Project: Generic Electronic 57 LED Hourglass Kit (TJ-56-205)

I couldn’t help myself, so once I finish one kit, it’s often straight onto another. This kit was sold as an electronic LED hourglass and was a little more expensive at AU$3.07 including postage. Compared to local suppliers, this is a bargain – one major supplier in Australia wants AU$8.95 for a two transistor flasher kit, so I thought I’d build one of these instead and try it out.

The Kit

Another clear plastic zip-lock bag. Maybe I should call these “fun bags” from now on … or maybe not. *looks awkwardly around in case anyone heard me say that*

Inside, there is a double-sided quarter page leaflet in Chinese with (presumably) instructions, PCB layout, bill of materials (BoM) and schematic. Already, we can see that the circuit uses a microcontroller, an obscure STC 15W201S, with the ability to be reprogrammed through a serial ISP header. This unfortunately means that the kit is probably not as educational as something with more traditional chips – the source code for the current program isn’t provided either. The circuit runs LEDs using a complimentary visual multiplexing matrix drive, so while not as complex as Charlieplexing, does at least allow for twice as many LEDs to be controlled as compared to regular visual multiplexing and makes debugging somewhat easier. Curiously, the BoM lists a capacitor that has no position and omits a USB connector.

The supplied components are, as listed in the BoM, so I suppose that’s what they intended to supply. It is a good idea, wherever you use microcontrollers, to have a bypassing capacitor installed near the chip itself but the leaflet makes no mention of a provision for it. The inclusion of an IC socket is a good feature, although I’d have to criticize them for including such a rusted, crusty push-button switch. There is also a quality control slip as well.

The PCB is a double-sided fibreglass type PCB with green soldermask on both sides and white silkscreen on the top. The pads are tin plated-through-hole, which makes this a pretty decent quality board for a kit. The board has the identifier TJ-56-205 on it, which suggests it is related to the heart flasher kit built in the last post.

Rather sadly, in the packing process, it seems that the board did suffer some abrasion but was not functionally damaged.

Construction and Testing

This kit is one that seems to require a substantial amount of work to complete –

57 LEDs x 2 connections
1 IC x 16 connections
1 switch x 5 connections
1 power jack x 3 connections
1 ISP header x 4 connections
1 switch x 4 connections
TOTAL - 146 joints

Construction was, however, very straightforward because the orientation of LEDs was consistent, the board was well labelled and the pads (while small) were tinned. The only thing it took was time – about 55 minutes for me to complete including distractions and photographing.

In the end, three spare LEDs were included, which is a nice touch. However, the negatives were the rusty push-button switch which was found to be somewhat intermittent in functioning, the lack of USB connector (for a more popular way to power the circuit), the lack of a capacitor position on the board, the “blind” use of a rather obscure microcontroller to perform all the functions and the almost unnecessarily thin traces (which might make damage more likely).

The unit, when operating, runs like this, displaying a rather long animation that simulates grains of sand falling from the upper to the lower portion. This “abruptly” resets to a full top-half once the bottom-half is full. The push button allows you to toggle between three speeds.It’s not particularly inspiring, but the LEDs are quite bright and the visual effect is somewhat eye-catching. It’s a shame that the microcontroller is a little obscure, otherwise the LEDs could be used to display something else entirely (or maybe a more flexible hourglass that actually functions as a timer).

Conclusion

While this kit was slightly more expensive than some others, it’s still very cheap by local standards. The cost is understandable, especially when you consider how many high-intensity LEDs were included and the quality of the double-sided fibreglass PCB. Construction was very straightforward, with only minor negatives in the fact that everything is hidden inside an obscure microcontroller with no source code, that the traces seem unnecessarily thin on the board, that a USB connector for easier powering was not included, that the push-button switch included was rusty and intermittent and that no position is provided on the board for the capacitor which I would deem good practice.

That being said, the tin plated board was a joy to solder to and the consistent LED direction made things a lot quicker than it would have otherwise been. The effect is also quite bright and visually distinctive. Having three spare LEDs and an IC socket makes this kit quite recommendable for someone who wants a decent amount of through-hole soldering practice, with a good chance of success.

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Project: Generic 18 Red LED Flashing Love Heart Kit (TJ-56-30)

Another day, another classic kit. It seems that LED heart-shaped displays are a timeless novelty, so after building a multi-coloured chasing heart and a breathing heart, why not a flashing one? For just AU$1.68 including postage, I managed to score this kit from eBay which is an upgraded version of the classic two-transistor flasher circuit.

