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Using Arduino => Project Guidance => Topic started by: xl97 on Oct 28, 2011, 07:15 pm

Title: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 28, 2011, 07:15 pm
I am trying to set-up a simple voltage meter using the Arduino Duemilanove..


First I need to make the voltage divider...

I am current using a PSU to power... but am trying to use 7.4v in my equation for figuring out the math (which I set my PSU to)...  (which doesnt seem to be working)..

Im sure I have it wired up wrong =(


alternately..once past the voltage divider.. I am un-sure if I need to use a resistor on each led.. or can I combine all negative leads and use 1 resistor for the total?


(to throw a wrench in all this.. but not needed.. I was hoping to maybe use different color leds too.. which of course reds/yellows have lower vF...I believe)..

anyways.. the voltage divider first I guess...

here is what I tried before posting:
(http://dmstudios.net/misc/voltageDivider_voltMeter.jpg)

I took the LEDs out of the picture.. until I can dial in the voltage.. (at least 'close enough'..doesnt need to be perfect)


using this simple code.. I watched the serial monitor:

Code: [Select]
//voltmeter tester/PLI

int voltagePin = 5;
int voltageVal = 0;           // variable to store the value read

void setup()
{
  Serial.begin(9600);          //  setup serial
}

void loop()
{
  voltageVal = analogRead(voltagePin);    // read the input pin
  Serial.println(voltageVal);             // debug value
}



if at 7.4v. I should be getting 1023..

and as I turn down lower.. the reading/output should also adjust (lower)..

however.. I dont get a change in the reading until I am under 5v+.. (which to me says the voltage divider is set up wrong)

as always.. I tried to read what I could (and understand).. and also give it a 'try' before asking to be spoon fed!  lol

Thanks
Title: Re: Voltage Divider & Voltmeter help
Post by: Grumpy_Mike on Oct 28, 2011, 07:23 pm
Quote
I am un-sure if I need to use a resistor on each led.. or can I combine all negative leads and use 1 resistor for the total?

You need a resistor on each LED. If the LEDs share a resistor then the brightness of the LEDs will depend on how many you have on.

Quote
however.. I dont get a change in the reading until I am under 5v+..

Then you have not wired it up in the way you have shown, the values look fine, although a volt meter with an input impedance of 1K5 is a bit of a poor meter.
Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 28, 2011, 08:21 pm
Hello Grumpy_Mike-

re: leds.

thanks.. makes sense.. as also takes care of the 'color' problem (as I can use a resistor to match that specific led's vF



re: wiring

I have +Vin (PSU +).. going to breadboard..   same row, next hole.. I have resistor (470 Ohm), in-line/same row after that second resistor leg (again same row).. I have the +Vout going to Arduino A5 pin.

still (same row) after the Vout to A5.. I have (last hole in row) 1 leg of resistor (1k Ohm)..going to another , new row.

In new row.. on 1 side of resistor.. I have -Vin lead (goes to PSU -).. on other side of resistor (same 1K Ohm) I have the -Vout to a GND pin on the Arduino


here is a different schematic.. more or less showing it how it is on the breadboard..  (maybe because Im using a BB I screwed it up?)   (yes, I know you dont like breadboards.. its all I have!)  LOL

(http://dmstudios.net/misc/voltageDivider_voltMeter2.jpg)



Also.. can you dumb this up a bit?  LOL.. or maybe give an explanation?

Quote

although a volt meter with an input impedance of 1K5 is a bit of a poor meter.


Im not  sure what you mean?  in any project "I" would use this in would be a 3.7v, 4.5v, 6v or 7.4 volt (the only battery packs I really use in my hobby)..  mostly 7.4v Li-Ions.. but others have been used.

