Go Down

Topic: Reading values from a NPC-100 Wheatstone bridge pressure sensor (Read 400 times) previous topic - next topic

Oliverhowden

Hello,
I was just looking for some advice on reading data from a sensor.
Generally I have worked with I2C sensors, though for this specific application this sensor is required. Here is the Datasheet.

Am I under thinking this by assuming I can connect this directly to the Arduino 5v power supply? Do I need a differential supply or is 0-5v okay?
I'm reading lots of mixed things about whether I need op amps or whether I am able to power and read this directly.

Thank you very much for you help.

Oliver

Wawa

The datasheet only shows the Wheatstone bridge, without electronics.
To use this sensor, you must add a stable excitation voltage for the bridge, a differential amplifier, and a high-resolution A/D.
Fortunately this is all available on a common/cheap HX711 breakout board.
Leo..

Oliverhowden

Awesome, exactly what I was after.
Thank you very much for your help Leo.

Cheers,
Oliver

Oliverhowden

Hi Leo,
I purchased the HX711 breakout (Bought it from a local supplier and turns out its actually a Keyes 234 140C0A) - but appears to be almost identical.

I've tried using the Bogde library and having some issues with the readings.
No matter which gain I set (Channel A 128, Channel A 64, Channel B 32), my readings are returned as:
HX711 reading: 8388607

I can't see many spots I could mess up in the wiring:

Arduino 5V -> VCC
Arduino GND -> GND
D4 -> DT
D5 -> SCK

My sensor is then connected directly to E+, E-, A+, A- (Tried B also).

Any suggestions on where I may be going wrong?

Cheers
Oliver

Wawa

Please read the "how to post" guide of the forum.
We need all the details to help you.
Code, pictures, which Arduino, etc.

Start by measuring the voltages on the sensor.
There should be about 4.25volt on E+
The two A inputs should be about half of that, with very little voltage (<20mV) between them.
Leo..

Oliverhowden

A-: 2.85V
A+: 2.57V
Voltage across A+/A-: 0.280v
E+: 5v
E-: 0.715V

Arduino using the Bodge HX711 Library
Code: [Select]
#include "HX711.h"

// HX711 circuit wiring
const int LOADCELL_DOUT_PIN = 2;
const int LOADCELL_SCK_PIN = 3;
const int GAIN = 64; // channel A, gain factor 64
HX711 scale;

void setup() {
 Serial.begin(57600);
 scale.begin(LOADCELL_DOUT_PIN, LOADCELL_SCK_PIN, GAIN);

}

void loop() {
 if (scale.is_ready()) {
   long reading = scale.read();
   Serial.print("HX711 reading: ");
   Serial.println(reading);
 } else {
   Serial.println("HX711 not found.");
 }

 delay(100);
 
}


I have tried both A/B channels with different gain to the same effect.

Thank you for your help

Wawa

280mV between A+ and A- is way to much.

E+ should be 4.25volt.
E- should be 0volt (Arduino ground).

Need to see the setup (as previously requested).
Leo..

Oliverhowden

Photos of the setup are attached to the previous post.
Is that enough?
Should I using arduino GND instead of E-?

Cheers

Wawa

Connections seem right. Are you sure the sensor is connected right? (can't see).

Did you measure with the black lead of the DMM touching the metal part of the USB socket (Arduino ground).

E+ must be ~4.25volt, and E- must be 0volt (no sensor connected).

They must stay the same with the sensor connected.
Leo..

Oliverhowden

I was gathering the voltages with the black lead on the GND pin, not the metal part of the USB port.

When measuring with the black lead on the metal casing, the voltage appears around 4V briefly on E+, then drops to zero.

My understanding was GND was okay to use instead of the metal casing.

Does this suggest a faulty Arduino?

Oliverhowden

I have just swapped to a MEGA instead, new readings with black lead on metal casing:

E+: -5v
E-: -0.7V

A-: -2.56V
A+: -2.9V

Wawa

E+ must be ~4.25volt, and E_ must be 0volt (ground), otherwise there could be a wiring/connection problem.
Did you properly solder the pins to the HX711 module?
Leo..

Oliverhowden

I have checked all connections and reflowed the joints.
Excuse my ignorance, though is it not the same result that the voltage across E+/- is 4.25v?

Wawa

Not sure what you mean.
There must be ~4.25volt across E+ and E-
But E- is directly connected via the HX711 module to Arduino ground.
There should be 0volt on E-
Leo..

Oliverhowden

When I check the continuity between E- and GND, there is a connection. When I check the voltage between E- (GND, on the HX711, or the metal casing), it remains around 0.7v.
There is 4.25V across E+/E-, though E- is not 0V.
Any suggestions on the next step I should take?

Thank you very much

Go Up