This 2-bit adder was a lot of work to build. It uses a total of thirty-six 555 timers and it does have the option of adding or subtracting numbers. It’s a rather unorthodox use of the part, depending more on the chip as an inverter and taking advantage of the fact that there’s an NPN transistor built into it. [cpu86] did use some PNP transistors to give him the ability to turn off some of the 555’s to get everything working correctly.
He explains the use of two’s complement in the subtracting feature but the process is just touched on very quickly. Luckily there’s a huge eagle schematic available with his project writeup so that you can follow along and really grasp how this thing works. We’ve generated a PNG and embedded it after the break for your convenience. You’ll find it just after the two videos of the device in action.