Electronic – Arduino: read different voltage values in a range from -12 to +12V

arduinoresistorsvoltagevoltage divider

I would like to trigger different functions in Arduino related to the following voltage values: -12V, 0V, 6V, 9V, 12V (the range is from -12V to +12V).

According to the value of the voltage read (-12V, 0V, 6V, 9V, 12V) I have to show instantly different light animations on a LED strip.

Considering that the Arduino is not able to read a such voltages directly, I suppose that probably I have to use a voltage divider, but I don't know how to calculate the resistors to use. Can anyone help me?

Thank you.

I was suggested also to use 1 dual op amp do the following:

Create inverting amplifier with a gain of 1, but insert a offset of 12V to the + (non inverting pin), this will max your -12 to 12V range change to a -24-0V range.
To the output of stage 1, connect another inverting amplifier with gain of 0.2 you would get a a positive voltage range of 0-4.8V which is fine to work with the arduino's 0-5V ADC tolerance.

What do you think about this idea?

Best Answer

Figure 1. The voltage levels to be detected.

Since you never have to detect 0 V you can treat any voltage below 6 V as -12 V.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Figure 2. Level converter circuit. D1 protects the ADC against negative voltage inputs. The R1 / R2 divider gives an output of 5/12 times the input voltage.

On your logic you could set thresholds of 10.5 V, 7.5 V and 3 V for each of the analog 'windows'. For a 10-bit ADC these would correspond to \$ 1024 \frac {10.5 \times 5}{12} \$ for the 10.5 V threshold, etc.

If you have the comparator then you could use this to detect each positive going edge - set the threshold to 3 V (1024 * 3 / 12) - and after a short delay sample the voltage.

Note that I chose 7k and 5k to make the voltage divider ratio blatantly obvious. Choose some standard values close enough to do the same job.

Related Solutions

Electronic – arduino – 12V supply to Vin Pin on Arduino

The very same schematic was used for the original Uno32 from chipKIT. As it happened, someone managed to blow it up by applying 15V to the barrel jack. That was perfectly within the specs of the regulator, but the comparator ejected all the Magic Smoke™ and ceased to work.

After some diagnosis of their board I did the same calculations as you and came to the same conclusion. 15V, while within the limits of the regulator, overwhelmed the input of the op-amp.

I subsequently recommended a pair of clamping diodes on that input (now in Rev C of the Uno32), and since then there have been no reports of dead boards from too much input voltage.

enter image description here

(Note: I don't know if it's just a labelling problem in the PDF version of the schematic, but the VIN on the upper diode should actually be Vcc so it's clamped to the same supply voltage as the comparator.)

Detect voltage by arduino

I am assuming you are going to simply measure the voltage drop across the LED and use that to determine if the LED is on, this will work but you will need a couple of extra components, you can damage your arduino if you feed in a voltage over 5V.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

In the above R1 and R2 form a voltage divider to reduce the voltage on the A0 pin of your arduino. We can calculate the maximum voltage you can use to safely stay under 5V on A0:

Vmax = 5.0 / (R2 / (R1 + R2))
     = 5.0 / (47000 / (100000 + 47000))
     = 15.6V

Finally read the value of A0 in your arduino code and use the following formula to convert that value into a voltage:

float r1 = 100000;
float r2 = 47000;

float vRaw = (analogRead(0) * 5.0) / 1024.0;
float vDrop = vRaw / (r2 / (r1 + r2));

Depending on the accuracy needed you may need to mess with the 5V value above and adjust r1/r2 to a measured value (depending on accuracy of the resistors you are using).

For a complete example see https://www.udemy.com/blog/arduino-voltmeter/

Best Answer

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Electronic – arduino – 12V supply to Vin Pin on Arduino

Detect voltage by arduino

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