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PICList Thread
'[OT]: LED with 1.5 V'
1998\02\02@025442 by Pasi T Mustalahti

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I was looking for a simple way to use a led with a NiCd cell.
LED needs about 1.45..1.7 V to give light. A NiCd cell gives 1.2..1.4 V.

I got some interesting ideas from here and we continued with them.
This is the last stage:


       ------------------------------------- +U (0.6..1.55 V)
       |                    |
       R20K              || <
       |                 || <
       ----------------> || <
       |               > || <
       = 10nF          |    /------>|--|
       |               ----| BC337     |
       |                    \          |
       |                    |          |
       --------------------------------------

The transformer is made of a ferrite toroid, size was about 4x3x2 mm (I
found it from my pocket). I made there about 10 + 50 r (even 10 + 10
worked fine). Capacitor was firs 1uF, but it was too large in size and we
used 10 nF in the last proto. R10K made a High Brigh LED run a bit too
bright to be reliable, so we changed there 20K.
It goes fine on a 3x5 hole Vero board.
It took about 30 10 minutes to make and costs about as much as a small cup
of coffee.
I'm going to use it as a flashlight at sea, when I want to see the map.
A high brigh LED gives enough light to read a book.
I needs 4 mA @ 0.6..1.0 V, 20 mA @ 1.4 V.
Our proto goes fine with 0.56 V !!
Do not use more +U than 1.5 V, because the this cannot regulate the
current. (and I don't want to add the series resistor)
If you don't get it running, change the lines going to the transformer. I
ALLWAYS make it wrong first time :)

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1998\02\02@035848 by John Payson

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> I'm going to use it as a flashlight at sea, when I want to see the map.
> A high brigh LED gives enough light to read a book.

One thing to note, by the way: even yellow LED's are monochromatic; yellow
is not the same thing as red plus green.  In many cases, LED's work fine
for illumination, but in many other cases the monochromatic light from an
LED turns out to be just the WRONG color.  I personally would suggest
either using a white LED or else a bi-color LED (use both elements).  Of
course, for the latter the circuitry would be a little more complex (to
make sure both elements light nicely) but the color will probably be more
useful.

1998\02\02@100118 by Bob Fehrenbach

picon face
John Payson <supercatspamKILLspamMCS.NET> wrote:
>> I'm going to use it as a flashlight at sea, when I want to see the map.
>> A high brigh LED gives enough light to read a book.
>
>I personally would suggest either using a white LED or else
>a bi-color LED (use both elements).

Considering the context of the application, I believe that red is the
color of choice because of its minimum effect on night vision, a
factor at sea at night.

--
Bob Fehrenbach    Wauwatosa, WI     .....bfehrenbKILLspamspam.....execpc.com

1998\02\05@165958 by Robert Nansel

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face
On 2/2/98 Pasi T Mustalahti <EraseMEptmustaspam_OUTspamTakeThisOuTUTU.FI> wrote:

-------8<--------
>
>        ------------------------------------- +U (0.6..1.55 V)
>        |                    |
>        R20K              || <
>        |                 || <
>        ----------------> || <
>        |               > || <
>        = 10nF          |    /------>|--|
>        |               ----| BC337     |
>        |                    \          |
>        |                    |          |
>        --------------------------------------
>
>The transformer is made of a ferrite toroid, size was about 4x3x2 mm (I
>found it from my pocket). I made there about 10 + 50 r (even 10 + 10
>worked fine).
-------8<--------

How many turns did you use on the transformer? What gage wire did you use?
And those dimensions: is that a 4 mm outside diameter, 3 mm hole, and 2 mm
thickness?

--RLN

1998\02\06@080503 by Pasi T Mustalahti

picon face
On Thu, 5 Feb 1998, Robert Nansel wrote:

{Quote hidden}

PTM: Really, I dont know. I found two small toroids from my pocket.
Another was black and another was violet. Black was about the size of a
asperin tablet and another about 10% bigger. Then I just walked my hands
spread and found some wire, about .1 mm diameter and 2 meters long. While
I was still walking I wound this wire to these toroids without thinking
anything. The transistor I had on my own taple among many others.

It looks like there is about 10 turns + 50 turns on the violet and 20 + 20
turns on the black toroid. My frien sain that 1:10 and hard ferrite should
be fine, but the device seems to work with what it gets.

I connected it to a AA-cell with 1.452 V in it at about 18:00 last friday
and left it there. This morning there was 0.73 V and the LED was just as
bright as in the begining. So it has lasted nov 7 days (minus 4 hours) and
still going strong.

