555
The 555 timer circuit is cool; I'm just following a "cookbook" application, but I have adjustible clock pulses which I can probably get to drive TTL circuits. I can't drive a relay coil directly, unless perhaps I use a very high input voltage, or a transistor, and I still don't see how the transistor would work, although there seem to be "cookbook" examples of that too. It's more fun to understand the theory behind something than to follow somebody's instructions.
