Gabriela Manogaran

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6 years, 9 months ago.

LED and LDR using NUCLEO L073RZ

Hi, I was trying out the following code to make an LED glow in the dark automatically. After the code is run, the LED keeps glowing and the LDR value is some absurd value.

  1. include "mbed.h"
  1. define LED_PIN PA_5 SETTING LED PIN TO D13
  2. define LDR_PIN PB_0 SETTING LDR PIN TO A0

DigitalOut myled(LED_PIN); AnalogIn myldr(LDR_PIN);

int main() {

while(1) { int ldrstatus = myldr; if (myldr >= 3000000000) { myled = 0; printf ("LED IS OFF \n"); } else { myled = 1; printf ("LDR READING = %d \n", myldr); printf ("LDR IS DARK, LED IS ON \n");

}

} }

OUTPUT IS: LDR READING :13244420 LDR IS DARK, LED IS ON

1 Answer

Martin Simpson
6 years, 8 months ago.

MBED AnalogIn as in your code would be a floating point number from 0 to 1.0

so just reading myldr will return a 0 to 1.0 or using the member function read_u16 i.e. myldr.read_u16(); will return a 16 bit number (normalized but thats another story)

Your code with the <code> tabs as per 'editing tips'

include "mbed.h"
define LED_PIN PA_5 SETTING LED PIN TO D13
define LDR_PIN PB_0 SETTING LDR PIN TO A0
DigitalOut myled(LED_PIN); AnalogIn myldr(LDR_PIN);

int main() {

while(1) { 
  int ldrstatus = myldr;
  if (myldr >= 3000000000) 
    {myled = 0; printf ("LED IS OFF \n"); }
  else
    { myled = 1; printf ("LDR READING = %d \n", myldr); printf ("LDR IS DARK, LED IS ON \n");}
 } 
}

edited code

include "mbed.h"
define LED_PIN PA_5 SETTING LED PIN TO D13
define LDR_PIN PB_0 SETTING LDR PIN TO A0
DigitalOut myled(LED_PIN); AnalogIn myldr(LDR_PIN);

int main() {

while(1) { 
  float ldrstatus = myldr;//NB changed int to float
   if (myldr >= 0.5f)//NB now checking against a floating point value (MID range) change to whatever value you want to activate light 
   { myled = 0; printf ("LED IS OFF \n"); }
   else
   { myled = 1; printf ("LDR READING = %4.2f \n", myldr); printf ("LDR IS DARK, LED IS ON \n");}//NB printf data handler is now a float type %4.2f 4 characters to 2 decimal places
  }
}

Hi,

I tried this and now the LED just keeps flickering. The terminal still shows the LDR reading as 0.00 . Is there any additional stuff I need to do to interface an LDR? Does the NUCLEO-L073RZ board not support this?

I am really new to this and would be very grateful if you could help me out.

posted by Gabriela Manogaran 02 Apr 2018

Check wiring on the LDR please see for an example

https://os.mbed.com/users/4180_1/notebook/using-a-photocell-to-determine-light-levels/

posted by Martin Simpson 09 Apr 2018