I am looking at an IR sensor that has the following requirement:
The receiver is open collector which means that you do need a pull up resistor. Most microcontrollers have the ability to turn on a built in pull up resistor. If you do not, connect a 10K resistor between the white wire of the receiver and the red wire.
I am trying to connect the sensor wire (white) to D4 and D3.
So does the Photon and Electron have a built in pull up resistor?
Note, however, that the pull-up is about 40K. This is suitable for most purposes, but if the device actually needs a smaller pull-up, you may need to add an external one. I2C, for example, always requires external pull-ups on SDA and SCL.
Also, the internal pull-ups (and pull-downs) are only suitable for 3.3V devices. Most of the IO pins are 5V tolerant in digital mode, but only when NOT using an internal pull-up or pull-down down.