I want to light ALL leds if full juice.. and go down on the number of leds lit after that..  (reaching my cut-off/threshold)...which would leave the leds only 1 lit.

what do you mean by 'poor'?

and how do you remedy that?

thanks
Title: Re: Voltage Divider & Voltmeter help
Post by: CrossRoads on Oct 28, 2011, 08:34 pm
"input impedance of 1K5 " just means your meter draws a lot of current - in this 7.4V/1500 = 5mA, could use 4.7K.10K and cut that down to 0.5mA.

7.4 to 470, to 1K to gnd. Junction of 470/1K should be 5.03V if batteries were discharged and down at 3.7V. (7.4*1K)/(.47K + 1K)
Fresh batteries will  be more like 8.4V tho, so 5.71V, dropping to ~5V when discharged.
Consider changing your R values to reflect that.
Title: Re: Voltage Divider & Voltmeter help
Post by: Grumpy_Mike on Oct 28, 2011, 08:45 pm
You want to look at the LM3914, that does what you seem to want to do with no arduino at all.
Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 28, 2011, 11:34 pm
(stupid session timeouts...lost my whole damn post!)   grrr..


anyways..

thanks for replies.. to quickly sum up what I had origianlly typed out.

I 'do' want to use an Arduino..  (but in the end.. Im hoping to NOT take up 10 I/O pins to do so...  (maybe a shift register?..going back to my other post...kinda works for same principle?)



@CR_

man you guys are just too smart/knowledgeable on this junk!..  Its another project just to decipher the answers!  LOL


1.)  Grumpy_Mikes comment..  (the meter itself is drawing excess current)... got it!  Although I wouldnt have an clue nor cared without you guys pointing it out..

2.) I think your saying..I can cut that 'pull' of excess current down by... ??  'adding'??  additional 4.7k & 10k resistors?  (where?.. and additional?..not in place of...right?)

3.) I think your saying I should re-calculate my resistor values.. based on Vin of 8.4v and not 7.4v.......yes?  (fully charged is more at 8.4v instead of 7.4v)


Thanks
Title: Re: Voltage Divider & Voltmeter help
Post by: Grumpy_Mike on Oct 28, 2011, 11:36 pm
Quote
I think your saying..I can cut that 'pull' of excess current down by... ??  'adding'??  additional 4.7k & 10k resistors?  (where?.. and additional?..not in place of...right?)

By using resistors where you have them but making them ten times the value you have them.

Quote
I think your saying I should re-calculate my resistor values.. based on Vin of 8.4v and not 7.4v.......yes?

Yes
Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 29, 2011, 12:31 am
(DOH!)...

ok.. so 'replace' with those values.. (I didnt even catch it was 10x the value either)..

thanks.

and then the second part.. if I re-calculate using a larger Vin..  wont those values ^^ need to change then?



I am running it from a PSU (stable Vin)  7.4V/1Amp..

So I will change out the resistors to be 4.7k & 10k respectively.. and lets me keep the draw form the 'meter' low...

but why @ a stable 7.4v from the PSU am I getting readings of 1023 from +5v - +7.4v??

Title: Re: Voltage Divider & Voltmeter help
Post by: CrossRoads on Oct 29, 2011, 02:42 am
Ah, I was thinking you had two Li-Ion batteries, which are 3.7V x 2 = 7.4V.  Just a coincidence I guess.  Sorry about that.

If the output is a stable 7.4V, how are you getting it down to 5V?  Think we're missing something there.

Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 29, 2011, 02:51 am
hi CR_

to clarify.. in the 'end' I will be using two li-ions (7.4v)..

but until then I am using a variable PSU set at 7.4v to mimic it...

(then I am dialing the PSU down from 7.v to see the serial monitor output)
Title: Re: Voltage Divider & Voltmeter help
Post by: CrossRoads on Oct 29, 2011, 03:26 am
Okay, then I stand by my recommendation to change the resistor values so that at 8.4V you have 5V at the resistor junction.

