Measuring voltage between 15+/-V and output to lcd.

Using Arduino Project Guidance
1

Using the analog pins and the ADC i want to measure voltage. So far I can measure positive voltage and get a reading using a potential divider. There is lots of info already on this but not so much on negative voltages. A suggestion i found says instead of grounding one end supply a voltage to that end but when i do this I get very strange readings just using analogRead().If i do this what should i include in my code? I have included the diagram i am trying to follow. The ratio of the divider is 10:1. Could that be the problem? Any suggestions appreciated. (getting values on the lcd is not an issue)

level-shifter.png
level-shifter.png1171×894 39.6 KB

2

conorsquash:
A suggestion i found says instead of grounding one end supply a voltage to that end

That is the right concept. You need to add a big enough voltage so that the -15v shows up as 0, or a little higher. With your 10:1 voltage divider the -15v should appear as -1.5 so try adding (say) 3.3v from a 3.3v regulator IC. That way the full range of +1.5 to -1.5 will become +4.8 to +1.8 (all within the Arduino's 0-5v range) and you know to use maths to subtract 3.3 to get the correct value.

...R

3

Something like the enclosed would do the job..

Allan

offset.pdf (18.3 KB)

offset.pdf (18.3 KB)

4

Robin2:
and you know to use maths to subtract 3.3 to get the correct value.

Thanks for your response Robin.. I have one problem that is i am not sure where to subtract the voltage in the code. If in subtract 682 (3.3 when converted) from the analogRead() value it messes up the result and i get values between -12 and +18v using a 9.4V battery . Any suggestion is appreciated

5

I hope this is correct ...

Suppose you are measuring -7.5 volts. And you have a 10:1 voltage divider giving -0.75v. And you sit that on top of +3.3v giving +2.55v.

The ADC will read that as 1023 / 5 * 2.55 = 522.

So you need to convert the 522 to 2.55 volts and then subtract 3.3 from it to get back to -0.75v. Something like this (avoiding the need for floating point maths)

adcVal = analogRead(A0);
volt100Val = 5 * 100 * adcVal / 1024; // should give 255 for adcVal = 522
                                                       // divide by 1024 is much easier for Arduino than 1023
trueVolt100Val = (volt100Val - 330) * 10 // should give -750 (or -7.5 * 100)

...R