If you still want a single transistor, your best bet is still the UJT
oscillator, or a phase shift oscillator.  Hartley, Colpitts, and Clapp
all use 1 transistor, but only work great above 1MHz.

The UJT oscillator produces a triangular waveform, but probably won't
give you a desirable duty cycle.  The phase shift oscillator should
produce a sine wave... more suitable for LED drive... and you can use
it with your 2N3904.
                        ______vcc
                       |
_______________________D
|                      |
|                 vcc  |           Something like this.......
|                  R   /
--C----C----C----C----|T           C-cap, R-res, T-NPN, D-LED
     R    R    R   R   \
     |    |    |   |   RS          RS-resistor for LED I limit
     ------------------------gnd


Your calcs should read something like:

T = RC x LN((vcc-0.6)/vcc)
F = 1/T

Just an estimate on the formula....  I could be way out to lunch.

Craig



> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Barry King
> Sent: Wednesday, September 13, 2000 1:39 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: not [OT]: One transistor LED flasher?
>
>
> > Hard to build a one transistor oscillator. Most are RF type
> > osc at high frequency. To get anything like a led flash
> > frequency you will need a massive inductor, we are talking
> > power transformer size here! Are you sure you want to do this?
>
> Uh, search for "unijunction transistor" and or "relaxation
> oscillator".  Should be easy with a unijunction, if you can still get
> them.  Mine (in a toy at home) has a 1972 date code...
>
> Any volunteers to show off a working circuit with one FET?  Extra
> credit for one BJT, especially if its a garden variety, say 2N3906 or
> 2N3904.  Ladies and Gentlemen, start your soldering irons!  :)
>
> Regards,
>
> Barry.
> ------------
> Barry King, Engineering Manager
> NRG Systems "Measuring the Wind's Energy"
> http://www.nrgsystems.com
> Phone: 802-482-2255
> FAX:   802-482-2272
>
> --
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