23. Photocell

Photocells, also known as photoresistors or light-dependent resistors (LDRs), are simple yet powerful components that can detect changes in light intensity. These components are commonly used in various applications such as automatic lighting systems, light-sensitive alarms, and other projects where light detection is essential.

Materials

Component	Image
Breadboard
Jumper wires
Arduino Uno R4 Minima
Photocell
Resistors (220 ohms for the LED, optional if using internal pull-up resistors for buttons)
LED (white, red, blue, or green)
74HC595 Shift Register

Instructions

1. Make the following connections using the breadboard and jumper wires.

Arduino to Shift Register:

• latchPin (pin 11) to ST_CP (pin 12) of the 74HC595.
• clockPin (pin 9) to SH_CP (pin 11) of the 74HC595.
• dataPin (pin 12) to DS (pin 14) of the 74HC595.

Shift Register to LEDs:

• Connect the output pins (Q0-Q7) of the 74HC595 to the anodes of the LEDs through current-limiting resistors (usually around 220 ohms).

Power Supply:

Connect VCC of the 74HC595 to a 5V power source. Connect GND of the 74HC595 to the Arduino's GND.

2. Paste the following code into your main Arduino sketch:

Code

int lightPin = A0;
int latchPin = 11;
int clockPin = 9;
int dataPin = 12;

void setup() {
  Serial.begin(9600);
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
}

void updateShiftRegister(byte leds) {
  // take the latchPin low so
  // the LEDs don't change while you're sending in bits:
  digitalWrite(latchPin, LOW);
  // shift out the bits:
  shiftOut(dataPin, clockPin, MSBFIRST, leds);
  // take the latch pin high so the LEDs will light up:
  digitalWrite(latchPin, HIGH);
}

void loop() {
  int reading = analogRead(lightPin);
  Serial.println(reading);

  byte ledsLit = 0;
  // Calculate the number of LEDs to light up based on the reading
  // and the thresholds for each LED, Calibrate the Sensor here
  if (reading > 900) ledsLit |= 0b00000001;
  if (reading > 912) ledsLit |= 0b00000010;
  if (reading > 924) ledsLit |= 0b00000100;
  if (reading > 936) ledsLit |= 0b00001000;
  if (reading > 948) ledsLit |= 0b00010000;
  if (reading > 960) ledsLit |= 0b00100000;
  if (reading > 972) ledsLit |= 0b01000000;
  if (reading > 984) ledsLit |= 0b10000000;

  Serial.print("Leds Lit: ");
  Serial.println(ledsLit, BIN);

  updateShiftRegister(ledsLit);
  delay(1000);
}

3. Connect your Arduino to your laptop using a USB-C cable and upload the code to the arduino.

4. Test! Vary the light environment around the photocell and watch the LEDs turn on and off!

5. Optional: You can calibrate the sensor by changing the threshold values in the If statements!

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22. The Serial Monitor	24. Shift Registers and 7 Segment Display