ULN2803 8-Channel Darlington Driver

One chip turns eight wimpy 5 V GPIO outputs into eight 500 mA load-driving switches with built-in flyback diodes.

Difficulty: Breadboard. Estimated build time: about 35 minutes. Estimated parts cost: about US$3.20. 4-part bill of materials with live supplier pricing. An 8-channel relay/LED-cluster driver built around the ULN2803A, an array of eight Darlington pairs in a single DIP-18 package.