This weekend I finished the Clock Circuit for the 8-bit computer that I’m building. The project uses prototyping boards (known as “breadboards”) instead of printed circuit boards and soldering. Breadboards are great because they make circuit building almost like Legos. You pop in wires and chips and can move them around and experiment without a lot of commitment.
This Clock is the heartbeat of the coming computer. Each time it pulses the computer will advance one instruction. The pulsing is controlled with a “555 timer“, one of the most ubiquitous ICs (integrated circuits; a chip) in the world. Invented 51 years ago, it uses a clever bit of circuitry to create a stable and consistent pulse. Water is often an analogy for electricity, and it functions a bit like a cup, filling with water, then tipping over to empty, then righting itself and doing it all over. So long as the flow rate is constant, so too will be the timing of the fill and empty.
The build includes a way to manually pulse the clock and to control which pulse method is output, the auto pulse or manual.
I took some liberties with the source material. I chose to color code my wires to make clear which wires belong to which part of the Clock. This helped me a lot when working out the outputs on the right side of the circuit.
The original design uses a potentiometer to control the speed of the pulse in the automatic (Astable) circuit. I built mine using parts I had on hand and a few I picked up on Amazon with some gift certificates. Unfortunately, I had a potentiometer but it was about 1/10the the resistance that I needed to get it to work properly in this circuit. I ended up leaving it out, and will order a proper one when I buy the parts for the next part of the computer.
This project sources from Ben Eater’s work. I can’t say enough amazing things about Ben, his work, and his ability to explain the complex in incredibly simple terms. Einstein said, “If you can’t explain it simply, you don’t understand it well enough.” It’s clear Ben understands this. I would encourage anybody who wants to learn about and understand circuits to begin here. Especially if you want to know what the heck your computer is ACTUALLY doing.
Here’s a video of the full Clock Circuit in action.