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 ...

Just a handful of 555 timers and some 74HC series logic work together to produce pulses with approximately correct timings. Operating the device is as easy as tweaking a few potentiometers ...