Repair water damaged Solar Garden Lamp

[Dave_s]

At
http://s1001.photobucket.com/albums...rrent=Water-inside-solar-garden-lamp-CORR.jpg
are pictures of my water damaged Solar Garden Lamps.

Pictures show the damaged Solar Garden Lamps I need to repair. The OEM
long ago discontinued this product making impossible to buy replacement
gaskets and parts for the electronics.

I am trying to repair these units.

I need to buy replacement resistors and diodes.

The resistors are smaller than normal 1/4W parts and the pad lead holes
are too small for 1/4W parts. Same problem with the diode. It looks
like a mini smaller 1N4001 package with much smaller diameter leads
holes in pads.

The transistor is marked STS but I cannot find any info for this part at
STS web site. Is this a FET or bipolar. Need datasheet to find ratings.
Thank you for any help. Dave_s

 

[Martin Brown]

At
http://s1001.photobucket.com/albums...rrent=Water-inside-solar-garden-lamp-CORR.jpg

are pictures of my water damaged Solar Garden Lamps.

Pictures show the damaged Solar Garden Lamps I need to repair. The OEM
long ago discontinued this product making impossible to buy replacement
gaskets and parts for the electronics.

I am trying to repair these units.

I need to buy replacement resistors and diodes.

Why? What makes you think they have failed? Passive components don't
often fail unless they are mechanically damaged or show obvious signs of
overheating. A DMM will confirm if that is the case. Usually these
devices have a couple of small inductors to generate the LED voltage.

The resistors are smaller than normal 1/4W parts and the pad lead holes
are too small for 1/4W parts. Same problem with the diode. It looks
like a mini smaller 1N4001 package with much smaller diameter leads
holes in pads.

The transistor is marked STS but I cannot find any info for this part at
STS web site. Is this a FET or bipolar. Need datasheet to find ratings.
Thank you for any help. Dave_s

Looks to me more like the NiCad has vented caustic electrolyte and a
quick wash and brushup followed by swapping the batteries for a new pair
will fix it (any pair of AA's for quick functional test).

Regards,
Martin Brown

 

[Hammy]

At
http://s1001.photobucket.com/albums...rrent=Water-inside-solar-garden-lamp-CORR.jpg
are pictures of my water damaged Solar Garden Lamps.

Pictures show the damaged Solar Garden Lamps I need to repair. The OEM
long ago discontinued this product making impossible to buy replacement
gaskets and parts for the electronics.

I am trying to repair these units.

I need to buy replacement resistors and diodes.

The resistors are smaller than normal 1/4W parts and the pad lead holes
are too small for 1/4W parts. Same problem with the diode. It looks
like a mini smaller 1N4001 package with much smaller diameter leads
holes in pads.

The transistor is marked STS but I cannot find any info for this part at
STS web site. Is this a FET or bipolar. Need datasheet to find ratings.
Thank you for any help. Dave_s

If after cleaning it up and changing the batteries they still don't
work. You could just buy new ones Home Depot usually has some on sale
every year for a couple of bucks each. I paid five bucks per about 3
years ago and they are still going even after being left out in the
winter. My dogs getting older so I leave them out in the winter so he
can see better.

I have everything I need here to test and fix them but for 5 bucks I
wouldn't waste my time. I would just get new ones unless they just
need replacement batteries. The batteries probably cost almost as much
as the lights.

 

[GregS]

They make 1/8th-watt, 1/10-watt resistors, and even 1/16th and 1/20th-
watt resistors.
All of these are physically smaller than standard 1/4-watt
resistors. Digikey carries them.

Not sure what the diode function is (no schematic), but it's probably
just being used to isolate the NiCad from the solar cell. (?)
If so, I'll bet any diode that fits will (should) work. Maybe a
1N914. They're tiny.

STS is probably the manufacturer. ??
Again, probably just used as a simple switch. So maybe a 2N3904 or
2N3906 would do?
Keep in mind these are all total guesses.

Personally, I would put a voltmeter on the solar cell first and make
sure there's some output in direct sunlight.
Would be a bummer to find the device basically works, but it's not
charging itself.
And as others have stated, I wouldn't "trust" a NiCad in the garden
for too many seasons....

Good luck.

I have had good luck with NiMH cells in the garden, but these types
of lamps need constant attention.

