Car related: resistance to voltage

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi,

I am replacing a non working gauge on my car with an Arduino lcd-screen.

The problem is:
The gauge sender is a resistor, one end connected to the vehicle ground, and from 0ohms to 100ohms. In order to read the sender at Arduino the signal needs to be converted to 0 - 5V, of course.

I bought a couple of LM741 opamps, as i thought they would do the job... this far i have tried two circuits... the first one (top of the pic) works close to it should, but both, the resistor and trimmer, get too hot.

Then i decided to keep it simple, and tried the circuit described second on the pic, now the 741 gets hot :o ... if i increase resistors on the 741input i would lose the 0-100ohm resolution.

I know this should not be very difficult setup, but beyond my knowledge :slight_smile:

Please help. A schematic to do the trick with 741 or other components would be great help. Thanks.

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2

Just use two resistors as a voltage divider!

3

thanks, i knew i am trying too difficult ::slight_smile:

4

There are many aspects involved in your project.

(1) Do you want to protect the Arduino from "spikes" from the car electric? Then you should not use the car voltage (which is presumably indicated by the +14V line.) If you have to, an at least 10k resistor should be added to connect to the ADC. Higher values are recomandable, but will noticably influence the ADC.

If you can, use the Arduino 5V to supply the "gauge sender".

(2) You have become aware of the current flowing ("...gets got"), so a 1k current delimiting resistor seems appropriate. The back side is a apparently very small voltage swing 0 to 100/1k * 5V = 0,5V.

However this is not really small! You can configure the Arduino to a 1,1 V reference for the analog reading, which will fit fine!

(3) Again voltage supply from the car: This has a second disadvantage: You will never know the exact voltage, so the reading is bound to a considerable error. This can be improved by a second reading of the car voltage, using a fixed 2k2 / 100 Ohms divider, in case you also use a 2k2 with the "sender resistor" to reduce the car voltage.

(4) I wonder why there are still 714 opamps available - they are the grand-grand fathers of current opamps which are less expensive and much much better... In your case no opamp is needed!