TTL homebrew CPUs

[Holger Veit]

Cameron Kaiser said:
>> > You're making me feel very old. When I was a kid, I used to make a
>> > neon
>> > bulb blink using one resistor and one capacitor.
>>
>>    Relaxation oscillators are fun.  I like the UJT version as well,
>> though UJTs are getting harder to find than they once were.  I have
>> two or three 2N2646 UJTs left.

A quick look here at a few distributors here lists at least old stock
availability, at a wide price range. Problem is IMHO that such old parts
are not ROHS conforming; thus they are delisted, e.g. at Digikey. Farnell
still offers them, at an obscene price, Reichelt in Germany here has a
fair price.

But UJTs are mainly one-trick ponys which is why they are primarily found
in basic courses in electronics :-) They had their time in the 60s and
70s.

> Relaxation oscillator?
>
> Tony may feel very old, but I'm just feeling very dumb. ;)

Now that is exactly the work principle that Tony has described with his
neon sawtooth oscillator: a resistor charges a capacitor in series. The
load to this voltage divider is the UJT or the neon bulb which have a
characteristics with a negative resistance at some point - once some peak
voltage is reached resistance will drop very fast and will discharge the
cap. Result is the sawtooth wave.

There are other circuits with a comparable characteristic, like the PUT
which is basically a SCR with an anode gate - you can make one with
_parallel_ coupled PNP and NPN transistors. Or with a bit more effort you
can utilize the same effect with a NIC or a gyrator.

-- 
Dr.-Ing. Holger Veit
IT-Management

Fraunhofer IAIS
Institut für Intelligente Analyse- und Informationssysteme
Schloss Birlinghoven
D-53757 Sankt Augustin
e-mail: holger.veit at iais.fraunhofer.de
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Fax. +49 2241 14 2342