Can't tell from the picture how much damage, but water cleans most green
stuff away, then protect.


greg

 

[Paul E. Schoen]

At
http://s1001.photobucket.com/albums...rrent=Water-inside-solar-garden-lamp-CORR.jpg
are pictures of my water damaged Solar Garden Lamps.

Pictures show the damaged Solar Garden Lamps I need to repair. The OEM
long ago discontinued this product making impossible to buy replacement
gaskets and parts for the electronics.

I am trying to repair these units.

I need to buy replacement resistors and diodes.

The resistors are smaller than normal 1/4W parts and the pad lead holes
are too small for 1/4W parts. Same problem with the diode. It looks
like a mini smaller 1N4001 package with much smaller diameter leads
holes in pads.

The transistor is marked STS but I cannot find any info for this part at
STS web site. Is this a FET or bipolar. Need datasheet to find ratings.
Thank you for any help. Dave_s

I think these lamps usually use a "joule thief" type circuit which can
generate the required 3 VDC or so for the white LED from mostly depleted
NiCd cells down to 2 VDC or less. But that would indicate a little
inductor/transformer, which I don't see in the photos. I made the following
"joule thief" circuit which actually regulates the LED current over a range
of battery voltages, and I have attached the LTSpice schematic. I actually
built the circuit and it drives two large white LEDs in series with as low
as 2 VDC, and it's fairly efficient.

What is also lacking is the charging circuit from the photovoltaic cells to
the battery. It might be just a Schottky diode.

Some components in the attached circuit may not be required, and values are
not critical. If you do a search you may find very simple circuits and even
one or two that use a bunch of turns of wire on a nail for the tapped
inductor. Maybe you can roll your own circuit as a deadbug and then
encapsulate it in electrical grade silicone.

Paul

--------------------------------------------------------------
Version 4
SHEET 1 880 680
WIRE 80 0 -112 0
WIRE 208 0 80 0
WIRE 80 16 80 0
WIRE 208 16 208 0
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WIRE -112 128 -112 0
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WIRE 208 176 208 112
WIRE 80 224 80 208
WIRE 80 224 0 224
WIRE 96 224 80 224
WIRE 144 224 96 224
WIRE 0 240 0 224
WIRE 96 256 96 224
WIRE 192 304 160 304
WIRE 288 304 272 304
WIRE 368 304 352 304
WIRE 496 304 368 304
WIRE 272 320 272 304
WIRE 496 320 496 304
WIRE -112 384 -112 208
WIRE 0 384 0 304
WIRE 0 384 -112 384
WIRE 96 384 96 352
WIRE 96 384 0 384
WIRE 208 384 208 272
WIRE 208 384 96 384
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WIRE 496 384 368 384
WIRE 368 432 368 384
FLAG 368 432 0
SYMBOL npn 144 176 R0
WINDOW 0 33 47 Left 0
SYMATTR InstName Q1
SYMATTR Value 2N2219A
SYMBOL ind2 96 112 R180
WINDOW 0 36 80 Left 0
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SYMATTR InstName L1
SYMATTR Value 800µ
SYMATTR Type ind
SYMBOL ind2 192 0 R0
SYMATTR InstName L2
SYMATTR Value 100µ
SYMATTR Type ind
SYMBOL LED 352 240 R0
WINDOW 3 22 69 Left 0
SYMATTR InstName D1
SYMATTR Value AOT-2015
SYMBOL res 64 112 R0
SYMATTR InstName R1
SYMATTR Value 1k
SYMBOL voltage -112 112 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 3
SYMBOL LED 352 112 R0
SYMATTR InstName D2
SYMATTR Value AOT-2015
SYMBOL schottky 16 304 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D3
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL LED 352 176 R0
SYMATTR InstName D4
SYMATTR Value AOT-2015
SYMBOL res 352 288 R0
SYMATTR InstName R2
SYMATTR Value 30
SYMBOL npn 160 256 M0
WINDOW 0 33 47 Left 0
SYMATTR InstName Q2
SYMATTR Value 2N2219A
SYMBOL schottky 352 288 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName D5
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL cap 256 320 R0
SYMATTR InstName C1
SYMATTR Value 100n
SYMBOL res 288 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 34 46 VTop 0
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL cap 480 320 R0
SYMATTR InstName C2
SYMATTR Value 10µ
TEXT 280 72 Left 0 !K1 L1 L2 1
TEXT -66 360 Left 0 !.tran 10m startup

 

[Martin Brown]

I think these lamps usually use a "joule thief" type circuit which can
generate the required 3 VDC or so for the white LED from mostly depleted
NiCd cells down to 2 VDC or less. But that would indicate a little
inductor/transformer, which I don't see in the photos. I made the following
"joule thief" circuit which actually regulates the LED current over a range
of battery voltages, and I have attached the LTSpice schematic. I actually
built the circuit and it drives two large white LEDs in series with as low
as 2 VDC, and it's fairly efficient.

