWM (capacitor?) cause I wouldlike to keep control of brightness by software; it's a good meaning to send visual alerts, by night :o

Anyway, I'll see for a hack of the LCD PCB, and if it's too hazardous (schematic is cheap for the serLCD-v2.. ^.^) I'll do as you say with another circuit to set the backlight. -.^

Hi -.^

I'm asking myself if there's a solution to eliminate backlight flickering when adjusting the brightness under full power on a SparkFun serial LCD (16x2)?
This flickering is not very strong, but in some conditions (neon lights for ex.) it's a clearly visible.

If I'm not wrong it's cause the backlight LED is drive by PWM.
So, is someone had trying to place a capacitor somewhere to "smooth" the PWM?

Or the best way is to use another power source, without PWM?