# Arduino Lesson 9. Sensing Light ## Overview In this lesson, you will learn how to measure light intensity using an Analog Input. You will build on lesson 8 and use the level of light to control the number of LEDs to be lit. ![](https://cdn-learn.adafruit.com/assets/assets/000/002/273/medium800/learn_arduino_overview.jpg?1396781232) The photocell is at the bottom of the breadboard, where the pot was in lesson 8. - [Next Page](https://learn.adafruit.com/adafruit-arduino-lesson-9-sensing-light/parts.md) ## Featured Products ### Photo cell (CdS photoresistor) [Photo cell (CdS photoresistor)](https://www.adafruit.com/product/161) CdS cells are little light sensors. As the squiggly face is exposed to more light, the resistance goes down. When it's light, the resistance is about ~1KΩ, when dark it goes up to ~10KΩ. To use, connect one side of the photocell (either one, it's symmetric) to power... In Stock [Buy Now](https://www.adafruit.com/product/161) [Related Guides to the Product](https://learn.adafruit.com/products/161/guides) ### Diffused Red 5mm LED (25 pack) [Diffused Red 5mm LED (25 pack)](https://www.adafruit.com/product/299) Need some indicators? We are big fans of these diffused red LEDs, in fact we use them exclusively in our kits. They are fairly bright so they can be seen in daytime, and from any angle. They go easily into a breadboard and will add that extra zing to your project. - Pack of 25... In Stock [Buy Now](https://www.adafruit.com/product/299) [Related Guides to the Product](https://learn.adafruit.com/products/299/guides) ### Premium Male/Male Jumper Wires - 40 x 6" (150mm) [Premium Male/Male Jumper Wires - 40 x 6" (150mm)](https://www.adafruit.com/product/758) Handy for making wire harnesses or jumpering between headers on PCB's. These premium jumper wires are 6" (150mm) long and come in a 'strip' of 40 (4 pieces of each of ten rainbow colors). They have 0.1" male header contacts on either end and fit cleanly next to each other... In Stock [Buy Now](https://www.adafruit.com/product/758) [Related Guides to the Product](https://learn.adafruit.com/products/758/guides) ### Half Sized Premium Breadboard - 400 Tie Points [Half Sized Premium Breadboard - 400 Tie Points](https://www.adafruit.com/product/64) This is a cute, half-size breadboard with 400 tie points, good for small projects. It's 3.25" x 2.2" / 8.3cm x 5.5cm with a standard double-strip in the middle and two power rails on both sides. You can pull the power rails off easily to make the breadboard as... Out of Stock [Buy Now](https://www.adafruit.com/product/64) [Related Guides to the Product](https://learn.adafruit.com/products/64/guides) ### Adafruit METRO 328 Fully Assembled - Arduino IDE compatible [Adafruit METRO 328 Fully Assembled - Arduino IDE compatible](https://www.adafruit.com/product/50) We sure love the ATmega328 here at Adafruit, and we use them _a lot_ for our own projects. The processor has plenty of GPIO, Analog inputs, hardware UART SPI and I2C, timers and PWM galore - just enough for most simple projects. When we need to go small, we use a Out of Stock [Buy Now](https://www.adafruit.com/product/50) [Related Guides to the Product](https://learn.adafruit.com/products/50/guides) ## Related Guides - [Adafruit Proto Screw Shield](https://learn.adafruit.com/adafruit-proto-screw-shield.md) - [LSM303 Accelerometer + Compass Breakout](https://learn.adafruit.com/lsm303-accelerometer-slash-compass-breakout.md) - [2.8" TFT Touch Shield](https://learn.adafruit.com/2-8-tft-touch-shield.md) - [Arduino GPS Clock](https://learn.adafruit.com/arduino-clock.md) - [Sending an SMS with Temboo](https://learn.adafruit.com/sending-an-sms-with-temboo.md) - [Smart Cocktail Shaker](https://learn.adafruit.com/smart-cocktail-shaker.md) - [Nokia 5110/3310 Monochrome LCD](https://learn.adafruit.com/nokia-5110-3310-monochrome-lcd.md) - [Ladyada's Bento Box](https://learn.adafruit.com/lady-adas-bento-box.md) - [Arduino Lesson 17. Email Sending Movement Detector](https://learn.adafruit.com/arduino-lesson-17-email-sending-movement-detector.md) - [Track Your Treats: Halloween Candy GPS Tracker](https://learn.adafruit.com/track-your-treats-halloween-candy-gps-tracker.md) - [Trainable Robotic Arm](https://learn.adafruit.com/trainable-robotic-arm.md) - [Metal Inlay Capacitive Touch Buttons](https://learn.adafruit.com/metal-inlay-capacitive-touch-buttons.md) - [Arduino Lesson 5. The Serial Monitor](https://learn.adafruit.com/adafruit-arduino-lesson-5-the-serial-monitor.md) - [Adafruit Data Logger Shield](https://learn.adafruit.com/adafruit-data-logger-shield.md) - [Arduino Lesson 6. Digital Inputs](https://learn.adafruit.com/adafruit-arduino-lesson-6-digital-inputs.md)