That is certainly the most common over here. You should not assume the
batteries are wired in series though - some of mine are in parallel. I
was quite surprised when it started working with only one cell replaced.
Adding the second cell in parallel made it brighter. I guess this allows
them to save on components with a corresponding loss of efficiency.

In the UK the very long summer twilight makes it worth adjusting the
switch on point to much darker than the default manufacturers setting
which assumes a latitude of 30 degrees or so.

In winter they die a horrible death outside here alternately freezing
and being totally discharged for days on end. The batteries do not like
this treatment at all. The ones installed on roadside safety signs like
"beware bend" always fail in the worst icy foggy weather.

Regards,
Martin Brown

 

[Sylvia Else]

In winter they die a horrible death outside here alternately freezing
and being totally discharged for days on end. The batteries do not like
this treatment at all. The ones installed on roadside safety signs like
"beware bend" always fail in the worst icy foggy weather.

Sounds like they'd be causing more accidents than they prevent if people
end up relying on them and they don't work when most needed.

Sylvia.

 

[Dave_s]

Paul and others,

http://s1001.photobucket.com/albums...rrent=Water-inside-solar-garden-lamp-CORR.jpg

My Solar Lamp has no inductors that suggest a "Joule Thief" circuit is
used. My LED is orange, not white.

I tried to use run your netlist with my LTSPICE 4. Only get error
message "multiple instances of SYMATTRIB".

I copied your netlist into NOTEPAD and saved as SOLAR_JOULE.CIR.
Can youi suggest what else I can do to run your LTSPICE JOULE_THEIF FILE"?

The pcboard looks full of water damage salts. After cleaning with
SIMPLE_GREEN and ALCOHOL, the repaired board does not make the LED turn on.
Is the base bias resistors, 15K and 100K setting the threshold for LED
turn-on at sunrise?

My schematic is suspicious since there is no current limiting resistor
between LED and SOLAR CELL VOLTAGE. They cannot possibly be running the
LED directly off the battery without current limiting?



Thanks for all your help. Dave_S

 

[Martin Brown]

Paul and others,

http://s1001.photobucket.com/albums...rrent=Water-inside-solar-garden-lamp-CORR.jpg


My Solar Lamp has no inductors that suggest a "Joule Thief" circuit
is used. My LED is orange, not white.

I tried to use run your netlist with my LTSPICE 4. Only get error
message "multiple instances of SYMATTRIB".

I copied your netlist into NOTEPAD and saved as SOLAR_JOULE.CIR.
Can youi suggest what else I can do to run your LTSPICE JOULE_THEIF FILE"?

The pcboard looks full of water damage salts. After cleaning with
SIMPLE_GREEN and ALCOHOL, the repaired board does not make the LED turn on.
Is the base bias resistors, 15K and 100K setting the threshold for
LED turn-on at sunrise?

My schematic is suspicious since there is no current limiting
resistor between LED and SOLAR CELL VOLTAGE. They cannot possibly be
running the LED directly off the battery without current limiting?

There must be a transistor though and it will therefore be current
limited by the base input current x gain. Have you traced the circuit
board to a diagram and checked the component values and track continuity
with a DVM ?

Unless you are really fond of it for sentimental reasons your best bet
is to buy a new one or make a joule thief variant from scratch to fit in
the same physical space.

Designs using a pair of NiCads will barely light an orange led in series
with a transistor - are you sure there isn't a small inductor hiding
somewhere? It might look to the untrained eye like a resistor with
unusual bands on.

Regards,
Martin Brown

 

[Hammy]

Paul and others,

http://s1001.photobucket.com/albums...rrent=Water-inside-solar-garden-lamp-CORR.jpg

My Solar Lamp has no inductors that suggest a "Joule Thief" circuit is
used. My LED is orange, not white.