5V = (8.4V x 10000)/(x + 10000)

solve for x.
Title: Re: Voltage Divider & Voltmeter help
Post by: scottydb4b on Oct 29, 2011, 04:56 am
Hi, I have a problem also with a arduino voltmeter/ammeter that is part of a power supply I made.
I have written all the code connected it up and it reads the volts and amps pretty accurately on an lcd.
My problem is when i connect a load to the power supply the voltage drop shown on the lcd is no where near what it reads on a multimeter. ie. if I set the voltage to say 6.00 volts connect a 3.9ohm load the lcd shows 5.8 volts where as the multimeter shows something around 5.00 volts. and doesn't drop much more with bigger loads either.
Does anyone know why this might be?
Title: Re: Voltage Divider & Voltmeter help
Post by: CrossRoads on Oct 29, 2011, 05:01 am
Mostl likely a  math error in your calculation from the analogRead to the voltage you display.
Maybe mismatched variable types?

Can't really say without seeing  your code tho.

6/3.9 = 1.53846 Amp, is that being displayed correctly?
Title: Re: Voltage Divider & Voltmeter help
Post by: scottydb4b on Oct 29, 2011, 05:48 am
thought the problem might be electrical but ill attatch the code for you to look at.

but I get about 1.1 amps when i do the maths there so its a little out.

cheers

Code: [Select]

#include <LiquidCrystal.h>
LiquidCrystal lcd(13,12,11,10,9,8);

// These constants won't change:
const int analogPin = A4;    // pin that the sensor is attached to
const int ledPin = 6;       // pin that the LED is attached to
const int threshold = 365;   // an arbitrary threshold level that's in the range of the analog input

int analoginput0 = 3;    //analoge pin 2
int analoginput1 = 4;    //analoge pin 5
float vout1 = 0.0;    
float vout2 = 0.0;
int value1 = 0;
int value2 = 0;
float R1 = 5100.0;    // !! resistance of R1 !!
float R2 = 2193.0;     // !! resistance of R2 !!
float vin1 = 0.0;
float vin2 = 0.0;

void setup(){
 lcd.begin (16,2);
 lcd.setCursor (5,0);
 lcd.print ("G'day");
 delay (1000);
 lcd.setCursor (6,1);
 lcd.print ("Mate");
 delay (2000);
 
   // initialize the LED pin as an output:
 pinMode(ledPin, OUTPUT);
  // initialize serial communications:
 Serial.begin(9600);

 // declaration of pin modes
 pinMode(analoginput0, INPUT);
 pinMode(analoginput1, INPUT);
 lcd.begin(16, 2);

}

void loop(){
 // read the value on analog input
 value1 = analogRead(analoginput0);   //voltage value
 vout1 = (value1 * 4.555)/1023.0;
 vin1 = vout1 / (R2/(R1+R2));
 

 value2 = analogRead(analoginput1);    //Amp output
 vout2 = (value2 * 4.555)/1023.0;    //4.555=voltage threshold (1023=4.555V);
 vin2 = vout2*2;  //vin2 = value2;
 
lcd.setCursor(12,0);
 lcd.print("  ");
 lcd.setCursor(8,0);
 lcd.print(vin1+0.1);
 lcd.setCursor(8,1);
 lcd.print(vin2);

 lcd.setCursor(0,0);
 lcd.print("Volts");

 lcd.setCursor(0,1);
 lcd.print("Amps");
 
  // read the value of the potentiometer:
 int analogValue = analogRead(analogPin);
 
  // if the analog value is high enough, sound alarm:
// if (analogValue > threshold)

  if (vin2 > 3.2)      // Set amp alarm value
 {
   digitalWrite;     //(ledPin, HIGH); //ledpin for led
   tone(7, 3);
   delay(100);
 }
   else{
    digitalWrite(ledPin,LOW);
   noTone(7);
   }

 delay(100);
}

(code symbols added by moderator)
Title: Re: Voltage Divider & Voltmeter help
Post by: CrossRoads on Oct 29, 2011, 06:16 am
Why is your threshold voltage 4.555 and not 5.0?  Power supply is a little low?

vout = readvalue * 4.55/1023, okay

vout = vin *R2/(R1+R2 ) with R1 connected to Vin and R2 connected to gnd
vout*(R1+R2)/R2 = vin , okay

This seems odd:
value2 = analogRead(analoginput1);    //Amp output
  vout2 = (value2 * 4.555)/1023.0;    //4.555=voltage threshold (1023=4.555V);
  vin2 = vout2*2;  //vin2 = value2;   <<< where does the 2 come from?  0.5 ohm shunt to ground or something?



