Hello,
in my car is temperature sensor. Output from this sensor is voltage, 0V to 12V. Arduino analog intput is maximum 5V. So how I should do coversation to my arduino board? Shlould I use only resistence to reduce voltage from 12V to 5V? Thanks
Thanks, well I have chosen 330 ohm and 470 ohm resistors. But primary voltage (12V in car) may change when engine in car running it shoud be about 13,8 V. Ouput voltage is changing than 5V. So I need stabilized max ouput 5 V.
Get a pot and put the in/out across your battery when the car is running (should be near the max voltage). Use a multimeter between the pot wiper and ground and adjust until the voltage is 5v. Then measure the pot resistances to see what the actual values are. If you go with your current voltage divider, put a 5v zener diode between the 5v point and ground such that the zener will disipate any voltage above 5.1v. Make sure the resistor values in your divider are high enough to protect the zener from too much current.
13.8v when idle, when driving the voltage should be 14.4v
try 10k from sensor to duino input and 4.7k from duino input to ground, this will give you about 4.7v when car is running, if you car has an ecu as nearly all do now have you thought of taping into that? the serial interfaces are quite cheap these days and you get alot more data than just temp, like revs, speed, battery voltage etc....
Thanks. My car is very old from year 12/1992. It hasn't ECU, it has a carburetor engine. It is Skoda Favorit engine.
Contrairy to what I use to think, zeners apparently require a certain minimum current flow to work properly. A high resistance voltage divider may not allow that minimum current.
thanks all..
I have read this thread and I ad the thread owner may have hoped for a little bit more accurate answers. No answer is wrong, but whey are just "good enough" (as Bill Gates said when Win 3.1 started to sell).
If he wants exact measurement, is there a way to make a analog inputScale circuit with a V+ ref to automatically adjust to? If it could galvanically isolate to, then we have the ultimate analog shield!
I'm not that skilled in electronics any more. Didn't think I had to remember the bits any more, only the parts...
Every channel could have three connections;
Extern+ (Accepting 0-28VDC),
Extern-,
Signal.
Automatically scaling of Signal:
ExternSignalMax = Extern+,
ExternSignalMin = Extern-,
ArduinoSignalMax = Arduino+,
ArduinoSignalMin = Arduino-,
To Richard Crowley : My english is bad, but i read your text again and i translate it word by word. My schema is wrong, it isn't function how i was thinking. So I will prepare it a try it without Zener diode.
Can I change in program input range?
Kuk1 - I'm not sure I understood completely, but the voltage from the temperature sensor is from 0 to 12vdc depending on temperature. What you wish is to make this 1 to 12 range to be from 0 to 5 vdc. As mentioned before, use a voltage divider to make 15 vdc max to 5 vdc max (comfortable overhead).
This way as the temperature varies, the voltage the Arduino sees will vary from 0 to 4.5 range. This will give you the analog reading you need to display temperature with the Arduino.
If you are wishing to power the Arduino from the 12 vdc on the car, then use a 7805 regulator to control the voltage to 5vdc.
Have fun.
Ken H>
My english is bad, but i read your text again and i translate it word by word. My schema is wrong
Try loading the page with http://translate.google.com/#
david
good 
Kuk, this is NOT for you, but someone else might become interested in Zeners; here is good article for the aleady advanced:
http://sound.westhost.com/appnotes/an008.htm
çat[ch305] thanks.