I tried to use run your netlist with my LTSPICE 4. Only get error
message "multiple instances of SYMATTRIB".

I copied your netlist into NOTEPAD and saved as SOLAR_JOULE.CIR.
Can youi suggest what else I can do to run your LTSPICE JOULE_THEIF FILE"?

The pcboard looks full of water damage salts. After cleaning with
SIMPLE_GREEN and ALCOHOL, the repaired board does not make the LED turn on.
Is the base bias resistors, 15K and 100K setting the threshold for LED
turn-on at sunrise?

My schematic is suspicious since there is no current limiting resistor
between LED and SOLAR CELL VOLTAGE. They cannot possibly be running the
LED directly off the battery without current limiting?



Thanks for all your help. Dave_S

If you still haven't found the datasheet for your transistors this may
be it if I read the numbers of your picture correctly.


http://www.datasheetcatalog.org/datasheet/auk/STS8050.pdf

 

[Paul E. Schoen]

Paul and others,

http://s1001.photobucket.com/albums...rrent=Water-inside-solar-garden-lamp-CORR.jpg

My Solar Lamp has no inductors that suggest a "Joule Thief" circuit is
used. My LED is orange, not white.

I tried to use run your netlist with my LTSPICE 4. Only get error message
"multiple instances of SYMATTRIB".

I copied your netlist into NOTEPAD and saved as SOLAR_JOULE.CIR.
Can youi suggest what else I can do to run your LTSPICE JOULE_THEIF
FILE"?

It is not a netlist (.cir or .net). It includes graphical information for
the schematic and should be saved as ASC. The inductor I used is a Cooper
DRQ74-101 which is a 10/40 uH coupled inductor in a 7.5mm SMT package. I
have attached a simpler version which delivers about 20 mA into a 6V LED
load with battery voltage from 2V to 5V, and 10mA with 1.5V.

The pcboard looks full of water damage salts. After cleaning with
SIMPLE_GREEN and ALCOHOL, the repaired board does not make the LED turn
on.
Is the base bias resistors, 15K and 100K setting the threshold for LED
turn-on at sunrise?

I did not see your schematic previously. It does not seem quite right. The
first transistor seems to be used to turn off the second transistor when
the solar cell is producing current. The large diode is usually used to
keep the battery from backfeeding into the photocell when it is dark. It is
possible that a second photocell could be used just to detect light, but
unless it is just a tiny device it does not seem correct. The 15k resistor
should be connected to the photocell, but the 3k resistor and LED should be
connected to the battery. The photocells might be connected in series to
give enough voltage.

My schematic is suspicious since there is no current limiting resistor
between LED and SOLAR CELL VOLTAGE. They cannot possibly be running the
LED directly off the battery without current limiting?

The battery is probably connected through a current limiting device to the
LEDs and the photocell. It also needs to be configured so that the
photocell charges the battery but disconnects the LED load when it is
providing current, and then connect the LEDs to the battery when it is
dark.

Here is a link to two circuits for solar powered garden lights:
http://talkingelectronics.com/projects/SolarLight/SolarLight.html

And here are more circuits, ranging from simplest possible to one that uses
a microcontroller:
http://www.evilmadscientist.com/article.php/solar

