This is just a very quick example on how to add a Voltage Divider to the ESP8266 Internal ADC input to increase the ADC input range.For more theory behind, please take a look at the previous related article: ESP8266 – Internal ADC

Today project:
Measure a voltage input range from 0-5V with the ESP8266 internal ADC

What do we need:

  • 2 Resistors for the voltage divider, R1=105.6k, R2=24.08k. I am using here precision resistors and the values are measured values with a proper calibrated bench meter.
  • a good, trustable, calibrated ok Multimeter.
  • some wires to connect all together.

I will not insist on connections, take a deeper look at the previous article about

 
Voltage divider schematic and Vout (ADC input voltage) formula

How do we do it :

1. Measure your Max desired input voltage
    In my case, Max Input Voltage (Vmax) = 5.1919 (measured)

2. Measure Resistors values:
R1 = 105.6k
R2 = 24.08k

3. Check if Full Scale Value at the voltage divider output is inside the ADC defined domain:

  • Calculated : Vout = (R2/(R1+R2))*Vin = 0.964072733 V
  • Measured = 0.96038V

Good enough for the precision we are looking for. ESP8266 ADC is 10bit only and not exactly the most accurate in town

4. Calculate Voltage Divider Ratio:

  • Vdivider Ratio= Max Input Voltage/Fullscale value =  5.405976676

5.  Read ADC value : 

      adcr = adc.read(0)
      print("    ReadADC     : "..adcr)

     Average result for adcr = 1017

THIS IS IMPORTANT !! It tell us that we are inside the ADC domain as adcr < 1024 !

IF adcr > 1023 then you need to adjust your voltage divider resistors to fit inside ADC domain!

6. Calculate LSB  in 2 ways to cross check that we have the right value:

  • LSB = Input Voltage read by multimeter/ADC readed Value =

=  5.1919 / 1017 =  0.005105113 V       OR

  • LSB = (ADC Input  pin read Voltage by multimeter/ADC readed Value/)*Vdivider Ratio =

                  = (0.96038/1017)*5.405976676 = 0.005105113 V        It looks that we have the right LSB Value for our exercise!

If you want to know also the ADC LSB, then
LSB = ADC Input  pin read Voltage by multimeter/ADC readed Value  =   
                =  0.96038/1017 = 0.000944346 V

Guess what’s happening if you multiply ADC LSB with Vdivider ratio 🙂

7. Software Implementation for the ADC read function:

 function readADC()
      ad = 0
      LSB = 0.005105113 --calibrate based on your voltage divider AND Vref!
      adcr = adc.read(0)
      ad= adcr*LSB        
      print("    ReadADC     : "..adcr)
      print("    Read Voltage     : "..ad)
      return ad
end

and some results in the terminal window:

> SENT: readADC()
readADC()
    ReadADC     : 1017
    Read Voltage     : 5.191899921
> 
Categories: ADC

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