The Kit

This kit looks like any other cheap eBay kit from China, in that it’s packed in a zip lock plastic bag. But already, from the beginning, we see an improvement – they included a leaflet!

The kit is built on a standard “boring” square PCB, being of the single-sided paper-resin variety. White silkscreening on the front gives clear indications as to how to mount the components and their values. The kit seems to have a designation of TJ-56-30.

The underside reveals lacquer-coated copper pads and green soldermask.

Included are the LEDs, transistors, resistors, capacitors and a set of terminal blocks for power connection. A quality control tag is added to the bag as well.

The included double-sided half-page of information is in Chinese (which, sadly, I cannot read). However, it does include the schematic, as well as the bill of materials and PCB design which is handy reference. We can already see that this is a standard collector coupled astable multivibrator design, albeit with three branches rather than the more usual two.

Construction and Testing

Lets do some quick sums to check how much work is involved in building this kit:

18 LEDs x 2 connections
6 resistors x 2 connections
3 capacitors x 2 connections
3 transistors x 3 connections
1 terminal block x 2 connections
TOTAL - 65 joints

This isn’t a particularly high number, which should make the kit quite suitable for a novice. In all, including photo taking and testing, it took me about 40 minutes from start to finish.

Overall, the construction experience was not bad, but could be improved. For once, I dislike lacquer coated paper-type boards for soldering, mainly because the lacquer can interfere with formation of the joints, thus requiring a little more heat and care when soldering. The type of board is also more sensitive to overheating, so this isn’t necessarily a good thing for a beginner to have to deal with. The fumes from this sort of board, while distinctive, aren’t exactly the most pleasant to deal with either. The pads were a little on the small side as well, making solder control critical. The shape and routing of the traces were almost haphazard, along with the directions of the LED polarities, making it highly probable that a mistake could be made necessitating extra work to resolve (increasing the chance of damaging the board). Finally, there were no spare parts and only exact quantities were provided. That being said, I suppose, given the low cost, it’s something to be expected and are only minor niggles.

The result is a heart which has two-thirds of the LEDs lit at any time, chasing around the perimeter in a way that is reminiscent of some of those LED restaurant signs. Note that the capacitors and terminal block don’t move in operation – that’s a side effect of perspective shift and post-processing to remove hand shake from the recorded video.

Finally, I simulated a simplified version of the circuit (omitting the parallel LEDs) in Analog Devices LTspice, which clearly shows the behaviour of the circuit (albeit substituting the transistors with “close enough” equivalents. If you want, the model file can be downloaded here.

Simulators often have problems (in my past experience) simulating these types of circuits as their stable oscillation only occurs when the components have some “mismatch”. It was interesting to see that, aside from the initial glitch, it seems like the simulator worked just fine.

Conclusion

This kit is nothing but a simple classic design upsized. The kit itself was complete, although the single sided paper-type PCB with lacquered copper was not the easiest to solder. The quasi-random directions of the LEDs and the odd trace patterns also were not helpful, however, it did come with Chinese instructions that included a schematic, adding to its educational value. For the price, it’s a decent amount of fun and soldering practice.

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Teardown: Generic Wall-Mountable Passive PoE Midspan Injector

For a lot of modern networked devices, Power over Ethernet (PoE) is a big convenience, allowing the one cable to power a remote device (be it an access point, camera, etc.) and supply the data connectivity to it.

While there is a standardized PoE in the form of 802.3af/at, which specifies negotiation rules and a 48V nominal system voltage, many lower cost devices (especially older ones) cannot justify the increased cost of integrating the necessary logic and power conversion circuitry. These utilize an alternative solution which is termed Passive PoE.

Passive PoE does not involve any negotiation whatsoever and supplies power at all times. In its earliest and simplest incarnation, it used the two unused pairs from Fast Ethernet to carry the DC voltage. This was informally standardized as pins 4-5 as positive and pins 7-8 as negative, similar to 802.3af mode B. Later on, Gigabit Passive PoE used transformers to allow both data and power to share the same pins, although these also come in a few variations.

Most passive PoE installations use a variety of voltages, with 24V being a common compromise but even voltages as low as 12V can be practical depending on run length. Cheap passive adapters (endspan injectors and splitters) can be purchased to convert some 2.1mm DC jack based equipment to run off passive PoE although the cheapest adapters are fast Ethernet type. This is often good enough for older wireless APs.

However, whatever you do, you should not apply passive PoE to a device not designed to take it, otherwise damage to the Ethernet port’s magnetics may occur!

