Tim wrote: one-bit-flip-flop boards I'm round package.) pretty sure they do I built something like this when I was probably something like 12 or 13 years old. I was purchased at Radio Shack as a kit. It was a four bit binary counter, with incandescent lights as on the Q outputs, (though discrete transistor drivers), and a photoresistive cell or pushbutton switch as the trigger. With the photoresistor as the trigger, when you waved your hand in front of the photocell, it would increment the counter, which was pretty cool. I distinctly remember the RTL IC's made by Motorola in the black plastic "blob" packages. I have vague recollection of the project being a mother board that had four small circuit boards that had the flip flop chip, transistor driver, and lamp (perhaps these were the boards that Tim mentioned). The mother board had a photocell, a toggle switch, and a momentary action switch with de-bouncing circuitry. You could trigger the counter with the photocell, or the momentary action switch. I think the thing ran off a 9V battery if I remember correctly. The photocell didn't have very fast response time, but I do remember putting it in front of a fan in a dark room with a flashlight shining through the fan blades, and made the counter go pretty fast. I think that I eventually damaged the chips by trying to make the thing count BCD rather than binary by adding some gating. That was the end of it. It was fun building and tinkering with, and educational. It was my first exposure to integrated circuits.