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Topic: 4-20 mA pressure transducer convert signal to 1-5 volts circuit diagram help (Read 2100 times) previous topic - next topic


As said, that resistor can be any value between zero ohm and 27k.
I would use a common 10k 1% metalfilm resistor (with the ADS1115).

Same for the 100ohm resistor. Stability is important, absolute value is not.
As long as it's less than 102.4ohm, otherwise you can't reach that 20mA with an Aref of 2.048volt.
A 1% resistor is less than 101ohm, so perfectly ok for the job.

I think you didn't understand the maths behind the protection resistor.
There is an almost INFINITE internal resistance on the pin, so NO current is flowing through the resistor
if... input voltage stays below the supply voltage of the A/D (normal situation).


I thought the current limit of every Arduino Uno IO pin is 40 mA?
That's the drive current (source/sink) of an output pin.

Don't confuse that with the fault current an input pin can take when you put more than 5.7volt or less than -0.7volt on the pin.
The ADS1115 has that specified (10mA), an Arduino not.


Ahh... I see that makes sense, a higher voltage than 5.7V
would deliver a higher amount of power that the pins may not be able to handle.


Application notes have indicated the diode protection current should be limited to 1mA.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


I got the 250Ω, 100Ω, 27kΩ, and 51Ω resistors in the mail.

I wanted to try the 250Ω resistor first and hook up my analog out straight to the Arduino.
Here is a screenshot of my setup (I have the 250Ω resistor hooked up in series).
I am at zero pressure so I thought my voltage readout would be 1V (based on math in previous posts)
but it was 0V.

Any thoughts?
I also tried to read from the positive lead going to the pressure transducer and the analog out pin but I all I got a was a voltage near 23.7 volts (I believe from the power supply).


+supply goes to +sensor.
-sensor goes to resistor.
other side of resistor goes to -supply.

+DMM goes to -sensor, through the 27k resistor.
-DMM goes to -supply (fail).



+DMM goes to -sensor, through the 27k resistor.
-DMM goes to -supply (fail)..

So I'll try this?
1.+DMM goes through +sensor
2. -DMM goes through -sensor through 27k resistor


As Wilbur said to Orville "Oh brother".
@knightridar, I can't see where those shiny, silver, crooked leads are connected on top of the shiny silver terminal strip.
Can't you draw and follow a simple wiring diagram?

Do i need to draw in wire colors?


Sorry I'll post a clearer picture later.
I get the diagram.

I'm trying to figure out what leads I need to connect to if I were to just use a Dmm and no arduino.

I was connecting +Dmm to arduino analog out and - Dmm to ground and got a 0 volt readout.


Try this:

Connect DMM black to 24V blue, DMM red to sensor black


Thanks. I realized what I was doing wrong.
I was measuring at both points after the resistor.

This time I have the leads to the DMM connected properly.

1. One negative lead connected before resistor from power supply ground
2. One positive lead connected after resistor to ground.

I've got a voltage readout of .972 volts at zero pressure.
.972V * 250 ohms =~ .0039 A

The multimeter accuracy could be throwing the result off.
The resistance I am also reading is 249.9/249.8 ohms (.01% tolerance resistor reads at .04%)
Fluke 11 DMM Specs


Good enough, you should see near 3V with 50 PSI. Whats next?


It worked. The multimeter reads 3.010 V at 344.738 kPa (50 psi)
But the Arduino input pin isn't reading it.

I just put in this simple code and raw value is being reported as 0 on the serial monitor.

Code: [Select]

int inputPin = analogRead(A0);

void setup()
  Serial.begin(9600); // open serial port, set the baud rate to 9600 bps
  Serial.println("/** Water pressure sensor measurement **/");

void loop()
  Serial.print("Raw Value = " ); // shows pre-scaled value


But the Arduino input pin isn't reading it.

You have connected the A0 resistor to Arduino ground.


I took a look at post #42 from @outsider and matched the wiring to that now.
I have the ground wire connected to the same location where the blue wire from the power supply is connecting.

The Arduino analog output wire doesn't have anything else connected there, except for the opposite side of that terminal where the sensor ground and 250 ohm resistor are connected.

This other post seems to show the same thing.
reading 4-20 mA pressure sensor

In another post of mine I have analog out connected right before 10 kohm resistors. hook up to ground.
I tried that and I got the same 0 results.

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