Each channel of ULN2803C consists of Darlington connected NPN transistors. This connection creates the effect of a single transistor with a very-high current gain. The very high β allows for high output current drive with a very-low input current, essentially equating to operation with low GPIO voltages.

The GPIO voltage is converted to base current through the 2.7-kΩ resistor connected between the input and base of the predriver Darlington NPN.

The diodes connected between the output and COM pin are used to suppress the kickback voltage from an inductive load that is excited when the NPN drivers are turned off (stop sinking) and the stored energy in the coils causes a reverse current to flow into the coil supply through the kickback diode.

In normal operation, the diodes on base and collector pins to emitter are reverse biased. If these diodes are forward biased, internal parasitic NPN transistors draw (a nearly equal) current from other (nearby) device pins.