{"id":1814,"date":"2014-09-13T10:09:48","date_gmt":"2014-09-13T02:09:48","guid":{"rendered":"https:\/\/gergely.imreh.net\/blog\/?p=1814"},"modified":"2014-09-13T10:09:48","modified_gmt":"2014-09-13T02:09:48","slug":"tibetan-prayer-wheel-circuit","status":"publish","type":"post","link":"https:\/\/gergely.imreh.net\/blog\/2014\/09\/tibetan-prayer-wheel-circuit\/","title":{"rendered":"Solar powered Tibetan prayer wheel circuit"},"content":{"rendered":"

There are all kinds of toys and fun gadgets around the house for most people that might work or be broken, but either way most people wouldn’t know how do they work inside. For example take this solar powered Tibetan prayer wheel. There are many of these<\/a> out there, and found one at home lying around as well. It didn’t seem to work, and thought why not take apart to take a look?<\/p>\n

\"Solar<\/a>
Solar powered Tibetan prayer wheel<\/figcaption><\/figure>\n

The inside of this Tibetan prayer wheels is pretty straightforward: taking off the bottom cover out come two ballast stones, and drivebox with a solar panel hanging off it. The drivebox is connected to the prayer wheel outside via a rectangular shaft to turn it.<\/p>\n

\"All<\/a>
The prayer wheel in disassembled form<\/figcaption><\/figure>\n

The drivebox contains an electronic circuit and a couple of cogwheels to turn the DC solar power into motion somehow. Originally the solar panel was not detached from the circuit board as it seems above, just after messing around with it for a while it came off…<\/p>\n

Electronics<\/h3>\n

The circuit has only a handful of components and they looked reasonably intact (some surface corrosion on the capacitors and lot of grime on the transistors, but nothing that seems to be a showstopper). The round black thing on the right is a spool of thin, enamel coated wires \u00a0– a custom inductor. The wires were spun in both directions, so it is actually two inductors intertwined. \u00a0The transistors are C945<\/a>\u00a0according to their marking (or 2SC945 in it’s full name, as I’ve been told). The two lines leaving the photo to the left connect to the solar panel.<\/p>\n

\"Closeup<\/a>
The circuit board inside (top view)<\/figcaption><\/figure>\n

The bottom of the circuit. From here it can be reverse engineered to see how does it actually work. The enamel coated wires were freaky to have a lot of extra length after their soldering point and been all over the circuit. Probably that would be good to cut short to make sure they don’t cause any shorts in the future… The resistors were desoldered so I can check their component value, the capacitor values were read off their housing, the inductor I didn’t touch this time, so their value is an outstanding question…<\/p>\n

\"Closeup<\/a>
The circuit board inside (bottom view)<\/figcaption><\/figure>\n

To figure out what the circuit does, it \u00a0helps to transcribe the diagram into schematic well. Then I messed up the legs of the transistors in the up-down flipping multiple times, so the circuit came out all wrong. Fortunately JohnnyThree at Reddit AskElectronics<\/a> helped (may he be blessed with lot of karma!:), not just to correctly transcribe the circuit but arrange the components so that it shows what the circuit really is: a variation on an astable multivibrator<\/a>.<\/p>\n

\"The<\/a>
The reverse engineered circuit schematic<\/figcaption><\/figure>\n

I fixed up the circuit in KiCAD<\/a> (good learning experience as well, and go open source!). The difference between the regular astable multivibrator and this circuit is C3 capacitor between the base and collector of Q2 (according to JohnnyThree it’s to curb RF interference), and using inductors L1 and L2 instead of pure resistors.<\/p>\n

Here are some additional resources if anyone’s interested:<\/p>\n