I have many times found ULN2003 and ULN2804 transistor arrays to be very useful components when I want to control some loads like relays, small electrical motors or light bulbs from TTL circuits. I have for example used ULN2803 for many applications where I have needed to control something with PC parallel port.
The interesting question that sometimes come to mind is what is actually inside of those ICs. Reverse engineering the die of a ULN2003 transistor array tries to answer this.
ULN2003 – practical example of «reading» microchip schematic shows nice microchip photos of ULN2003 – array of Darlington transistors – and tells how the IC is build. Despite it’s simplicity this microchip is still widely used and mass manufactured. ULN2003 contains 21 resistors, 14 BJT transistors and 7 diodes.