If no external resistor is added these inputs activate at about 2V. Forward current should not exceed 10mA, and this caps the maximum input voltage at around 5V. Add external resistors if you need to apply higher input voltage.
Combine this Tibbit with terminal block devices – #20 (nine terminal blocks) or #21 (four terminal blocks).
There are three green LEDs which are connected to three control lines. LEDs light up for the LOW state of control lines (i.e. when current passes through the diodes of the opto-couples).