# Source: https://learn.adafruit.com/photocells/testing-a-photocell.md

# Photocells

## Testing a Photocell

The easiest way to determine how your photocell works is to [connect a multimeter in resistance-measurement mode](http://learn.adafruit.com/multimeters/ "Link: http://learn.adafruit.com/multimeters/") to the two leads and see how the resistance changes when shading the sensor with your hand, turning off lights, etc. Because the resistance changes a lot, an auto-ranging meter works well here. Otherwise, just make sure you try different ranges, between 1MΩ and 1KΩ before 'giving up'.

![](https://cdn-learn.adafruit.com/assets/assets/000/000/454/medium800/light_cdslitmm.jpg?1396763180)

![](https://cdn-learn.adafruit.com/assets/assets/000/000/455/medium800/light_cdscovered.jpg?1396763190)

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## 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...

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We sure love the ATmega328 here at Adafruit, and we use them&nbsp;_a lot_&nbsp;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 <a...></a...>

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### Digital Multimeter

[Digital Multimeter](https://www.adafruit.com/product/71)
This is a basic multimeter, I've played with it a bunch and I think its a great addition to a toolbox. It's low cost and simple to use with a big clear display and all the measurements you need:

- AC/DC Voltage measurement
- Current measurement, from 1uA up to...

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This is a cute, half-size breadboard with&nbsp;400 tie points, good for small projects. It's 3.25" x 2.2" / 8.3cm&nbsp;x 5.5cm&nbsp;with a standard double-strip in the middle and two power rails on both sides.&nbsp;You can pull the power rails off easily to make the breadboard as...

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