Building a Gigatron: Power and clock
Previous post: Building a Gigatron: Capacitors
There's power coursing through the Gigatron. The power isn't doing a lot right now, but it's there and it is powering a red LED.
It took two goes to get the power working. I was quite pleased with the first attempt, but it proved a dud. Luckily the fix was absurdly simple - I hadn't soldered the power pins on the USB socket, so power wasn't even leaving the USB.
Doing the power and clock has been a change from the capacitors. Suddenly there are different components and just finding them in the huge bag of bags took some time. Some of the new components require more care, usually because they're polarised (they have a positive and negative side) and have to be soldered the right way around.
Along with the now-familiar capacitors, the power circuit has
- zener diode (black cylinder with silver stripe), which I think protects the Gigatron if there's a power surge.
- multi-fuse (big orange disk), also known as a resettable fuse. Another component to protect against power surges. "Resettable" seems to mean that it can work (break the circuit) multiple times. That's excellent because the (normal?) fuses in my house have to be replaced if they blow.
- supervisory circuit (little black thing near LED), which somehow monitors the power supply and maybe does something? The articles I found were either vague or more technical than I understood, so this is pretty mysterious right now.
The last bits and pieces of the power circuit are some resistors and the red LED (labelled a "blinkenlight") to prove it's working.
The clock circuit has even more new components, including an exciting first - the first integrated circuit! The IC is a 74HCT04, which its datasheet says is a "hex inverter". The inverter takes 6 input signals and for each signal produces the opposite output. That's a logical Not, with True becoming False and vice versa, so this is a 6 bit Not gate. I don't currently understand why it's in the clock.
The other notable part of the clock is the "crystal", which looks more like a silver lozenge. This seems to be a "crystal oscillator", which has a tiny bit of quartz crystal inside the metal package. I really wanted to open it up and see inside, but had only bought one and had to put the still-whole crystal into the Gigatron instead.
The crystal is designed have a particular frequency, in this case 6.25Mhz. Remarkably, it's this oscillation that provides the clock signal for the Gigatron! According to Wiki, the clock signal "oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits." I had no idea that's how a computer clock signal works and it was a delightful discovery. I was definitely one of today's lucky 10,000.