Power source switching between two power sources

1/*
2 *arduino realy controlled by light sensing.
3 *Sense surrounding   light intensity with BHV1750. 
4 *Relay module:
5 *  if Logic Low: NC--Com connected
6   *  if Logic High: NO--Com connected
7 *If it is dark, connect power source A (Normally   Closed), eg. AC power adapter. 
8 *If is is bright, connect power source B (Normally   Open).eg, Solar panel generation.
9 */
10
11#include <Wire.h>
12#include <BH1750.h>
13BH1750   lightMeter;
14
15// constants won't change
16const int RELAY_PIN = 3;  // the   Arduino pin, which connects to the IN pin of relay
17const float LIGHT_THRESHOLD   = 250.;  //Threshhold for power source selection
18
19// the setup function runs   once when you press reset or power the board
20void setup() {
21  Serial.begin(9600);
22   
23  // initialize digital pin as an output.
24  pinMode(RELAY_PIN, OUTPUT);
25   // start with relay pin low - source A
26  digitalWrite(RELAY_PIN, LOW);
27   delay(3000);
28  
29  Serial.begin(9600);
30  Wire.begin();
31  lightMeter.begin();
32   Serial.println(F("BH1750 Test"));
33  
34}
35
36// the loop function runs   over and over again forever
37void loop() {
38  float lux = lightMeter.readLightLevel();
39   Serial.print("Light: ");
40  Serial.print(lux);
41  Serial.println(" lx");
42
43   if (lux < LIGHT_THRESHOLD) {
44    digitalWrite(RELAY_PIN, LOW);    
45  }
46   else {
47    digitalWrite(RELAY_PIN, HIGH);
48  }
49delay(3000);
50
51}
52

1/*
2 *arduino realy controlled by light sensing.
3 *Sense surrounding  light intensity with BHV1750. 
4 *Relay module:
5 *  if Logic Low: NC--Com connected
6  *  if Logic High: NO--Com connected
7 *If it is dark, connect power source A (Normally  Closed), eg. AC power adapter. 
8 *If is is bright, connect power source B (Normally  Open).eg, Solar panel generation.
9 */
10
11#include <Wire.h>
12#include <BH1750.h>
13BH1750  lightMeter;
14
15// constants won't change
16const int RELAY_PIN = 3;  // the  Arduino pin, which connects to the IN pin of relay
17const float LIGHT_THRESHOLD  = 250.;  //Threshhold for power source selection
18
19// the setup function runs  once when you press reset or power the board
20void setup() {
21  Serial.begin(9600);
22  
23  // initialize digital pin as an output.
24  pinMode(RELAY_PIN, OUTPUT);
25  // start with relay pin low - source A
26  digitalWrite(RELAY_PIN, LOW);
27  delay(3000);
28  
29  Serial.begin(9600);
30  Wire.begin();
31  lightMeter.begin();
32  Serial.println(F("BH1750 Test"));
33  
34}
35
36// the loop function runs  over and over again forever
37void loop() {
38  float lux = lightMeter.readLightLevel();
39  Serial.print("Light: ");
40  Serial.print(lux);
41  Serial.println(" lx");
42
43  if (lux < LIGHT_THRESHOLD) {
44    digitalWrite(RELAY_PIN, LOW);    
45  }
46  else {
47    digitalWrite(RELAY_PIN, HIGH);
48  }
49delay(3000);
50
51}
52