Build a light-seeking sunflower

Optional

This lesson has free STL files. Print the parts yourself with one of these 3D printers.

Build a light-seeking sunflower – EEMENTOR STEM™ lesson

Young sunflowers turn to follow the sun during the day. Solar panels on tracking stands do the same, so they make more electricity. In this lesson you build a flower that looks for the light: a servo turns it slowly from one side to the other while a photoresistor measures how bright it is in each direction. Then it turns back to the brightest direction and waits. When the light changes, it looks around again.

What you'll learn

How to scan with a servo and take a measurement at every step
How to find the biggest value in a set of readings
How to average several readings to get a steadier value
How to react when a value changes by more than a set amount

What you need

UNO R3 board and USB cable (UNO R3 Starter Kit)
Servo motor (UNO R3 Starter Kit)
Photoresistor module (45-in-1 Sensor Kit)
3 female-to-male jumper wires, or female-to-female and male-to-male wires plugged together
A torch or a phone light to test with
The printed parts from the 3D printing box below (servo stand, sunflower head and petals), or the cardboard version described there
Breadboard and jumper wires (UNO R3 Starter Kit)

3D printing · free STL files

Print the sunflower

There are three parts: a stand for the servo, a head that sits on the servo shaft and the flower itself. The photoresistor module rides on the flower, so the sensor always looks where the flower looks.

⬇ Servo stand STL · 64 × 48 × 25 mm ⬇ Flower head STL · 32 × 30 × 50 mm ⬇ Petals STL · 76 × 95 × 10 mm

Print settings and how to put it together

Print settings: PLA, 0.2 mm layers, 3 walls, 20 % infill. No supports. Print the petals in yellow, or add a colour change in your slicer at 1.2 mm for a two-colour flower.

Push the servo into the stand from the top, with the cable through the gap, and fix it with the two servo screws.
Upload the sketch once without the head, so the servo goes to 90 degrees.
Press the single-arm horn into the recess under the head, fix it with the horn screw and push the head onto the servo shaft, facing forwards.
Slide the petals into the slot at the top of the head. Add a drop of hot glue if they are loose.
Push the photoresistor module onto the peg in the middle of the flower, with the sensor at the top, facing forwards. Connect it with three female-to-male jumper wires and leave them loose enough for the head to turn.

No 3D printer? Cut a flower out of card and tape it to the servo horn, with the photoresistor module taped in its middle.

Wiring

From	To
Photoresistor module S	UNO pin A0
Photoresistor module + and −	5V and GND
Servo orange / yellow wire	UNO pin 9
Servo red wire	5V
Servo brown / black wire	GND

Sensor modules from different makers can label and order their pins differently. Always follow the labels printed on your module.

The sketch

#include <Servo.h>

const int LIGHT_PIN = A0;
const int SERVO_PIN = 9;
const int STEP = 5;                          // degrees between measurements
const unsigned long RESCAN_EVERY = 60000;    // look around again every minute...
const int BIG_CHANGE = 60;                   // ...or sooner if the light changes this much
const bool BRIGHTER_IS_HIGHER = false;       // see "Check your sensor"

Servo head;
int bestAngle = 90;
int bestLight = 0;
unsigned long lastScan = 0;

// Average of 8 readings, turned so that a bigger number always means more light
int readLight() {
  long sum = 0;
  for (int i = 0; i < 8; i++) {
    sum += analogRead(LIGHT_PIN);
    delay(2);
  }
  int value = sum / 8;
  return BRIGHTER_IS_HIGHER ? value : 1023 - value;
}

void scan() {
  Serial.println("Looking for the light...");
  bestLight = -1;
  for (int angle = 10; angle <= 170; angle += STEP) {
    head.write(angle);
    delay(angle == 10 ? 500 : 80);           // wait for the servo to arrive
    int light = readLight();
    if (light > bestLight) {
      bestLight = light;
      bestAngle = angle;
    }
  }
  head.write(bestAngle);
  lastScan = millis();
  Serial.print("Brightest at ");
  Serial.print(bestAngle);
  Serial.print(" degrees, light level ");
  Serial.println(bestLight);
}

void setup() {
  Serial.begin(9600);
  head.attach(SERVO_PIN);
  head.write(90);
  delay(1000);
  scan();
}

void loop() {
  int light = readLight();
  if (abs(light - bestLight) > BIG_CHANGE || millis() - lastScan > RESCAN_EVERY) {
    scan();
  }
  delay(200);
}

Check your sensor

Open the Serial Monitor at 9600 baud and wait for "Brightest at … degrees".
Shine a torch at the flower from one side. Within a second it should look around and stop facing the torch.
If it turns away from the torch instead, change BRIGHTER_IS_HIGHER to true and upload again. Photoresistor modules are wired in two different ways, and this one setting covers both.

How it works

scan() is a classic "find the biggest" loop. It starts with bestLight = -1, so the first reading always wins, and then keeps a new reading only if it is brighter than the best one so far, together with the angle where it was measured.

In loop() the flower measures the light where it is pointing. If it is very different from the brightest value it found (because a cloud came or a lamp was switched on), or if a minute has passed, it scans again. Waiting between scans saves the servo from constant buzzing, just as a real solar tracker only moves every few minutes.

Try this next

Make the scan faster or more precise by changing STEP. What is the trade-off?
Put the flower on a windowsill for a day and log the best angle every 10 minutes. Can you see the sun moving?
Add a second servo for up and down, like the radar stand, and track in two directions.

Troubleshooting

The flower turns away from the light: change BRIGHTER_IS_HIGHER, see above.
It scans again and again: the light is flickering (some LED lamps do) or BIG_CHANGE is too small. Try 120.
The wires stop the head turning: route them behind the stand and leave a loose loop.
The board restarts when the servo moves: servos draw bursts of current. Use a good USB port or a powered hub, and check that the servo's red wire is on 5V.
A printed part is too tight or too loose: printers differ by a few tenths of a millimetre. Sand or file a tight hole, and fix a loose part with a drop of hot glue or a strip of tape.