Title: Re: Voltage Divider & Voltmeter help
Post by: scottydb4b on Oct 29, 2011, 08:13 am
The 4.555 is the voltage I got when I measured the analogue input variable power supply and multimeter, so 1023 is reached at roughly 4.555 volts not 5 volts exactly, unless its the multimeter telling me different.

and I think i just added the *2 in there just to make the amps close to what it is ment to be. But yes i got a 0.5 ohm shunt on the ground rail.

Im no coding expert and just improvising where i can.

thanks for the help though.
Title: Re: Voltage Divider & Voltmeter help
Post by: Grumpy_Mike on Oct 29, 2011, 11:06 am
What does the +5V pin measure? The 4.555V is a bit low, maybe your meter is off? All measurements are relitave to the voltage on the +5V pin.
Title: Re: Voltage Divider & Voltmeter help
Post by: dc42 on Oct 29, 2011, 11:36 am

So I will change out the resistors to be 4.7k & 10k respectively.. and lets me keep the draw form the 'meter' low...

but why @ a stable 7.4v from the PSU am I getting readings of 1023 from +5v - +7.4v??


My guess is that either the resistors you are using are not really 470 ohms and 1K (e.g. the one you think is 1K is actually 10k, or the one you think is 470 ohms is actually 47 ohms), or you are using one of those breadboards that has a break in the middle of the 5v and gnd rails and you haven't taken account of that break (so the 1k resistor is not connected to ground).
Title: Re: Voltage Divider & Voltmeter help
Post by: scottydb4b on Oct 29, 2011, 02:42 pm
Yeh the 4.55 might be a bit low but at the end of the day i have to calibrate it to something and thats the reading i get @ 1023, its really no big deal i can change it to 5 but is less accurate, according to my meter. But its doesn't have anything to do with the voltage drop issue, or lack of it should i say.
Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 29, 2011, 02:52 pm
ack!.. you thread stealer!  =)

anyways..


@ CR-

5V = (8.4V x 10000)/(x + 10000)

should be:

5v = (8.4v x 10000) / (6800 + 10000)

right?



Im still a bit unclear that even with the other resistors in place for a the 7.4v setting.. 'why' it didnt represent a 7.4v 'value' in the serial print?

I mean it was steady 1023 from +5v up to 7.4v..??

I understand why change the resistors.. (since in the end I'll be using 2 x 3.7v li-ions)..

but what was wrong with it currently.. at the 7.4v??  


In my mind.. changing out these resistors will do NOTHING for me?  (but are only to be put in place because in the end I'll be using a 2 x li-ions)

the question (for me) still remains and is unanswered..

WHY with the current set-up of resistors/voltage divder.. and a 7.4v source am I NOT getting any changes in my analogRea() from 5v to 7.4v?
Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 29, 2011, 02:54 pm


So I will change out the resistors to be 4.7k & 10k respectively.. and lets me keep the draw form the 'meter' low...

but why @ a stable 7.4v from the PSU am I getting readings of 1023 from +5v - +7.4v??


My guess is that either the resistors you are using are not really 470 ohms and 1K (e.g. the one you think is 1K is actually 10k, or the one you think is 470 ohms is actually 47 ohms), or you are using one of those breadboards that has a break in the middle of the 5v and gnd rails and you haven't taken account of that break (so the 1k resistor is not connected to ground).



sorry I missed this post.. (DOH)..


is there a way I can meter my resistors? to accurately find out the values?

or I suppose I could also try to look up the color bands?  (always hate that)..


any better way?

Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 29, 2011, 03:22 pm
ok..

I took BRAND NEW 4.7k Ohm and a 10k Ohm resistor (fresh out of the pack)...  and put them in place

4.7k in between the +Vin and +Vout

the 10k between + and the -   (same as illustrated in the picks.. (but the values changed)

and I 'again' get the same results?  once I turn the PSU up.. and I get to +5v.. Im already capped at 1023 analog value??

what the 'heck' am I missing here? lol..

I dont even have the LEDS in the mix or part of the project until I can fix the analog read values to be 'somewhat' near my ranges..??

I mean damn.. I only have 2 wires going to the Arduino!!!  LOL  GND and A5

some resistors..and two leads going to my variable PSU..



Title: Re: Voltage Divider & Voltmeter help
Post by: dc42 on Oct 29, 2011, 04:14 pm
It sounds to me that you do not have the 10K resistor connected as you intend. Can you post a photo of your wiring?

If you disconnect the 4k7 resistor, do you get a stable reading of zero from the analog pin?
Title: Re: Voltage Divider & Voltmeter help
Post by: runaway_pancake on Oct 29, 2011, 04:19 pm
How about borrowing that 10K pot from your LM386 project.
Since you're just experiencing, using a pot will make things a little easier.

Edit -- corrected drawing
Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 29, 2011, 05:01 pm

It sounds to me that you do not have the 10K resistor connected as you intend. Can you post a photo of your wiring?

If you disconnect the 4k7 resistor, do you get a stable reading of zero from the analog pin?



hi-

her is the image I made of it.. (using a breadboard)

(http://dmstudios.net/misc/voltageDivider_voltMeter2.jpg)

I'll see if the camera is charged (usually isnt)...  and post one up of a 'real pic'..if it'll help.


@runaway_pancake-

I'll grab another 10k post I have laying around to use..

and look at your diagrams.

thanks
Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 29, 2011, 05:26 pm
@runaway_pancake-

I did as your diagram outlined..and it works 'perfect'...

I even left the meter included when I hooked up the Arduino just to keep an eye-on readings.

SO goes back to..  I must be doing something wrong on the breadboard set-up?

here is a pic:
(http://www.dmstudios.net/misc/vDivider_attempt1.jpg)


thanks for the help guys!.



Title: Re: Voltage Divider & Voltmeter help
Post by: dc42 on Oct 29, 2011, 05:59 pm
Your 4k7 resistor is shorted out because you have both ends in the same row of the breadboard.
Title: Re: Voltage Divider & Voltmeter help
Post by: xl97 on Oct 29, 2011, 07:24 pm
got ya!.   :)
thanks.
I have also corrected my diagram to show my error/fix

(http://dmstudios.net/misc/voltageDivider_voltMeter3.jpg)


I am now going to go back and change out the resistors to match for 8.4v input.

Thanks to everyone for your help.   :D
Title: Re: Voltage Divider & Voltmeter help
Post by: CrossRoads on Oct 29, 2011, 09:38 pm
So you've had 7.4V going into your analog pin all this time?  That's not good.
Does the chip feel warm when powered up now? If so, you likely blew the pin.
And if not, it may be partly damaged and fail on you eventually.

"is there a way I can meter my resistors? to accurately find out the values?"
Yes, just measure them with your multimeter. You have gold bands on the resistors? I think that's 10% tolerance (maybe 5%, I'm  not sure which).
If you know the actual values, you can use that in your math for more accurate results.
Title: Re: Voltage Divider & Voltmeter help
Post by: runaway_pancake on Oct 29, 2011, 10:58 pm

I would just add: Ohmmeter your resistors out of circuit,
Never* put an ohmmeter on a live/powered circuit.

(* For your meter's sake.)