Interesting simple design, although it only offers like 4! = 24 different combinations. To make more combinations possible by using logic gates you could probably assign each of the 4 keys a 2-bit ...

a simple nixie clock made with logic gates only. In this build, the mains 50Hz is used as a time base instead of a 32KHz crystal that most readers here may use. In the very long term, this clock ...

If not, you are in for a treat, because this incredibly easy-to-use tool is now available for Immediate Download as Freeware! This really is an ultra-cool application. First of all it has a schematic ...