In article ,
The Natural Philosopher writes:

Hoew about a unijunction with the LED in its collector?

It would be hard (maybe impossible) to get the on-off ratio
50%. Also, the with the output being saw-tooth rather than
square wave in a conventional unijunction oscillator, the
LED would fade off each time. BTW, there's no 'collector'
in a unijunction transistor -- I presume you mean the drain?
(I think it's about 30 years since I last used a UJT, and
that was to generate a timebase for an oscilloscope I was
building from an old telly;-)

--
Andrew Gabriel