Reducing voltage to AnalogPin

Other Hardware General Electronics
1

Hi Guys,

I am sending a signal to pin 0 and using the readAnalog function. However, the range of the sensor (straingage - INA125) is from:

147 to 850. I tried putting a resistor on the line going to pin 0, but it made no difference to the signal whatsoever. Can someone please explain why this is the case?

Thanks,

Cameron.

2

How is the strain gage wired up?

3

Direct to the INA125 and the two outputs join together to go straight to pin zero.

I just tried doing a voltage divider and that worked, but i don't want to half the voltage just lower it, so that it can stay in the 1024 range.

4

Sounds like it id in range you just want to reduce it to have the strain gauge go from 0-1024 you can use the Map() function for that. However if you want to reduce the voltage without the current draining voltage divider, You can use two transistors with the reduced voltage signal to npn trans base ground emitter collector to base of second transistor and 10k to new v+ wire this to 1k to base of second trans. pnp ground collector 10k to new v+ on the emitter and a reduced analog signal will be on the collector. But a optoisolator powered with the reduced voltage is a much simpler solution. and cheaper.

5

But a optoisolator powered with the reduced voltage is a much simpler solution. and cheaper.

That will not work because the transfere function of an opto is very non linear.

Not sure that other suggestion will work either cair to draw a schematic of it so we can see?

6

I'm sorry this must be a very dumb question:

But, I don't understand why putting a resistor on a line going to an Arduino pin doesn't reduce the voltage?

Thanks,

Cam.

7

Please post a schematic of how you are wired up.

8

Resistor in series alone will not reduce the range of the output - you need two of them making a resistor divider for that.

9

But, I don't understand why putting a resistor on a line going to an Arduino pin doesn't reduce the voltage?

The series resistor does not reduce the voltage because there is essentially no current flowing into the analog input pin. The input pin has an equivalent resistance of about 100 megohms to ground.

10

Hi, CrossRoads,

I got the INA125 now working :slight_smile: with it getting a range from 0 grams to 500 grams, but I have another problem with the reading fluctuating. But, I started a new post on that: AnalogRead - readings are erratic - Microcontrollers - Arduino Forum

If you could help that would be great :slight_smile:

Thanks,

Cameron.

11

Done.