TTL homebrew CPUs

[Ethan Dicks]

On 7/10/07, Dave McGuire <mcguire at neurotica.com> wrote:
> > Relaxation oscillator?

>    It's an oscillator built around a capacitor and something with a
> breakdown voltage, a neon lamp for example.  The capacitor charges
> slowly through a resistor at a calculable rate, and then (partially)
> discharges through the neon lamp quickly when its ionization
> threshold is reached and it conducts...then the cycle repeats.
> What's neat about it is
> that the blinking light is actually an active part of the oscillator,
> rather than just an indicator that displays the oscillator's state.

When I was a kid, my dad had a blinkenlights box on the bookshelf he
built in college (somewhere between about 1958 and 1964) - it was a
bakelite box, about 6"x9"x3", with a 3x3 grid of neon bulbs sticking
out the front, and some flying-lead-mounted components on the inside,
I presume nine caps and nine resistors, all of varying values, and an
old snap-end radio battery (B battery?)  You snapped a battery in,
then the lights would blink in a random and pleasing fashion.

I doubt he still has the box (I last remember it in the mid-1970s),
but I wouldn't know where you could find a battery these days that
could light a neon bulb - unless you wanted to gang a bunch of 9V
batteries together, or make run a DC-DC converter off of some smaller
value.

Not a complicated circuit, but fascinating to a 7 year old.

-ethan