[U2] is a ULN2803A-series 8-channel Darlington transistor array from Texas Instruments. It serves as a high-current, high-voltage driver interface between low-power logic and inductive or resistive loads.
Key Functions and Characteristics
Darlington Pairs
Eight NPN Darlington pairs, each with a built-in 2.7 kΩ series base resistor for direct TTL/5 V CMOS interfacing.
Collector-emitter voltage rating up to 50 V and collector current up to 500 mA per channel.
Built-in Clamp Diodes
Integrated common-cathode diodes for inductive-load suppression (relays, solenoids, motors).
Parallel Operation
Individual outputs can be paralleled for higher current capability.
Package
Available in 18-pin SOIC (as used in your design) and through-hole DIP variants.
How It Operates in a Circuit
Input Stage
A logic‐level signal (e.g., from an MCU GPIO) is applied to one of the eight inputs.
The series base resistor limits input current, allowing direct interfacing without extra resistors.
Darlington Amplification
The input drives two cascaded NPN transistors, yielding high current gain.
Output Stage & Protection
When activated, the Darlington pair pulls the output low, sinking up to 500 mA.
Clamp diodes divert inductive kickback to the common cathode (VCC), protecting the transistors.
Common Applications
Relay and solenoid drivers
Lamp and LED display drivers
Hammer drivers in printing and impact-dot applications