Easy question for you veterans

Ok, I know that the analog pins shouldnt have a value that exceeds V+ on the arduino board, but what if i need an analog reading on something from say, and car sensor? V+ fluctuates between +12 and +14.3V,

now that can be regulated to a more stable 12V via a LM7812 Voltage regulator, but now I cant use a 0-12V analog value for an analog pin.

From an electronics point of view, i know i can use a Comparator (an op-amp set up to compare 2 different input voltages and amplify the difference) but this is going to take up much needed space. I see a ARef pin on the arduino, and im wondering if this is what i need…i have no idea what that pin is for. Anyone?

Does your car sensor output a value between 12V and 14.3V?

You can reduce higher voltages to lower voltages relatively easily using a voltage divider.

The AREF pin is for setting an (optional) external reference for the A/D converter other than the existing options of 5V and 1.1V. Any input voltage values outside the range of 0V to AREF (either 5V or 1.1V) will not give meaningful readings, and values above 5V or below 0V will damage your board.

yow was up im just saying this cause i have to say something before i can post a topic. i know i know NOOB!!! right well see yeah hahahhahahaha

yow was up im just saying this cause i have to say something before i can post a topic. i know i know NOOB!!! right well see yeah hahahhahahaha

… :o

Mike I think this translates to:-

I know nothing about this topic and I am only posting this because the rules of the forum dictate I have to have replied to a topic before I can start a new thread of my own. Now I have done this I am off to post that thread, goodbye.

The thread in question might be:-
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1244914801/8#8

Of course I could be wrong, I was never any good at foreign languages. :wink:

This is one of those cases where a voltage divider (two resistors) is a good idea… since the A/D input presents a constant load… so a voltage divider would work ok. You could measure really high voltages with the right pair of resistors.

Grumpy - The thing that annoys me about this kind of post is all they have to do is to go into the Exhibition Section, look for something they like and post “Hey this is really cool. I like it”. Post 1 done !!

Well, lets see…

ok, so the sensor throws between 0V and 12V, possibly an absolute max of 14.1V

so, if i use a voltage divider, most of the range (times .5) would give me a decent value, once it gets to about 12 V, the Voltage at the divide point will be around 6V, and can go as high as 7 with Battery+ Voltages from the alternator going as high as 14.1!

I want to take readings from various Auto sensors in a car and report on them via LCD or whatever, like Engine temp, Fuel level, Vacuum pressure, MAF sensor reading, MAP reading, RPM’s, blah blah blah

most of these sensors have 3 wires, one is GND, the other is +12V( or whatever voltage V+ is at), and the last is an analog value between the two. I want that analog value, and give it a meaning.

Say I measure the Engine coolant temp sensor voltage, I get 8.5V. I can get out a thermometer and measure the temp and get 175 Degrees, I know that when that sensor outputs 8.5V, the temp is about 175 degrees. so by measuring the voltages at different levels, I can almost calibrate the readings in this way. mind you on a vehicles electronics there isn’t much to say as far as precision is concerned…but approx values are ok for my cause.

My problem is that i have the sensor readings in analog values between 0v and 12/14V.

I need to Cap out the max at 12V, and bring the entire scale down to Arduino level…0V to 5V…

or do I?

can I use the Aref and connect it to B+ on the car and use the analog pins directly on the analig inputs? im almost sure I can’t…

some useful analog reference! sheesh!

With my electronics background, the only solution I can think of is to make a comparator that will Not Amplify, but the opposite of amplify or limit the Voltage range, but this has to be done on an analog level due to the sensor voltage readings. I was hoping I could use the Aref on the Arduino, but i can’t see how anyone has used the damn thing.

Thanks BTW, im still tryin to figure this shit out!

Later fellas!

I understand what you mean with the Voltage divider…

The values dont have to match, I could use a 3K for the top and a 1K for the bottom and get a different range. I suppose i could also use a Variable resistor to get the setting just right too.

looks like i have some tinkerin to do

I’m not sure this is really needed but maybe you could use a baseline voltage divider to set aref. Run a 1M resistor from the car’s “12V” to aref and a 470K from there to ground. If you use the same two values for the other voltage dividers that would give you 100% of the range.

I have used 1M/470K voltage dividers to pick up signals from the tach and neutral sensor signals on a motorcycle. These are both off/on sorts of things so I haven’t needed to worry about the intermediate voltages.

By the way, does a car have a regulated 12V buss somewhere or does every circuit have to do its own regulation?

By the way, does a car have a regulated 12V buss somewhere or does every circuit have to do its own regulation?

The standard CAN-BUS cable includes an optional V+ line. I suspect this is used to carry regulated power in automobiles.

I could certainly be wrong and, even if I’m correct, there could be variations between (and maybe even within) manufacturers.

  • Brian

I’m pretty sure cars do not have a regulated bus. You pretty much get raw battery power, which means it can be:

  • really low when cranking (<9V)
  • negative if someone hooked up the battery backwards
  • 24V if a helpful tow truck operator wants to give you a REALLY good boost and uses two batteries in series

  • temporary >120V load dump if the battery becomes disconnected from the alternator while charging

Cars are definitely an environment for “rugged” electronics :wink:

might be of assistance: http://sites.google.com/site/adifferentlemming/Home/projects/arduino/voltagedividerstutorial.pdf?attredirects=0

I would protect the analogue inputs with something like this:-
http://www.thebox.myzen.co.uk/Tutorial/Protection.html

As it might spend considerable time above the 5V I would up the series resistor to 100R as well as having the voltage divider.

I want to take readings from various Auto sensors in a car and report on them via LCD or whatever, like Engine temp, Fuel level, Vacuum pressure, MAF sensor reading, MAP reading, RPM’s, blah blah blah

most of these sensors have 3 wires, one is GND, the other is +12V( or whatever voltage V+ is at), and the last is an analog value between the two. I want that analog value, and give it a meaning.

You have 12-14.5 volts going into the ECU which regulates all of those sensors down to 5 volts max. I would pull some to check but all of the ones I have seen are like that.

I am building a similar set-up for Vacuum/boost, O2, coolant temp, intake air temp, voltage.

Jim
Holland, MI