Paul

====================== JouleThief1.ASC ==========================

Version 4
SHEET 1 880 680
WIRE 80 0 -112 0
WIRE 208 0 80 0
WIRE 80 16 80 0
WIRE 208 16 208 0
WIRE 80 112 80 96
WIRE 208 112 208 96
WIRE 256 112 208 112
WIRE 368 112 320 112
WIRE 560 112 368 112
WIRE -112 128 -112 0
WIRE 368 128 368 112
WIRE 208 176 208 112
WIRE 560 192 560 112
WIRE 368 208 368 192
WIRE 80 224 80 192
WIRE 144 224 80 224
WIRE 80 256 80 224
WIRE 240 304 144 304
WIRE 368 304 368 272
WIRE 368 304 320 304
WIRE 400 304 368 304
WIRE 496 304 400 304
WIRE 496 320 496 304
WIRE -112 384 -112 208
WIRE 80 384 80 352
WIRE 80 384 -112 384
WIRE 208 384 208 272
WIRE 208 384 80 384
WIRE 400 384 208 384
WIRE 496 384 400 384
WIRE 560 384 560 256
WIRE 560 384 496 384
WIRE 560 416 560 384
FLAG 560 416 0
SYMBOL npn 144 176 R0
WINDOW 0 33 47 Left 0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL ind2 96 112 R180
WINDOW 0 36 80 Left 0
WINDOW 3 36 40 Left 0
SYMATTR InstName L1
SYMATTR Value 20µ
SYMATTR Type ind
SYMBOL ind2 192 0 R0
SYMATTR InstName L2
SYMATTR Value 20µ
SYMATTR Type ind
SYMBOL LED 352 208 R0
WINDOW 3 76 25 Left 0
SYMATTR Value AOT-2015
SYMATTR InstName D1
SYMBOL res 64 96 R0
SYMATTR InstName R1
SYMATTR Value 4.99k
SYMBOL voltage -112 112 R0
WINDOW 123 0 0 Left 0
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SYMATTR InstName V1
SYMATTR Value 2
SYMBOL LED 352 128 R0
WINDOW 3 74 35 Left 0
SYMATTR InstName D2
SYMATTR Value AOT-2015
SYMBOL res 384 288 R0
SYMATTR InstName R2
SYMATTR Value 27
SYMBOL npn 144 256 M0
WINDOW 0 33 47 Left 0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res 336 288 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 34 46 VTop 0
SYMATTR InstName R3
SYMATTR Value 499
SYMBOL cap 480 320 R0
WINDOW 3 31 47 Left 0
SYMATTR InstName C2
SYMATTR Value 10µ
SYMBOL schottky 256 128 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D3
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL cap 544 192 R0
SYMATTR InstName C1
SYMATTR Value 220n
TEXT 272 40 Left 0 !K1 L1 L2 1
TEXT -96 408 Left 0 !.tran 2m startup

 

[Dave_s]

Paul,

Ok, your LTSPICE circuit runs and simulates after renaming your
circuit from "*.cir" to "*.asc".

I followed some of your links and will follow them all later.

Your mailnote was 'terrific', loaded with much useful info.

My schematic does look possibly wrong and incomplete and I will more
carefully recheck and correct.

I just read that it may be common to operate the "LED without a current
limiting resistor" due to solar cell voltages being small.


My lamp circuit certainly seems different than ANY of the "Joule Thief"
style circuits shown. No where on the visible exposed pcboard are any
inductors, transformers or capacitors.
Makes unlikely that my Lamps use a "Joule Thief" circuit unless there
is another circuit board inside the lamp in the chamber with the solar
cells. Frozen Phillips head screws presently prevent me from looking at
the exact wiring of the solar cells and NiCd cell wiring and looking for
a 2nd circuit board.

I have much work to do and will advise results later.
Hope to provice an accurate schematic of my Malibou Solar Lamps.

Many thanks, Dave_s

 

[Paul E. Schoen]

Paul,

Ok, your LTSPICE circuit runs and simulates after renaming your circuit
from "*.cir" to "*.asc".

I followed some of your links and will follow them all later.

Your mailnote was 'terrific', loaded with much useful info.

My schematic does look possibly wrong and incomplete and I will more
carefully recheck and correct.

I just read that it may be common to operate the "LED without a current
limiting resistor" due to solar cell voltages being small.


My lamp circuit certainly seems different than ANY of the "Joule Thief"
style circuits shown. No where on the visible exposed pcboard are any
inductors, transformers or capacitors.
Makes unlikely that my Lamps use a "Joule Thief" circuit unless there is
another circuit board inside the lamp in the chamber with the solar
cells. Frozen Phillips head screws presently prevent me from looking at
the exact wiring of the solar cells and NiCd cell wiring and looking for
a 2nd circuit board.

I have much work to do and will advise results later.
Hope to provice an accurate schematic of my Malibou Solar Lamps.

Many thanks, Dave_s

I think some of your resistor values are also wrong.

I made an LTSpice circuit that is similar to what you have. I changed some
resistor values to match the components I selected, and to make it work as
it should. I used 100 ohm internal resistance for the photocell, and 2.4
VDC for the NiCad battery. I used 2N3904 NPN transistors, and a 30k base
drive which current limits the LED to about 15 mA. This corresponds to a
Beta of 285, which is about right, but it can vary from 100 to 300, so the
base resistor or the transistor may need to be selected. Or you can add an
emitter resistor and a couple other components to make a more accurate
current limited source.

