Thursday, August 11, 2016

ESP8266 - Internal ADC 2 - the easy way example



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
>






Thursday, August 4, 2016

ESP Basic -3 - PCF8574 I2C I/O 8 x independent switches example



8 x Independent switches driver Example :






  
   After doing last time the Mutually exclusive 8x Switch driver example, now is time for a 8 x independent switches implementation .


What we will need: 


      This is how is looking the setup on a breadboard, for a better visibility:





Software implementation:

Something to remember: 

  • PCF8574 can SINK but NOT SOURCE much current - 100uA only (it cannot output high, if you want). Look at the above example how is connected the LED for SINKING current.
  • Each of the 8 GPIOs have a minimum guaranteed sinking current of 10 mA per bit at 5 V.
  • Each pin needs its own limiting resistor to prevent damage to the device!! keep under 25mA/pin.
  • Maximum device limit sink current in about 80mA. If you need more, look after PCA8574 (200mA max sink current!)

For more details please take a look at the PCF8574 Datasheet


ESP8266 Basic code: 
let address = 32 'PCF8574 I2C Address

i2c.begin(address)
ss = 0 xor 255 'XOR - Bit masking for the desired I/O pins
i2c.write(ss)
i2c.end()
button "1", [1]
button "2", [2]
button "3", [3]
button "4", [4]
button "5", [5]
button "6", [6]
button "7", [7]
button "8", [8]
button "OFF", [9]
wait

[9]
i2c.begin(address)
ss = 0 xor 255 'XOR - Bit masking for the desired I/O pins
i2c.write(ss)
i2c.end()
wait

[1]
i2c.begin(address)
ss = ss xor 1   'XOR - Bit masking for the desired I/O pins
i2c.write(ss)
i2c.end()
wait

[2]
i2c.begin(address)
ss = ss xor 2
i2c.write(ss)
i2c.end()
wait

[3]
i2c.begin(address)
ss = ss xor 4
i2c.write(ss)
i2c.end()
wait

[4]
i2c.begin(address)
ss = ss xor 8
i2c.write(ss)
i2c.end()
wait

[5]
i2c.begin(address)
ss = ss xor 16
i2c.write(ss)
i2c.end()
wait

[6]
i2c.begin(address)
ss = ss xor 32
i2c.write(ss)
i2c.end()
wait

[7]
i2c.begin(address)
ss = ss xor 64
i2c.write(ss)
i2c.end()
wait

[8]
i2c.begin(address)
ss = ss xor 128
i2c.write(ss)
i2c.end()
wait

This is how is looking the interface in the Web Browser after running the above program:



PS: credit for the XOR Bit masking goes this time to Forlotto from the esp8266.com forum who posted quicker than me this elegant solution :)