Photocouplers are much like Optoisolators, and have an internal LED that shines upon a phototransistor to electrically isolate two circuits. Now the two circuits can be completely isolated, and this is very handy for noisy devices such as Servo's/Motors.

    Here's what a Photocoupler looks like on the inside in a practical circuit;

    Keeping in mind that the internal LED has a maximum current of 10mA with this Photocoupler, a 390 Ohm or greater resistor must be put in series with it. Notice the 10K pull up resistor, this means that the signal will be inverted, that is, when the input is low, the output is high and vise versa.