Drop voltage when I connect to Analog In pin

Using Arduino Sensors
1

Hi guys I´m again.

I have a trouble when I try to read a voltage of a LM324 output. I was searching information about this problem but I don´t have luck.

I have a circuit with this OpAmp to read an a load cell, this part not have problem, I reads without problems the output of the sensor but when I connect the output to the arduino the voltage drops on the output of the LM324 and the reading in serial port is 0

I was joined the ground of the power supply with the Arduino´s GND.

The schematic of the LM324

The arduino code:

void setup(){
    Serial. begin(9600);
}

void loop(){

Serial.println(analogRead(A0));
delay(800);
}

Thanks

AMPLIFICADOR_INSTRUMENTALmod.png
AMPLIFICADOR_INSTRUMENTALmod.png800×440 19.4 KB

2

It sounds like the pin you are trying to read from is in an output mode? In looking at the 324 data sheet from TI ( http://www.ti.com/lit/ds/symlink/lm124-n.pdf ) note that while typical source output from the op-amp is 40ma, the sink capability is only about 20 microamps (I had not realized it was that low :o ) Why are you trying to read the output of an op-amp via a serial port?

3

Let us see a schematic of your circuit and a gook good image of your wiring. :confused:

.

4

Thanks for your answer gpsmikey.

Yes the 328 pin is in analog read

I need to make a digital scale. in the V1 and V2 I connect the wheatstone bridge outputs, result of the half bridge load cell and others two resistors.

After this the LM324 amplifies the signals of the wheatstone bridge and I try to catch this reading trough the ADC of the arduino

If I put in the lm324 output another OpAmp with follower mode ?

Thanks!

5

Things look ok. Shouldn't be a problem with the op amp circuit. Sounds like a weak output. Put a 10 meg resistor across the output and see what you measure. If it goes to zero, then you better start tracing through your wiring.

6

A gook image? :smiley:

7

Are you using the LM324 with dual power supplies or single?

8

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The full schematic

9

If I put a LED on the output of the LM324 turns on proportionally to the presure of the load cell.

And if I measure the current in thos moment I have 27mA and the ouput voltage is 2.5 V

10

And you are sure that Vo is positive with respect to ground and the amp is not oscillating?

11

768×1024 104 KB

12

Oh well, I won't insult your intelligence by asking if the black probe is on ground :slight_smile: , do you have another 324?

13

Yes, the black wire of the multimeter is on the black wire of the power supply and the red wire of the multimeter is on the output of the LM324.

14

It´s work now !

Thanks to all!

I leave the entire schematic for feedback.

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15

Very dangerous circuit for the Arduino (pin).
The opamp output can swing 20volt, and can deliver 20mA.

Load cell is connected wrong in both diagrams.
Load cell must have a ground reference to keep the opamps inside the common mode range.

Why dual supply.
That opamp has an input common mode range that includes ground, and the output that can go to ground.
Load cell output is (should be) 6volt anyway.

So single 12volt supply.
Load cell:
Top to 12volt (should be well regulated/stabilised to 10volt).
Bottom to ground.
Right to top opamp +input.
Left to lower opamp +input.
Loose the buffer opamp (last opamp).
4k7 resistor between output of the third opamp and the 100n cap (Arduino pin protection).

All of this should be done with the much better INA125.
Or easier with a HX711 breakout board.
Leo..