Digital read on voltages between 1 and 31 volts?

Question

Hello Arduino comunity!

I am working on a circuit that uses an Arduino Mini to read values from certain points of a boost/buck converter circuit to determine things like input state, output state, overheat and so on. The circuit is working all right, and the only part left to do is to connect the digital pins of the Arduino to the different points in the boost/buck regulator.

The problem is however that I need to read from a range of voltages between 1 and 31 volts with the Arduino, which isn't possible, because not only will it damage the controller if the voltage exceeds 5V, but values below 3V will be regarded as false.

I thought of putting some potential dividers, which would obviously not do the trick, and I also thought of using a WS78L05 voltage regulator per input pin (because each pin might have a different voltage from the rest) but that isn't dealing with voltages <=5V and 31V exceeds its rating as well.

Lastly I thought of using transistors, which seemed like a good idea, but I would require 2 transistors per input wired as follows:

^{simulate this circuit – Schematic created using CircuitLab}

But it is impractical to do for 4 inputs and there is limited space available on the PCB too :(

Is there any IC that encapsulates the circuit above, or is there any other way to achieve what I want to do?

Thanks very much :)

Answer 1

Using a simple voltage divider (51K / 10K as you have worked out already) fed directly into an ADC input will give you a reading of 32 on the ADC at 1V input. That should be enough to give a reliable threshold detection.

If you want to have higher sensitivity at lower voltages you can increase the output of your voltage divider to more than 5V and then clip it with a zenner diode. For instance if you reduce the 51K to 33K you get 7.2V out of the voltage divider. A 5V zenner diode can then clip it to 5V. Your 1V input (which would be 0.233V on the output of the divider) would now read 47 on the ADC. Anything over 21V would then read 1023.

^{simulate this circuit – Schematic created using CircuitLab}

Theoretically you could get away with not having the voltage divider at all and just using the zenner and one resistor to clip the incoming voltage to 5.1V. That would give you the greatest sensitivity, but also the greatest current flow through the zenner.

^{simulate this circuit}

Size R1 to limit the current at your maximum voltage to within the rating of the zenner.

Answer 2

Use two quad lm339 voltage comparator chips to make an analog to digital convertor-you will need 2 to read 31 volts accurately. Its a standard ADC circuit, with this common lm339 is, so a search will find circuit-it may need a simple modification to combine the two circuit.

Answer 3

The junction of a 51KΩ, 10KΩ voltage divider will feed an Arduino ADC input 5.08 V at 31 V input, and 0.16 V at 1 V input. With a 5 V reference, I think the first of those will give 1023 counts, and the latter, 33 counts. This is from 1023*(1/5)*10/(10+51) ≃ 33.54. That may be enough counts for reliable detection, depending on how noisy the environment is.

For more sensitivity at lower voltages, if the first reading is less than 225 counts, one can switch to the 1.1 V internal ADC reference and take another reading. (1023*1.1/5 ≃ 225.06.) With a 1.1 V reference, a 1 V input to the divider produces 152 counts, and a 6.7 V input produces about 1023 counts. (1023*(1/1.1)*10/(10+51) ≃ 152.46 and 1.1/(10/61) ≃ 6.7.)

To change the ADC reference voltage, say analogReference(DEFAULT) for Vcc reference, or analogReference(INTERNAL) for 1.1 V reference.