## Light Sensors | Features | Phototransistor | Photoresistor | |-----------------|---------------|---------------| | Sensitivity to light | Low | High | | Directional | No | Yes | | Resistance changes? | No | Yes | <!-- The performance of the phototransistor can be superior to that of the photodiode for some applications in view of its gain. As a rough guide, where a photodiode may enable a current flow of around 1µA under typical room conditions, a phototransistor may allow a current of 100µA to flow. --> ### Phototransistor A phototransistor conducts more or less current as a function of light level. The signal is just the current times the resistance R. Sensitivity can be changed by changing the value of R in the range 1 kOhm - 100 kOhm. We have phototransistors that are sensitive in the visible and others sensitive in the near infrared. ![phototransistor](./phototransistor.png) LDRs (light dependent resistors) are very popular and cheap, but not very accurate. See [Adafruit's](https://learn.adafruit.com/photocells) documentation. ![ldr](./ldr.png) ### Photoresistor A photoresistor changes its resistance as a function of light. As it receives more light its resistance decreases, and the voltage at the point labeled A0 decreases. Any of the analog inputs A0 - A5, could be used (see the AnalogReadSerial example for Arduino code). ![photoresistor](./photoresistor.jpg)