Reading Battery Voltage on AO

Need Help With My Project
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4

So why does it still fail when I use a AAA battery?

1.59v before the voltage divider and 0.79v after the divider. So only 0.79v on the A0 pin … :frowning:

5

What happens is you serial print the actual analogRead(A0) value before you do the maths on the result?

Pete.

1 Like
6

Damn! I was sure the code was correct… :shushing_face:

7

Oh dear, now I think my maths may be at fault … :woozy_face:

Error
Error1121×912 144 KB

8

Where does the 4.97 in your equation come from? Seems a bit random to me.

Pete.

9

I was expecting the ADC voltage to be the same as the board voltage(5v). To be sure I got my old Fluke meter out and measured it.

10

No, the ADC is like the other pins, 3.3v

Pete.

1 Like
11

Oh, so that should be a lot nearer 3.3v :crossed_fingers:

12

Changed it to 3.45v and it gives me a pretty accurate reading now as long as the voltage I’m measuring isn’t above 6v. Is that because of the 3v limit on the A0 pin?
So for higher voltages(50v+) I’m going to need a much better voltage divider, one with multiple resistors as the one I’m using now only halves the voltage?

13

As I said earlier. 3.3v on the pin gives maximum ‘deflection’ of 1023.
If you calculated the voltage divider so that your nominal 50v input give a 50% ‘deflection’ then 512 would be the corresponding reading.
Meaning that you’d be getting roughly 0.1v resolution.

If you sized your voltage divider so that 50v gives an output of 1000 on the sale of 0-1023 then the resolution would be 0.05v, but you’d only have a 1.15v margin at the top end, so 51.15v would give you maximum ‘deflection’ with a maxed-out reading of 1023.

Pete.

1 Like
14

Could a pentiometer be used as a voltage divider?
Give me an easy way of calibrating the whole thing for high and low voltages ?
I have a couple somewhere, I guess I could just try it … :stuck_out_tongue_winking_eye:

EDIT.
No, same problem…

15

Yes, potentially :smiley:
But, TBH you are better-off using high tolerance fixed value resistors to give the maximum stability.

If you want to measure two different voltages using the same device then you’re probably better using an ESP32 which has multiple pins that can be used as analog inputs. A couple of things to watch out for though, some of those pins can’t be used the same time as WiFi (those in the ADV2 group), and the default is that you’ll get 4095 as the maximum output for a 3.3v input.

Pete.

1 Like
16

Damn those resistors!

Well, I think I have the solution, thanks to a little online calculator I found… :stuck_out_tongue_winking_eye:

Resistors
Resistors699×127 28.8 KB

17

Hmmm, not sure about 1.9 Meg as a suitable R1 value.

Pete.

18

Finding a 1.9Meg is difficult so I’m thinking two 1.0Meg is close enough as I can get my hands on those easier?

19

Ok, I’m convinced my code and maths are good.
Its just my voltage divider that sucks.
Works well for <6.3v but is useless for >6.3v

#define BLYNK_PRINT Serial
#include <ESP8266WiFi.h>
#include <BlynkSimpleEsp8266.h>

char ssid[] = "*************";
char pass[] = "*************";
char auth[] = "*************";

int value = 0;
float voltage;
float R1 = 67600.0;
float R2 = 67500.0;

BlynkTimer timer;

void myTimerEvent()
{
  voltage = value * (3.3 / 1024)*((R1 + R2)/R2);
  value = analogRead(A0);
  
  Serial.print("A0 Value = ");
  Serial.println(value);

  Serial.print("Voltage = ");
  Serial.println(voltage);
  
  Blynk.virtualWrite(V5, voltage);
}

void setup()
{
  // Debug console
  Serial.begin(9600);
  Blynk.begin(auth, ssid, pass);
  timer.setInterval(1000L, myTimerEvent);
}

void loop()
{
  Blynk.run();
  timer.run(); // Initiates BlynkTimer
}

So, I can save this as a “good” working code and start working on how to integrate it into my EPever Controllers & Blynk code …

Feel free to tell me my code is “pants”. But if you do please tell me why so I can try and fix it, because I think its great … :stuck_out_tongue_winking_eye:

20

You’re calculating your result before you’re taking your reading!

I think your choice of resistor values look far too high to me. I’d be expecting values in maybe the 50k - 200k range.

Pete.

1 Like
21

Ooops. I wondered why the first serial print was zero … :woozy_face:

To be honest I was too. But its not what this says … :thinking:

22

According to this:


you should be aiming for a current drain of between 1ma and 5ma (0.001 to 0.005 amps).

Plugging those values into your calculator gives this:

image

Pete.

1 Like
23

hey guy’s, just an FYI but your voltage divider formula is backwards.
the correct formula is Vr2=Vs*(R2/(R1+R2)), as per Ohms Law