# Source: https://learn.adafruit.com/basic-resistor-sensor-reading-on-raspberry-pi.md

# Basic Resistor Sensor Reading on Raspberry Pi

## Overview

![](https://cdn-learn.adafruit.com/assets/assets/000/001/320/medium800/raspberry_pi_shade.jpg?1396770987)

We've already covered how to use an Analog-to-Digital Converter chip with a Pi. These chips are the best way to read analog voltages from the Pi. However, there's a way to read many sensors&nbsp; **without** an ADC! By measuring the sensor as a resistor that is used to 'fill up' a capacitor, we can count how long it takes. It's not nearly as precise as an ADC and its a little flakey (since it depends on the Pi timing&nbsp;itself which can vary based on how 'busy' the computer is)  
  
The way we do this is by taking advantage of a basic electronic property of resistors and capacitors. It turns out that if you take a capacitor that is initially storing no voltage, and then connect it to power (like 3.3V) through a resistor, it will charge up to the power voltage slowly. The bigger the resistor, the slower it is.&nbsp;  
  
This technique only works with sensors that act like resistors. however, there are quite a few fun sensors that act this way: [photocells](http://adafruit.com/products/161 "Link: http://adafruit.com/products/161"), [thermistors (temperature sensors)](http://adafruit.com/products/372 "Link: http://adafruit.com/products/372"), [flex sensors](http://adafruit.com/products/182 "Link: http://adafruit.com/products/182"),[force-sensitive resistors](http://adafruit.com/products/166 "Link: http://adafruit.com/products/166"), and many more.  
  
It cannot be used with sensors that have a pure analog output like [IR distance sensors](http://www.adafruit.com/products/164) or [analog&nbsp;accelerometers](http://www.adafruit.com/products/163).

- [Next Page](https://learn.adafruit.com/basic-resistor-sensor-reading-on-raspberry-pi/how-it-works.md)

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