> Am looking for a one transistor LED flasher circuit. Does such an animal
> exsist?
>
well, here is a relaxation oscillator, the frequency is
probably too fast but you could fiddle with the LC oscillator
section and see if you can get it to go.....

alice

Author: Bryan Hord


>From this explanation it seems like the circuit should work
with a higher
voltage LED. Has anyone tried to power a blue LED from a
single cell?

snip%<-----------------------------------------------------
>>> On 2/2/98 Pasi T Mustalahti wrote:
>>>
>>>       -------8<--------
>>> >
>>> >
>        ------------------------------------- +U (0.6..1.55
V)
>        |                    |
>        R20K              || <
>        |                 || <
>        ----------------> || <
>        |               > || <       ^^ <---the led
>        = 10nF          |    /------>|--|
>        |               ----| BC337     |
>        |                    \          |
>        |                    |          |
>        --------------------------------------
>>> >
>
snip %< ----------------------------------------------------
>The circuit is best described as a self-oscillating flyback
converter.
>Each time the transistor turns on, it charges the
transformer with
>current until it saturates. When it saturates, the voltage
induced in
>the base winding decreases, causing the transistor to turn
off. The
>energy stored in the transformer is then dumped to the load
(LED in this
>case).
>
>For a given frequency of operation and transformer core, the
circuit will
>deliver approximately constant *power* to the load,
regardless of the
>load voltage. The power is distributed in pulses having the
energy that
>the transformer core can hold before it saturates. Since it
always
>charges until it saturates, the output power doesn't depend
on the input
>voltage, if (and this is a big "if"), the frequency doesn't
vary.
>
>If the DC supplied to the transistor base circuit is
adequate, the
>circuit will oscillate continuously at the highes practical
frequency
>(depends on the input voltage and the inductance of the
winding). More
>likely, Pasi's circuit is running in "relaxation mode". The
base current
>required by each cycle discharges the capacitor somewhat, to
less than
>Vbe so the transistor doesn't turn on right away after the
transformer
>voltage reaches zero (all energy having been delivered to
the LED).
>There is a delay during which the resistor has to charge up
the capacitor
>to start the transistor conducting again. So the frequency
probably
>decreases significantly with input voltage as the current
available thru
>the resistor decreases. Varying the resistor should vary the
frequency,
>and thus the brightness of the LED. A diode in parallel with
the
>transistor base (to keep it from going too far negative)
would supply
>current to the capacitor while the transistor is off, and
probably get
>the circuit to oscillate continuously rather than in
relaxation mode.
>However, the resulting output power would likely be too
high, and
>difficult to control.
>
>However, the experiment shows that it had satisfactory
performance over
>the life of the battery. The self-regulating properties of
the circuit
>are apparently working well enough.
>

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