Problem with using analogRead to measure the voltage signal of a photodiode

Other Hardware Sensors
1

Hi, guys

Recently I'm using a photodiode to measure the intensity of a CW laser. (Voltage signal) The circuit diagram of the photodiode device is shown below:

The "Vin" terminal in the diagram is connected to Vin pin on Arduino, and Arduino is supplied by a 12V source.

When I used a multimeter whose input impedence is 10 Mohm to measure the voltage difference between the output and ground, I got 3.81V.
However, when I used pin A10 on Arduino mega2560 to read the voltage, I got 5V. (The output and ground in the diagram was respectively connected to A10 and GND on Arduino.)

Here is my code:

int pin = 10;

void setup() {
  pinMode(pin, INPUT);
  
  Serial.begin(115200);
  Serial.println("Now start");
}

void loop() {  
  float measure = analogRead(pin);  
  measure = measure/1023*5 ; 
  Serial.println(measure); 
  delay(1000);
}

FYI:

  1. When I used both Arduino and multimeter (in the same time) to read the output voltage, I got 3.81V (steady) on my multimeter and 4~5V (fluctuating) on my Arduino.
  2. When I connected a 1 Kohm resistor between output and ground, Arduino got 0V. (still measuring the voltage difference between output and ground)

Does anyone know what is happening? Please help me.

2

That is not a useful way to use a photodiode.
Here are some ideas for useful circuits (the first will explain why yours doesn't work):
http://ecee.colorado.edu/~ecen4827/hw/hw1/PhotodiodeAmplifers.pdf
http://physlab.lums.edu.pk/images/1/10/Photodiode_circuit.pdf

3

When I used a multimeter whose input impedence is 10 Mohm to measure the voltage difference between the output and ground, I got 3.81V.
However, when I used pin A10 on Arduino mega2560 to read the voltage, I got 5V.

This is because the input impedance of an arduino pin is greater than 10M. So your meter is dragging down the voltage.
However if you connect the resistor to Vin you are putting too much voltage on the pin. Fortunately for you the resistor and reverse bias diode restrict the current so that the anty-static protection diodes kick in and protect the input pin. This is not something you should use.