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Step 2: Wiring Up and Testing the Phototransistor

Picture of Wiring Up and Testing the Phototransistor

Normally, you would need a resistor in series with the phototransistor across power, but I knew that the Arduino had the ability to enable an internal pull-up resistor on any of the pins. I suspected that I could take advantage of that to hook up the phototransistor to the Arduino without any additional components. It turned out my hunch was correct!

I used wires to connect the phototransistor to the GND and A5 pins on the Arduino. I then created a sketch that set the A5 pin as an INPUT_PULLUP. This is normally done for switches, but in this case it provides power to the phototransistor!

#define SENSOR A5
void setup() {
void loop() {
  // Read analog value continuously and print it

This sketch prints values to the serial port corresponding to the ambient brightness. By using the handy "Serial Plotter" from the "Tools" menu of the Arduino IDE, I can get a moving plot of ambient light! As I cover and uncover the phototransistor with my hands, the plot moves up and down. Nice!

This sketch is a nice way to check whether the phototransistor is wired up with the right polarity: the phototransistor will be more sensitive when hooked up one direction versus the other.