I had a little time this evening, so I thought, why not crack open a kit? After all, I did buy a stack of cheap eBay kits to keep me amused and give me something to write about … so I decided to pick out a kit with a slight twist.
The kit itself is an NE555 timer and CD4017 counter based LED chaser. This is a pretty classic configuration and one I’ve seen from the Funway 2 days. It is also advertised under the name of “running flow LED light”, which is an interesting translation.
However, the twist is that this time, I’ll be building the kit using mostly SMD components. For the princely sum of just AU$1.00 including postage, you can challenge yourself to some non-trivial amount of SMD soldering. I didn’t even know they still made 555 timers in SMD packaging!
Say hello to my new friend! Zip-lock bags, loose components and no instructions. Nothing new here, but that’s just the realities of low-cost kits.
As the project uses SMD components and 3mm LEDs, the physical size of the PCB is not very big. It is a quality fibreglass board, double-sided, tin plated with blue soldermask and white silkscreen on the top. It is marked 1918m and YL-117 which might have something to do with the creator. The components are pretty well marked, although there are a few subtle issues – the LED symbols have mostly been lost to the pads, the resistor numbering is pseudorandom, the text for the NE555 may have you mounting it upside down and the CD4017 is labelled as a CD4B17. There are also some dark stray marks on the rear which may be due to abrasion in sorting.
Aside from that, you get your SMD resistors, capacitors and ICs in cut-tape format, which is fairly standard. The LEDs are in their own zip-lock bag, and the trimmer and header pins are packed loose. That’s all there is to it – but admittedly, that’s quite a lot for AU$1 posted!
Construction and Testing
A quick compute shows the total number of joints needed to complete the kit, noting that a lot of them are surface mount joints:
10 LEDs x 2 connections 12 resistors x 2 connections 2 capacitors x 2 connections 1 CD4017 x 16 connections 1 NE555 x 8 connections 1 trimpot x 3 connections 1 power x 2 connections TOTAL - 77 joints
Construction is probably the more interesting part of the kit, so I won’t bother reverse engineering the kit – after all, it’s the same as the Dick Smith Funway 2 LED Chaser which consists of a 555 timer as an oscillator driving a counter chip with each output hooked to an LED and resistor.
It makes most sense to start with the surface mount resistors first. As the row of LED resistors are the most numerous and equal-valued, I started with them first. Taking care, I first tinned the pads on the board with a little solder (as I don’t have any solder paste). I then extracted the resistors from the tape (carefully, so as not to lose them) by peeling off the plastic and shaking them out one by one. With a fine set of tweezers, I placed each resistor in approximately the right position, and then waved a heat gun slowly over the whole board with the airflow set low so as not to blow the resistors away. Slowly, you see the resistors “twitch” into their position, although some might need a subtle nudge.
Voila, the resistors are mounted. Not the most pretty job and I’ll be the first to admit I used a little too much solder and left the heat on long enough for the solder to start oxidizing. It’s not bad enough to be fatal by any measure though, especially seeing as I’ve never been trained to do SMD.
I repeated the same process for the capacitors and resistors at the top. Then, next comes the chips. Instead of using the “reflow” type process, I decided to solder these by hand using a fine iron and fine solder. This was because the legs were wide enough to permit this and I felt more comfortable doing it this way …
… although the result does look ugly with too much solder (again) but not too bad. The chance of bridging with SMD chips is quite high especially without careful control of solder and a fine tip, so reflow may still be preferable. Do note the text orientation on the silkscreen versus the actual pin 1 orientation of the NE555 chip could be a trap for inexperienced constructors!
After that, getting the through-hole components mounted completes the kit. I found that some of the through-holes are a big on the large side, resulting in solder flow-through the board which varied depending on how long you heated the joint. The last LED’s footprint was also slightly chopped off, suggesting just how “tight” the design is and possible limitations in quality control during manufacturing. It is, however, just a minor defect.
In the whole kit, the only spares you get are one of each type of resistor. I guess LEDs are a bit too expensive to give you spares of … but that’s fine.
In the end, total construction time was 40 minutes including time for photographs and distractions. The result is an LED which chases from one end to the other, with the speed configurable using the trimpot.
As with other versions of the kit, if you wanted fewer states, you could hook-up the next state to the reset line on the CD4017 to truncate the count. This would have sufficed to create the “windmill” kit constructed earlier in a way that would not require a microcontroller.
A quick analysis of the 555 shows it has an astable vibrator configuration that results in:
C = 1uF R1 = 2kR R2 = 10kR - 60kR f = 65.591Hz - 11.828Hz Chase Rate = 6.5591Hz - 1.1828Hz
For AU$1 posted, it’s a good skills tester or practice kit to get some experience doing SMD soldering while building what I consider to be a classic kit. The board quality is quite good and all the parts were included with a spare resistor of each sort (in case one pings out and gets lost). It’s best done with a hot air gun and tweezers at the minimum, but it’s also possible to build it with two soldering irons or even one if you’re willing to accept a slightly less pretty look. I was quite surprised to find the venerable 555 timer available in SMD form and fairly pleased that I was able to complete the kit straightforwardly. I wasn’t that confident starting out, but look at the result!
In the meantime, I’ve still got quite a few more kits on the pile for when I have some time to build and analyze them … so you’ll probably be seeing a few more in the near future.