Counting output voltage from SENS-43UV on Uno without changing AnalogReference()

[drmse87]

Hi guys, Arduino beginner here!

I have a SENS-43UV sensor, connected to A2 pin on my Arduino Uno. The Arduino reads the sensor (reporting very low values indoors, ~5-6, but outdoors in the sun it was higher, 13-15, so obviously it reacts to sunlight).

According to the guys over at Build an Arduino UV Index Meter | DroneBot Workshop,

If you are using an UV sensor that will never give out over a volt under normal conditions you are actually wasting a lot of accuracy with the above arrangement. A range of 0 to 1 volt will only use the digits between 0 and 204, leaving 820 unused digits.

To address this, they suggest changing analogReference() to somewhere around 1 volt:

That way the entire 1024 bits of resolution will be available for your sketch.

However I use other analog sensors as well and would not like to change analogReference(). My question to you is, how can I convert the analogOutput of the sensor to corresponding voltages/UV indexes, even though it will admittedly not be as accurate?

UV1 – 227 mV
UV2 – 318 mV
UV3 – 408 mV
UV4 – 503 mV
UV5 – 606 mV
UV6 – 696 mV
UV7 – 795 mV
UV8 – 881 mV
UV9 – 976 mV
UV10 – 1170 mV (1.17 volts)

Usually it is a matter of multiplying like so:

sensorvalue * 5 /1024

is it not?
Any help would be greatly appreciated!

[Railroader]

What other analog sensors do You use, the range of their output?

[hammy]

Yes, if the outputs of your other sensor is say 5v , then you could use a potential divider on those and use the 1.1volt reference, which is the preferred route anyway .
Note if you don’t use this then the analog input uses the supply voltage ( ~5v ) as its reference and that is likely to vary a bit !

[drmse87]

Thanks for your answers Railroader and hammy!

I removed the other analog sensors (easiest that way I think!) and then changed analogReference like so in setup() method:

  analogReference(INTERNAL);

Now I get sensor values of 45-70 indoors, so it seems to be working on the new 1024 resolution. My question to you is, how do I convert these readings to mV? Or is this actually mV?

[Railroader]

Measure the voltage at ref pin. Tell us.

[JCA34F]

Measure AREF voltage with accurate DVM, let's say it is 1.08V.

float aRef = 1.08;
float mV = ADC_value * aRef / 1024.0 * 1000;
Serial.println(mV);

Say your ADC reading was 450, then, 450 * 1.08 / 1024 * 1000 = 474.6mV.