Ya gotta start somewhere!
Meet my first computer, a COSMAC ELF, handbuilt by yours truly in (gulp!) 1976, guided by a construction article in Popular Electronics magazine.

I "maxed it out" by decoding all the address and data lines, along with all the internal status signals so I could truly see what was going on inside the CPU while it was running a program.

The card cage -- the yellow/orange thing with the slots -- was scavenged from the Ford People Mover at Fairlane, where I worked at the time.
This is a shot of the magazine's version, which is much simpler than my adaptation. It features only the bare necessities -- 8 toggle switches to enter the program, load, run & input switches, and a hexadecimal display to see the program contents. It contained a massive 256 BYTES of memory - about space enough to store one cbrillow-sized run-on sentence.

Thanks to Bill Richman for letting me use his scan of the photo from the original construction article. Check out his very cool website!!
Here's a closer look at the front panel. The dark rectangular area in the center is an optical filter, behind which was located the hexidecimal display.

The original machine sported one switch and one light emitting diode (LED) as input/output devices. No kidding - the first programming problem was to write a routine which would monitor the switch and turn on the LED if the switch was depressed! (Depresses me just thinking about it!) On my machine, the output LED is in the "box" on the left, and is labeled "Bingo!", kind of a reward for correctly coding the program.

Note the Single Step switch, which is knocked askew on the right side of the panel.
The front of the card shows the CPU -- the long white chip, lower center -- and several other large chips. The two to the right of the CPU are the original memory: two 128-byte RAM chips. The two on the left side are additions from the first "upgrade" -- a 2k static RAM and a 2K eprom, which kept the machine's main program (an upgrade in itself) stored when power was off.

The shiny silverish thing is the crystal which controlled the CPU's clock frequency, which on mine was 4 megahertz. Modern CPUs run at up to about 2 gigahertz -- 500 times faster!

The rest of the chips are for decoding, latching and driving the many LEDs on the front panel.
The back of the card shows the wiring. This is called "wire wrap" technique. Each of the pins on a chip socket has a square post, around which up to 3 connecting wires may be wound. Interestingly, no matter the complexity of the circuit or the number of chips, a well-designed layout should never have more than 2 wires connected at any point!

In wire wrapping, about 1 inch of the wire conductor is exposed, and then wrapped about 10 times around the post to form the connection. I used a manual wire wrapping bit inserted into an electric draftsman's eraser to save a little work.

Yes, you CAN make it look neater than this -- but it's literally not recommended. Running the wires parallel and pretty can lead to noise problems in the circuitry.
Hey -- who stole my 3 key?

This was the first upgrade I put in - a "hex" keypad. Without delving into esoteric (and boring) discussions about computers' number handling capability, suffice to say that they don't deal with decimal numbers the way we do. Most computers like to talk "hex" -- short for hexidecimal -- which is base 16. In hex, there are 16 digits rather than the 10 we're accustomed to in decimal, or base 10. That's what the letter keys are for. O thru 9 are represented just as they are in decimal, but 10 through 15 are represented by A to F. It all sounds very strange, but it's very logically attuned to the way computers work with bits and bytes.

The keypad greatly simplified programming, replacing setting 8 toggle switches representing a single digit with a simple keypress. Ah, progress!
For easy servicing, the wires from the circuit card were routed through a connector pair to the front panel switches and LEDs. Also seen are a couple of switches scrounged from the door open/closed status circuits of the People Mover. I don't remember what the did on my computer -- probably just additional input devices.

The additional slots in the card cage were for future expansion, where I'd hoped to control my whole house (and half of the civilized world) from my computer. It never happened, but I'm still working at the world-domination thing...
Here's the power supply, a basic full-wave rectifier with a humongous filter capacitor (again, People Mover salvage) and a heat-sinked (sunk?) 5 volt IC regulator.

So what did I actually DO with this thing? About the most sophisticated use I ever put it to was to wire a relay to the "Bingo!" output, and connected it to my phone line (at work!!!!) to dial telephone numbers. Pretty ironic, considering my general aversion to talking on the telephone, eh? I abandoned my little sweetie when a newer, sexier, pre-fab single board computer came along --- the MOS Technolgy KIM 1.

The ELF was a great learning tool, allowing me to learn programming at the rawest machine code level, where everything is bits, baby, bits. And, with the reconnection of a few wires that have been knocked off in the intervening years, I'd bet she'd still run!