Those solar-powered lawn lights

[Mark Hammer]

A number of stores that send us flyers in the daily newspaper were advertising solar lawn lights for a dollar this long holiday weekend, so I thought I'd pick up a couple and see if there was anything interesting inside them.  These are the units that have a solar cell on the top, and a white LED popinting at the ground.  They're about the size of a turkey baster, and you stick them into the ground.

Initially, I thought they might just be a source of relatively inexpensive solar cells (though I'm sure someone here will direct me to cells that are cheaper than $1).  BUt they have turned out to be more interesting.

Measuring the voltage available from the solar cell, it appears to put out around 1.85vdc in household fluorescent light; I imagine more in broad daylight. A couple of these in series could make a decent solar battery charger for your wireless mouse or whatever.  The LED is the 3mm type but appears to be even brighter than the superbright 5mm ones I got from Tayda.  The circuitry is rather simple, or rather the complexity is well-hidden in one of those epoxy blobs-on-a-board that presumably contains any logic for turning the LED on when it gets dark and turning it off when it's light out.

What I found most interesting, though, was that this little cheapo contained a 1.2V Ni-Cad battery that is approximately half the size of an AA and is rated at 150mah.  That is, if you stick two of them end to end, they take up just a little more space than a penlight (assuming the spring contact can absorb the minimally extra length)...except that a pair provides 2.4v instead of 1.5.  That means a 4AA-pack would be able to supply 9.6v of rechargeable powe, and a quartet of these batteries, taking up the space of 2AA could conceivably power something like an electret mic or a simpler onboard FET buffer, for quite a while before needing recharging.

I'm sure others could think up more creative uses for them.  I was personally unfamiliar with this size/format of battery, so I thought I'd let you know.

[deadastronaut]

hi mark,

one of my other hobbies is messing with those solar lights, solar dancing flower mechanisms, joule thiefs..

great little circuits.....the best little solar panels by far are the ones from danciing flowers and neko cats...3volts..put them in series etc..

funny you should mention those black blobs...today i burnt the goop and scrapped off carefully, but found nothing under it, just tiny pcb traces...strange.

but from what i can tell it is a joule thief circuit, i have a joule thief on breadboard at the moment, 1.5 AA battery running 3x 3v RGB colour changing leds..

i have had lots of leds running off of the 1.5v AA battery....i think the max i had was 30 or so....even on a dead battery for days.....its an amazing circuit

http://www.aronnelson.com/gallery/main.php/v/chickpea/joule+thief1.jpg.html
http://www.aronnelson.com/gallery/main.php/v/chickpea/joule1.jpg.html
http://www.aronnelson.com/gallery/main.php/v/chickpea/P200112_13_20.jpg.html

i decided to make the joule thiefs into lamps with smaller toroids.
http://www.youtube.com/watch?v=kPCBwD7l9u0

i'm working on a solar charging version with a couple of panels in series.and as many leds as possible

my latest discovery which is kinda cool are solar bug lights...i have 3 of them, (also joule thiefs) you stick them to your window, they charge, then at night they light up indoors (colour changing.)
and only  £1.00 each...ive taken those apart and the RGB leds are really bright...some have ldrs in, some don't..i think its built within the panel.

solar bugs..

http://www.youtube.com/watch?v=9x0cFYt1TEg
http://www.youtube.com/watch?v=XYIHyi6k_qA

with the cover off..
http://www.youtube.com/watch?v=Ml8B8SaokSM

great fun. and free energy too. what's not to like.

try out the joule thief..amazing little circuit.

[PRR]

> around 1.85vdc in household fluorescent light; I imagine more in broad daylight.

The volt/light curve is log. 10% of real light gets you 90% of max voltage, or nearly.

It's right-near 0.6V per cell, so this is probably a 3-string.

The *current* is proportional to light. Bright fluorescent can't deliver 1/10th the current of the mid-day sun. (Run a junction into a virtual-ground amplifier, the output voltage IS the light intensity; with diode feedback it is near log(light) which is more useful for the 1:10,000 range of common light intensities. Photo-Meter.)

You are in Canada? As driveway lights, in Maine, I find they don't work well most of the winter. Days too short, nights too long.

Half-C cells are an old thing in NiCads, half-AA is not a huge shock. (My big number-light takes full AAs, I have not cracked the turkey-baster jobs yet.)

The "circuitry" could be quite simple, tho tenth-Volts matter. The die may really be too small to find by eye.

Yes, 9.6V 150mAH for four bucks is interesting.

[duck_arse]

I traced the circuit of one of those solar lights, and it had a Ge transistor in. improved efficencies, or something. I can't find the diagram now, of course.

[PRR]

The one (of 3) basters that still works has just one full-AAA battery.

When it dies I'll autopsy and find out how it runs a 4V LED with a 1V battery.

[Gus]

Here is a link for led lighting
http://www.diodes.com/applications/led_lighting.html

[PRR]

Thanks, Gus.