The LED is a 3500 mcd device and you can read A4 in its light.
It costs about nothing, so you can make some tries. I'm interested if you
find some optimisation to this. Or a SIMPLE method to make a current
limiter without spending current.
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1998\02\06@105935 by Robert McAtee

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Gee that's a hard way to get a led to work off of 1.5 volts. I've got leds
blinking for over a year using LM3909's a 100uf cap. amd a 1.5 volt C cell.
Then again maybe you want a steady light???  ==Mac==
============================================================================
===
{Quote hidden}

1998\02\06@135123 by Mike Keitz

picon face
On Fri, 6 Feb 1998 13:47:24 +0200 Pasi T Mustalahti <spamBeGoneptmustaspamBeGonespamUTU.FI>
writes:
{Quote hidden}

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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1998\02\07@073716 by paulb

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Robert McAtee wrote:

> Gee that's a hard way to get a led to work off of 1.5 volts. I've got
> leds blinking for over a year using LM3909's a 100uf cap. amd a 1.5
> volt C cell.   Then again maybe you want a steady light???  ==Mac==

 Two things.  1) Yes, I think an apparently steady light was what was
desired.  You can do that with the 3909 by using a small cap, but then
it's rather dim.  Using shunt resistor(s) in its bias line will increase
the frequency also, and increase brightness, however 2) The 3909 is
actually not as efficient as the toroid circuit.  And it won't work to
as low a voltage.

 Another reference: Whether the inductor circuit operates in relaxation
or continuous mode depends on the turns ratio of the transformer.  As
well as other things.

 Cheers,
       Paul B.

1998\02\09@035449 by Pasi T Mustalahti

picon face
On Thu, 5 Feb 1998, Robert McAtee wrote:

> Gee that's a hard way to get a led to work off of 1.5 volts. I've got leds
> blinking for over a year using LM3909's a 100uf cap. amd a 1.5 volt C cell.
> Then again maybe you want a steady light???  ==Mac==

PTM: I didn't have a LM3909 in an easy reach.
I wanted to have as much steady light out of a HiBright Led as possible.
This uses about all the juice in a cell: this morning, after 9 days  of
light there was 0.599 V in the cell and still the LED was rather bright.
0.599 is very near what we normally need between base and emitter of a
trasistor, so now it goes interesting...

{Quote hidden}

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1998\02\18@163359 by Octavio Nogueira

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>
>        ------------------------------------- +U (0.6..1.55 V)
>        |                    |
>        R20K              || <
>        |                 || <
>        ----------------> || <
>        |               > || <
>        = 10nF          |    /------>|--|
>        |               ----| BC337     |
>        |                    \          |
>        |                    |          |
>        --------------------------------------
>
>The transformer is made of a ferrite toroid, size was about 4x3x2 mm (I
>found it from my pocket). I made there about 10 + 50 r (even 10 + 10
>worked fine). Capacitor was firs 1uF, but it was too large in size and we
>used 10 nF in the last proto. R10K made a High Brigh LED run a bit too
>bright to be reliable, so we changed there 20K.


Ok, I used this circuit to make 22V from 5V, what I need now is -22V
from 5V. Is there any way this circuit could be modified to do that?

Cheers,

Octavio
======================================================
Octavio Nogueira  - e-mail:   RemoveMEnogueiraEraseMEspamEraseMEmandic.com.br
http://www.geocities.com/~oct_nogueira
"ProPic" Production PIC Programmer Windows under US$20
======================================================

1998\02\18@183554 by Mike Keitz

picon face
On Wed, 18 Feb 1998 18:40:36 -0300 Octavio Nogueira
<RemoveMEnogueiraspam_OUTspamKILLspamMANDIC.COM.BR> writes:

>Ok, I used this circuit to make 22V from 5V, what I need now is -22V
>from 5V. Is there any way this circuit could be modified to do that?

It is simple.  Turn the circuit "upside down," using a PNP transistor.
The collector of the transistor will now fly to a negative voltage.  The
inverting version does need to work a little harder since the input
voltage is not added to the output voltage.

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1998\02\19@050148 by Russell McMahon

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There are several ways but the "simplest" cct wise is -

Use a PNP transistor.
Feed Ground to where U+ is now and U+ (0.6-1.5v) to where
ground is now.
Reverse the polarity of the output diode.
Voila (I hope).

This is out of my head but looks OK - try it and tell us
what happens :-).



{Original Message removed}

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