12v sensor needs a ground?

Other Hardware General Electronics
1

I have a sensor that runs on 12VDC.
When I connect my voltmeter up to it I connect the voltmeter ground and the sensors 12v ground together and voltmeter red cable to the Signal wire of the sensor.
This works.
When I connect my arduino from Analog 0 to Signal on the sensor and +and- on the sensor to a 12v wallwart the arduino bounces all over the place. I assume I need the grounds to be common to the signal can complete the loop?
So since the walwart is 12V can I do that or will it torch the arduino?

2

forgot to say. The sensor is a 0-5VDC range sensor on the signal wire.

3

Yes, you need the grounds connected together. The signal coming from the sensor is only 0V-5V so it's OK that the wall-wart is 12V. You can always add a 1k (or so) series resistor (sensor-->1k resistor-->analog 0) for extra safety.

--
The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

4

perfect. Thanks for taking the time to reply.

5

still getting a nasty bounce. Maybe Ill try the resistor to to smooth it.
Also going to add the voltmeter back inline (in series) to see if the output from the sensor is jumping. (i dont think so)

at 5 second intervals:

0
160
0
33
0
0
1023
79
0
607
1023
319
631
776
442
683
411
0
1023
73
550
0
1023
0
0
892
0
119
0
0
0
0
0
0
0
0
0
0
0
0
4
1023
0
1023
0
1023
951
0
913
0
1023
1023
1023
727
1023
479
789
927
948
817
560
109
0
6

check with the voltmeter in parallel and please post the code for help you

7

still getting a nasty bounce.

Don't understand this.
Please post your code.

8 
void setup() {
  Serial.begin(9600);
}
void loop() {
 Serial.println(analogRead(0)) ;
delay(5000);
}

just trying to see what the arduino is reading.

9

OK, can you explain this

Maybe Ill try the resistor to to smooth it.
Also going to add the voltmeter back inline (in series)

please?

10

yep had a brain freeze and had to figure out inline again... Now to dig out a resistor. i should really use that 100 drawer case and separate these things.

11

Maybe just drop the jargon and explain what you have and what you want to do.

12

map a linear sloped value
http://arduino.cc/forum/index.php/topic,59869.0.html

you do a small program with
Code:

Serial.println( analogRead(1) ) ;

and note the value for your two known values (these have to be integers, but if they are 0.05 you just use 500 then you know that you divide the reported integer value by 10000 to get the float value).

Say the Arduino say 315 and 873 for the saline values 0.05 and 1.2 (I have no idea if these values are reasonable - I'm just making some up)
Then write the real program where the output is
Code:

Serial.println( map(analogRead(1), 315, 873, 500, 12000 )/10000.0 ) ;

this will then correctly linearily scale even when your input is below 315 or above 873.

Now we just need to get your circuit to yield a "reasonable" voltage to put into the Arduino. Remember the Aref pin voltage defines what 1023 is.

13

map a linear sloped value

A "value" is a point - it can't have a slope without reference to another "point"

14


this is what I need to do right?

Yea I want to
Serial.println( analogRead(0) ) ; (ValueA) At a known salinity of 1000uS and then
Serial.println( analogRead(0) ) ; (ValueB) At another known salinity of 2000uS.

and then

Serial.println( map(analogRead(0), ValueA, ValueB, 1000, 2000 ) ) ;

or at least thats what I think I need to do :wink:

15

I don't know what your diagram is saying - can you post a circuit diagram?

16

i think this is a schematic not a circuit diagram... sorry Im not at the circuit diagramming stage yet.

sensor powered by 12VDC
Sensor signal -> voltmeter+ -> voltmeter- -> 1kResistor -> Arduino A0
Arduino Grd -> 12VDC supply ground

17

No no no. Your voltmeter is connected very very very wrong.

Do you know the difference between Current and Voltage?

And your circuit is complete BS.
Try to:
-use angled wires in your drawing don't make them cross eachother
-add "+", "GND" and "SIG" signs so we know what is what
-add voltages, 3.3V, 5V, 12V
-not to use terms that you don't understand

And finally, what is that sensor? Type, manufacturer, link to datasheet

Now, what is wrong with the placement of the meter? You really need to assimilate with the basics of the electronics.

Cheers,
Kari

18

ok measure volts, voltmeter in parallel.
Measure current(amps) ammeter in series.
so..
I got a solid 1.865VDC.

When I sprinkled salt in the water it went up as expected.
It is currently 1.890VDC.

The arduino is reading:

935
0
439
0
0
0
0
0
0
0
0
1023
0
1023
985
0
1023
1023
1023
931
1023
319
266
0
1023
0
1023
0
0
728
0
0
0
0
0
0
4
609
0
723
0
935
0
1023
1023
632
1023
184

when running at 5 second intervals. lowering and raising the interval of delay doesnt change it.

The sensor doesnt have a datasheet. It was custom made for me by somebody. They took one of their current sort of home brew designs and converted it to a 0-5VDC signal. Not a lot of testing was done on it and the guy doesnt have an arduino. he was just using a voltmeter. Using a voltmeter it works.

For example:
I get a reading of 1.890VDC.
My linear slop has a formula of (.2054 +y) / .0008 = x
so X=2619.25
X=Total Disolved Solids in a liquid in microsiemens
My Hanna TDS meter reads 2550uS
Thats a difference of 69.25uS or 34.625 Parts Per Million. Pretty darn close!!
Definitely in the "good enough for government work" range. When accounting for the accuracy/resolution of both the Hanna meter and the custom meter and temperature compensation in both.

So sorry to be long winded, I always hope these posts help somebody else as I get most of my knowledge from reading somebody elses posts. :wink:

19

I put in a 1k resistor between sensor signal and arduino. Voltage dropped just a little and is stable on the voltmeter but arduino is still giving erradic results.

I decided to try a different input as well. A5. same thing.

Im going to get out my Uno and try it. Maybe I dropped this Duemolove a few to many times.

20

A resistor does not "smooth" a fluctating voltage. A resistor-Capacitor circuit will do so.

IF (I am making assumptions here) the voltage meter is showing a steady voltage, and the Arduino is showing erratic values I can think of two possible causes: You ar not using the right pins (and the program is reporting an opencircuit/nonsense value) or your physical construction of the circuit is not solid enough, there is a loose connection.

We might get this kind of problem if the sensor is not outputting a single steady voltage level, but some pulsetrain or there is some other "noise" on the line. The voltage meter takes long samples, ie. it sort of shows an average voltage level. The Arduino ADC is a lot faster and might truly sample the voltage. Only way to know is to use an oscilloscope. (Or use an RC circuit...but then we move into a whole new set of problems)

Could you get your friend to write a little/short description of how/what his sensor is doing, which you then include here?

(NB: Have you tried the Scandianvian Forum where you can write in Finnish?)