The LilyPad Light Sensor is a sewable breakout board with an ALS-PT19 light sensor built in and ready to use right away.
Each sensor outputs voltage between 0V and 3.3V depending on the level of ambient light shining on it.
As more light is applied on the sensor, more current will flow from the board through the signal tab to the microcontroller you connect the sensor to.
If the sensor receives no light, no current will flow through it. In a typical indoor lighting situation, the sensor will output around 1 to 2V.
Adding this board into your next e-textile or wearable project will really open up the possibilities by being able to incorporate light level sensing capabilities.