Try the ASC file and see how it turns off the LED when the photocell comes
up to about 2.2V.

Paul

======================= SolarLight.ASC ============================

Version 4
SHEET 1 880 680
WIRE 400 32 48 32
WIRE 192 96 -48 96
WIRE 272 96 192 96
WIRE 304 96 272 96
WIRE 400 96 400 32
WIRE 400 96 368 96
WIRE 192 112 192 96
WIRE 48 128 48 32
WIRE 272 160 272 96
WIRE 400 160 400 96
WIRE -48 192 -48 96
WIRE 192 224 192 192
WIRE 192 224 144 224
WIRE 272 256 272 224
WIRE 48 272 48 208
WIRE 80 272 48 272
WIRE 48 288 48 272
WIRE 192 304 192 224
WIRE 208 304 192 304
WIRE -48 384 -48 272
WIRE 48 384 48 368
WIRE 48 384 -48 384
WIRE 144 384 144 320
WIRE 144 384 48 384
WIRE 272 384 272 352
WIRE 272 384 144 384
WIRE 400 384 400 224
WIRE 400 384 272 384
WIRE 400 400 400 384
FLAG 400 400 0
SYMBOL Misc\\battery -48 176 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 2.4
SYMBOL Misc\\cell 400 160 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 84 Left 0
SYMATTR InstName V2
SYMATTR Value PWL(0 0 .1 4)
SYMATTR SpiceLine Rser=100
SYMBOL schottky 368 80 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName D1
SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL LED 256 160 R0
SYMATTR InstName D2
SYMATTR Value QTLP690C
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL res 176 96 R0
SYMATTR InstName R1
SYMATTR Value 30k
SYMBOL npn 208 256 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn 80 224 R0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res 32 272 R0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL res 32 112 R0
SYMATTR InstName R3
SYMATTR Value 30k
TEXT -82 394 Left 0 !.tran .1 startup

 

[Paul E. Schoen]

I made a regulated version which provides no more than 20 mA to the LED at
Vbatt=3.0, 15mA at Vbatt=2.2V, and works down to Vbatt=1.8V at which point
the LED current drops to about 1.6mA. But the forward drop on the LED model
is about 1.9V at 20mA so it is the limiting factor and the circuit is
essentially giving it full battery voltage. It fades away to almost nothing
at 1.6V which is dead battery anyway.

This circuit uses the zener property of the LED itself to perform current
regulation. There was a discussion some time ago about current regulators
and this design was presented.

Paul

=========================== SolarLightCurrReg1.ASC
============================

Version 4
SHEET 1 880 680
WIRE 544 32 -80 32
WIRE 128 96 -176 96
WIRE 384 96 128 96
WIRE 448 96 384 96
WIRE 544 96 544 32
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WIRE 544 160 544 96
WIRE -80 176 -80 32
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WIRE 384 240 384 208
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SYMATTR InstName V1
SYMATTR Value 2.2
SYMBOL Misc\\cell 544 160 R0
WINDOW 123 0 0 Left 0
WINDOW 39 24 84 Left 0
SYMATTR SpiceLine Rser=100
SYMATTR InstName V2
SYMATTR Value PWL(0 0 .1 4)
SYMBOL schottky 512 80 R90
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SYMATTR Description Diode
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SYMATTR Value 20k
SYMBOL res -96 160 R0
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SYMATTR Value 20k
SYMBOL pnp 336 256 R180
WINDOW 0 41 29 Left 0
WINDOW 3 34 55 Left 0
SYMATTR InstName Q3
SYMATTR Value 2N3906
SYMBOL res 112 368 R0
WINDOW 0 33 25 Left 0
WINDOW 3 27 85 Left 0
SYMATTR InstName R4
SYMATTR Value 2k
SYMBOL res 432 240 R0
WINDOW 0 33 25 Left 0
WINDOW 3 27 85 Left 0
SYMATTR InstName R6
SYMATTR Value 100k
SYMBOL res 368 368 R0
WINDOW 0 33 25 Left 0
WINDOW 3 27 85 Left 0
SYMATTR InstName R7
SYMATTR Value 10
SYMBOL diode 240 320 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName D3
SYMATTR Value 1N4148
TEXT -344 320 Left 0 !.tran .1 startup