The ULN2803A is a circuit module used to drive high current loads, it can directly drive a relay and also be used for level conversion. Each ULN2803A has eight load interface and can drive up to eight high-current load. "VEX" and "5V" interface can be used for an external power supply. The 595 chip is used for the ULN2803A cascade and the function of its corresponding interface "595IN" and "595OUT" is to connect the Arduino and to implement the cascade between the ULN2803A.

Its maximum input voltage is 30 V, maximum output voltage is 50 V.

For example, when you use the SCM or MCU as the output current signal, which is too small to direct drive big load, then you can choose this ULN2803A module.

ULN2803A module's 4 up road and 4 down road is load (such as LED lamp tape) slot, each slot can be connected with one LED lamp tape. So every ULN2803A module can drive 1~8 LED lamp tape.

ULN2803A module's power supply is divided into two parts, one is VEX POWER driven green terminal on the module with the mark "VEX" , which is the power of driving ULN2803A , and it requires users to connect the external POWER supply, when the requirement voltage of driving load is bigger than 5 V , you will use this VEX interface. The ULN2803A VEX interface can connect with the highest 30 V voltage.

 

Note: if the voltage of drive load is greater than 5 V, the jumper position is shown as below :
 

Another part is the interface that provide ULN2803A chip internal logic level 5 V power supply module interface, which is also the 595IN interface of this module, it is used to connect with the IDC-6 interface of Arduno IDC shield module, then getting 5 V logic level from Arduino derectly through connecting the 595IN interface with IDC-6 interface.
 

If the voltage of driving load is equal to 5 V, logic level is equal to the voltage of driving load, at this time, we may don't need external power supply, and can directly use logic level of 595IN port to drive the 5 V load.

Note: If the voltage of driving load is equal to 5  V, the jumper position is shown as below, and without external power supply :

 

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