The concept of passive PoE is fairly straightforward and endspan injectors can be purchased in pairs quite cheaply (about AU$2 a pair). But what if you don’t want to inject and remove power at the ends of the Ethernet cable? Maybe you have voltage drop issues, or you have a more convenient source of power closer to the device? In that case, you need a midspan injector.

The Device

On eBay, for about AU$1.59 posted you can have a “wall mountable” midspan Fast Ethernet passive PoE injector.

The unit has a plastic body with a hole on the top for a power indicator LED. The wiring is printed on the casing for reference.

A 2.1mm DC barrel jack receives power from your power pack to inject to the cable. The PoE port supplies the data (two pairs used for Fast Ethernet) and power (over the unused pair) to a device expecting passive PoE or a splitter that you supply. The LAN port provides only the data and is not connected to power.

It is “wall mountable” owing to a small hole eyelet in the casing, although it does seem a little flimsy.

The unit is held together with one screw covered by a QC label I had removed. How does it look inside? Lets see.

Teardown

The screw was the only thing stopping you getting inside.

The unit is built on a paper-type single-sided PCB to reduce costs. Inside, there are the Ethernet jacks, the power jack, an LED, a 10kohm resistor and two jumper wires. This is probably the second simplest design that could be used – the simplest would have omitted the power indication altogether. The PCB is marked XLY-POE2(20150717) suggesting a company by the initials XLY manufactured the unit, this is probably the second version designed 17th July 2015.

A quick calculation seems to show the resistor is well sized – at a maximum 802.11af voltage of 57V, about 55V is put over the resistor resulting in a current of 5.5mA (very safe for the LED). At a low of 12V, with 10V over the resistor, the current is just 1.0mA which should result in a dim but visible glow.

Unfortunately, the unit doesn’t employ any safeties – no polyfuse protection against short circuits, no diode protection against reverse polarity. As a result, be sure to use a limited-power supply which will only source a safe amount of power in case of a fault and confirm the polarity before using it. It would be trivial to add it with a knife and soldering iron though.

However, because of its simplicity, the unit could also be used as a splitter by plugging in the power+Ethernet lead into the PoE port and extracting the power from the DC jack (by making a male to male cable). You can also use it as an indicator as to whether PoE power is being delivered to a passively-powered Fast Ethernet device just by plugging the powered lead into the PoE port and seeing if the LED lights. Or you could use this as a crude Fast Ethernet only cable joiner in an emergency. For the most part, most endspan passive PoE kits can’t do this as they lack an LED.

A look at the underside shows green solder resist and very oddly shaped thick chunky traces. It might guard against delamination during soldering, but it might not have a good effect on the data signals.

Tracing the path of various signals, the positive power is in red, the negative power is in dark blue. The LED to resistor connection is in yellow. The remaining data pins are colour coded green, orange, purple and sky blue. The light-blue overlay shows where the LED, resistors and jumper wires on the other side are.

This is a mess. In fact, this is where endspan units might be better as they are based on a crimped RJ45 end and CAT5 or better cable soldered to a socket which isn’t particularly complex. However, as we can see in this design, the data lines are all different lengths. The traces are also shaped quite oddly with some data lines needing to jump over to the other side and back. This would probably cause both skew and impedance mismatches which could affect the data integrity slightly. Being unshielded, it could also introduce interference into the network in the case of being near high EMI/RFI sources.

Luckily Ethernet is extremely tolerant of dodgey cabling. Low rates of errors will be corrected through retransmission – I’ve even had success running 100Mbit/s Fast Ethernet for a while on CAT3 “voice grade” cabling which was only suitable for 10Mbit/s just on a whim. So I don’t think this design will cause problems per-se, but it’s not the best situation for an Ethernet signal. Of course, using two pairs for power in a dedicated fashion will preclude Gigabit Ethernet from working.

Conclusion

This inexpensive mid-span passive PoE injector is very much as minimalist as it can be, with the exception of the inclusion of a power LED. It can be useful in a pinch as an injector, a splitter (with your own male-to-male lead), diagnostic tool to check for PoE presence or to forcibly strip out any passive PoE to safely connect non-passive-PoE-aware devices or as an emergency Fast Ethernet cable joiner. The design isn’t exactly the safest, so ensure you are using a limited power supply with the correct polarity. The design also isn’t the best for ensuring the integrity of an Ethernet signal, but it should be “good enough” that it would work in most cases. Definitely nice to have as a spare in the toolbox.

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