I’ve always been someone to enjoy constructing electronics kits from a young age. Even though I’ve become more experienced at soldering and more knowledgeable about electronics, I still find constructing new kits therapeutic and enjoyable for a bit of weekend fun.
Unfortunately, local sources of kits are relatively few and expensive. As a result, I went trawling around eBay on the lookout for some rather cheap kits to build in my spare time. I knew from previous experience that these do vary significantly in quality and challenge, but given the price, it’s good fun all-round.
This particular kit is sold as an electronic LED windmill kit, for AU$2.55 shipped, complete with a complement of 3mm red LEDs. It’s so cheap that I couldn’t resist, despite my expectation that it would be simple and not particularly educational.
As with many Chinese kits, this one was packed in a zip-lock bag as a loose assortment of components and PCB. No instructions are provided, although none are really needed.
The kit is pretty simple – all identical red 3mm LEDs, all identical 1kohm resistors, all identical switches, one two-way header pin assembly and a single 8-pin DIP. It’s a bit of a shame that no socket was included, as this is a common “insurance” policy against beginners mistakes.
The PCB is a nice FR4-style fibreglass double-sided plated-through-hole PCB with green solder resist and white screen-printing clearly showing the polarity and placement of components. There are two push-button switches, labelled in Chinese. The PCB seems to have a number of 31063AP48-171119, suggesting to me the design was made 19th November 2017.
The underside is also solder masked and all connections are tin plated.
Construction and Testing
A quick calculation shows that the total amount of joints that need to be made are 64 –
19 LEDs x 2 pins 4 resistors x 2 pins 2 switches x 4 pins 1 header x 2 pins 1 IC x 8 pins Total: 64 joints
The construction process took me around 35 minutes counting interruptions to photograph and other distractions. While constructing the kit, I found the pads and traces to be slightly on the thin side, making it a little more difficult than necessary (especially if you have a chunky soldering tip or thick solder). Because of the small holes, the LEDs would not easily mount flush due to the “bulge” in the legs, requiring the bulge be snipped off before inserting into the board. The process was rather repetitive and a little frustrating given the semi-random orientation of LEDs which made finding clearance between legs to solder a bit of a difficulty. The lack of a matching power connector or leads means that you will need to have some on hand and a power supply to test the kit. But the bigger annoyance is that with just two pins to solder, it can be difficult to get into place without resorting to tricks like tack-soldering one pin, soldering the other properly and then coming back to touch-up the first pin.
Nonetheless, the kit was constructed just fine.
After construction, there were six spare LEDs. I tested the kit, and that’s when I realized that these were included for a reason.
With a longer exposure, it seems that D3, D7 and D15 had failed. They were placed in the correct orientation and soldered well without excess heat (I’m not a beginner), but it seems the plastic that these LEDs are made of are like cheese. The units fell into bits during the desoldering process and are extra sensitive to heat. They may even not have been functional prior to being installed as I didn’t test them prior to construction. I replaced the three LEDs with three spares and it finally worked as intended.
The kit expects 5V DC and has no reverse polarity protection. In fact, it seems to operate from about 3.6V through to 5V, with diminished LED brightness at the low end. It cycles through three LED configurations to produce a windmill effect.
I spent a little time reverse engineering the circuit to see how it worked.
The circuit is quite simple – the middle LED is powered independently and remains solid. The other LEDs are separated into three banks of six LEDs all connected in parallel. Each bank is connected with a 1kohm resistor in series for current limiting. An STC15F104W microcontroller directly drives each LED bank in sequence to create the effect of a windmill. Each channel is driven with a total of approximately 3mA of current. One pin is left unconnected – maybe a four-step sequence is possible depending on the program loaded onto the microcontroller, unless this pin is reserved or limited in function.
The left button increases the speed of rotation, whereas the right button decreases the speed of rotation. I measured the output of one of the pins to determine the period of each cycle.
It defaults to a middling speed with a 411.1ms cycle time at power up.
The maximum and minimum speed seem to be 209.1ms and 612.8ms per cycle respectively.
In total, 11 speed settings seem to be available, with a ~40.3ms difference between steps. This suggests that internally, maybe there is a 75Hz timer of sorts (i.e. each channel clock period is 40.3ms/3).
It was a pretty cheap and generic kit without instructions or much in the way of educational value. It is a bit of an exercise to solder with the small pads, small holes, thin tracks and highly-sensitive LEDs. What you get is a slightly underwhelming three-step animation which could have been achieved in discrete digital logic using a counter chip and oscillator. Instead, a pre-programmed microcontroller is used which simplifies everything but also shows you how many simple functions are done in microcontrollers almost needlessly.
It was cheap, and it did keep me occupied for a while. I suppose if you need a microcontroller that toggles three outputs at the rates above, then maybe this could be useful …