Light Loop

Sep 26, 2017 00:00 · 326 words · 2 minutes read Hardware

The Features

The goal of this project is to use analog input and digital input to drive digital output.

This is accomplished by detecting when a photoresistor, an analog input, gets a certain amount of light. When that happens, it triggers an animation on the opposite end of the LEDS which travels through them back towards the photoresistor. When the photoresistor picks up enough light from the animation that has reached it, it triggers the start of the animation again. This essentially creates an infinite feedback loop.

The loop can be stopped by blocking the light from the photoresistor, as demonstrated in the video. It can be initialized by pressing the pushbutton which acts as a digital input that turns on the last LED, placed next to the photoresistor. This light causes the photoresistor to start the animation.

A pentiometer, another analog input, can be turned to speed up or slow down the animation.

The 74HC595 Shift Register is used to control 16 LEDs, each digital outputs, from a single digital output pin. Two shift registers are chained together to control 8 LEDs each, as instructed by Arduino’s Shifting Out guide. Adafruit’s Shift Register guide was helpful as well.

The Code

The full code for this is available here.

To control 16 LEDs with two shift registers, two bytes are stored in an array. Setting an LED on or off is abstracted by a function that maps the LED index to the corresponding byte, and updates the bit in that byte:

#define NUM_LEDS 16

byte leds[2];

void setLed(int led, int onOrOff) {
  int shifterIndex = led / LEDS_PER_SHIFTER;

  int ledToSet = led % LEDS_PER_SHIFTER;
  if (onOrOff == 1)
    bitSet(leds[shifterIndex], ledToSet);
    bitClear(leds[shifterIndex], ledToSet);

To send the corresponding bits to the LEDs via the shift registers, they are pushed in bytes in descending order:

void updateShiftRegister()
   digitalWrite(latchPin, LOW);
   shiftOut(dataPin, clockPin, MSBFIRST, leds[1]);
   shiftOut(dataPin, clockPin, MSBFIRST, leds[0]);
   digitalWrite(latchPin, HIGH);