TTL homebrew CPUs
holger.veit at iais.fraunhofer.de
Tue Jul 10 13:45:11 CDT 2007
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
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
> 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
Institut für Intelligente Analyse- und Informationssysteme
D-53757 Sankt Augustin
e-mail: holger.veit at iais.fraunhofer.de
Tel. +49 2241 14 2448
Fax. +49 2241 14 2342
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