I came across this in a “99 cent” discount store, for a dollar. The UPC code is 39277 75124. They also had the standard garden lights for $2, with the plastic stake to poke into the ground. I bought one of those and opened it up, and found that they had gone cheap, and used a AAA cell, instead of the earlier ones with an AA cell.
With this garden rock, I decided that for a dollar, I could get a solar photovoltaic cell, a white LED with a reflector and a battery, so it was worth the investment. I figured that it would have a AAA cell inside just like the $2 garden lights. The thought never occurred to me that the case was so small that the AAA cell might not fit inside (but I did managed to squeeze one in diagonally).
Everything was underneath the solar cell, which was hot glued into the case. I pried it off gently with a small screwdriver so as not to chip or break the solar cell. The circuit board isn’t worth anything to me; the black blob on it covers all the silicon devices, and the 470 uH choke is a poor choice for a Joule Thief due to its high resistance and high losses.
But the rechargeable cell is really a big disappointment. I was expecting something bigger than a little green button cell. I don’t see how these could run for more than an hour or so after the sun sets. My guess is that the circuit was designed for lower current drain on the button cell, and much lower LED light output, of course – that’s the only way they can make the button cell last for more than an hour or so. If so, then replacing the button cell with a larger AAA cell will not increase the light output, and there’s no way to modify the circuit for more light output. The label says it’s a 1.2V AG13 rechargeable cell, and it’s not replaceable. It says 40 MaH, and charge is 4 mA for 14 hours.
The circuit shuts off the LED when there’s light on t he solar cell, and it’s quite sensitive. Even the light inside the house is enough to shut it off. And this light is too low to recharge the battery. This leads me to conclude that if the experimenter wants to experiment, the circuit needs to be replaced.
Should I go back and buy a few more? I really like the reflector, which costs two dollars if I order them from online. That’s really expensive considering the companies that make these rocks are paying only a few cents for them. So I think it’s worth buying them just for the reflector, if I can get it out without damaging it. The solar cell is worth a few dollars. The switch can be reused. But the button cell, well, if I could get three of them and put them in series, and charge all three with a 4V solar cell, then they might be able to run a single LED. I think what might be a solution is to put a new board inside, and use a bigger battery. The case is really ugly, so I may salvage the solar cell and LED/reflector and use some other container, like an Altoids tin. I’ll have to do some measurements on the solar cell to find out how much current it can put out in sunlight. I may need two or more solar cells to charge a discharged AA cell during the daytime.
