SMD Soldering Practice Kit, or How I Learned to Stop Worrying and Love the Cheap Chinese Kit

This is not an Instructable about soldering. This is an Instructable about how to build a cheap Chinese kit. The adage is that you get what you pay for, and here is what you get:

  • Poorly documented.
  • Questionable part quality.
  • No support.

So why buy and make one?

  • Super inexpensive.
  • Interesting circuit.
  • Learn troubleshooting!

If you look at my other Instructables, you see that I design and sell kits. Why would I take time and effort to to document someone else's? I'm not sure, but I hate the idea of someone trying one of these and then giving up on electronics because of a bad experience. Electronics is hard enough without throwing a bunch of confusion in the mix. And maybe, if you get hooked, you'll buy one of mine.

This Instructable is specific to the "SMD Rotating LED SMD Components Soldering Practice Board Skill Training Kit" from Bangood, but the principle apply to any project. There are a lot of kits out there, but I like this one because:

  • Practice area separate from working circuit.
  • Interesting circuit demonstration (555 timer and Decade Counter).
  • Useful reference information on the back.

Step 1: Parts

"The whole point of the [low-cost kit] is lost... if you [don't document it]! Why didn't you tell the world, eh?"

To successfuly build an electronics project, you will generally need a schematic and a bill of materials (BOM). The schematic shows you how the circuit works, and the BOM shows you what parts are used. The kits from Bangood came with no documentation, and the website has only a partial schematic with the wrong reference numbers.

The best information I've found comes from an Ebay listing which provides the schematic and a board layout with component references and values on it: https://www.ebay.com/itm/2Sets-DIY-SMD-SMT-Compone... While it has a BOM, there is no reference numbers on it, so it is not much help to us. The hand-drawn images are both quaint and informative.

The best BOM I've found was from a forum post https://forum.banggood.com/forum-topic-240555.html), and even it is a bit scattered, so here is my synthesis:

Realize that for the practice areas, the values don't mater, only the package size. If you decide to do the practice areas first, be sure to set aside the components needed for the working circuit, namely:

  • R48, R49, C27, C28, and R61-64.

Step 2: Power

"Your average Ruskie doesn't take a dump without a plan," so we are going to build and test the working circuit in steps. First, we need to get the power sorted. The Bangood website lists 3-12V, but I doubt either the 555 or the CD4017 will run reliably at 3V. I used a good 5V power supply, but a cutting up an old USB cable, phone charging cord, or using a 9V battery would also be good sources.

Side note: Powered with a 3V lithium, the 555 portion of the circuit worked, but not the Decade Counter.

Step 3: 555 Timer

The 555 Timer is claimed to be "the most popular integrated circuit ever manufactured" and should be a part of any hobbiest tool set. The first part of the Wiki article makes for good reading: https://en.wikipedia.org/wiki/555_timer_IC

In this circuit, it provides a regular signal of about 3 cycles per second to flash the LEDs. Each pulse should light LED D1, and the actual timing of the on and off cycles are controlled by the resistance of R48 & R49 and the capacitance of C27. You can actual calculate the cycles using maths, or just plug the values in at http://www.ohmslawcalculator.com/555-astable-calcu...

  1. Solder U1, carefully observing pin 1 orientation. This is usually indicated on the chip by a dot or slash and the divot on the silk screen. Check the data sheet if you are unsure: http://www.ti.com/lit/ds/symlink/ne555.pdf
  2. Solder R48 ("205"), R49 ("103"), and R50 ("471" or "331"). Resistors are black in color and have no orientation so can be soldered in either direction.
  3. Solder C27 and C28. Ceramic resistors are brown and have no orientation or value markings.
  4. Solder D1 LED, carefully observing orientation.
    • Markings vary, but generally a green tint on the lens marks the cathode, or negative side which corresponds to the thicker line on the silkscreen.
    • The bottom of the LED may have an arrow or tee that points toward the cathode.
    • Most multi-meters have a diode mode that will help identify polarity and color of the LED.
  5. Attach your power and energize the circuit.

If you are not greeted by a rapidly blinking LED, do not loose faith. That is why I am here, and you are here.

  1. Visually inspect (with magnification if you have it) each solder joint and retouch any suspects.
  2. Verify the orientation of U1 and D1.
  3. With your multi-meter, verify you have approximately 5V at the power pads and that the polarity is correct (red positive, black negative, voltage reading positive value).
  4. With multi-meter black probe remaining on negative, place the red probe on the top pad of the LED.
    1. If you are getting a cycling voltage, the 555 is working and your LED is suspect (solder joints or orientation).
    2. If you are not getting voltage, put the red probe on U1 pin 8 (top left), and look for about 5V. If you are not getting voltage there, go back and check your power supply and solder joints.
    3. De-energize the circuit and check continuity (the beep mode) between:
      1. U1 pin 8 and the positive power pad.
      2. U1 pin 1 (bottom left pin) and the negative power pad.
  5. If all else fails, don't give up. Take a close-up photo and post something in the comments to get some help.

Step 4: Decade Counter

The CD4017 decade counter is another venerable chip worth knowing about. It will take the clock signal from the 555 timer and sequentially light one of ten LEDs at a time. Let's wire it up with just one LED for starters:

  1. Solder U2, carefully observing orientation as with the 555 chip. If in doubt, check the data sheet: http://www.ti.com/lit/ds/symlink/cd4017b.pdf
  2. Solder R51 ("331" or "471") in place.
  3. Solder D2 in place in the correct orientation as before.
  4. Power the circuit up and observe D2 blink once for every 10 blinks of D1.

If you are not getting D2 to blink, the trouble shooting is basically the same as before:

  1. Visually inspect (with magnification if you have it) each solder joint and retouch any suspects.
  2. Verify the orientation of U2 and D2.
  3. With your multi-meter, verify you have approximately 5V at the power pads and that the polarity is correct (red positive, black negative, voltage reading positive value).
  4. With multi-meter black probe remaining on negative, place the red probe on the positve pad of the LED D2.
    1. If you are getting a cycling voltage, the CD4017 is working and your LED is suspect (solder joints or orientation).
    2. If you are not getting voltage, put the red probe on U2 pin 16 (top left), and look for about 5V. If you are not getting voltage there, go back and check your power supply and solder joints.
    3. De-energize the circuit and check continuity (the beep mode) between:
      1. U2 pin 16 and the positive power pad.
      2. U2 pin 8 (bottom right pin) and the negative power pad.
  5. If all else fails, don't give up. Take a close-up photo and post something in the comments to get some help.

If all is good, you can either solder the remaining LEDs/resistors in the circle in, or move on to the next section.

Step 5: Transistor Switch

The decade counter and 555 circuits are great for driving signals and one LED, but to drive multiple LEDs, you need a little help. This is where transistors, another great addition to your knowledge tool box, come in. Again, a little wiki reading is good: https://en.wikipedia.org/wiki/Transistor

For this circuit, the "clock out" signal of the CD4017 is applied to the base of the transistor (through a resistor and diode) which in turn allows current to flow from the collector to the emitter. This should turn the four corner LEDs on for five clock cycles, and off for five.

  1. Solder D1 (orange with black end) in place with the black end (cathode mark) downward to match the thicker silk screen line.
  2. Solder R61 (black "103") above D1.
  3. Solder Q1 (black with three legs)..
  4. Solder D16 LED observing polarity.
  5. Solder R65 (black "471" or "331").

Power the circuit on and observe LED D16 cycle. If it is not lighting, you know the routine:

  1. Visually inspect (with magnification if you have it) each solder joint and retouch any suspects.
  2. Verify the orientation of D1 and D16.
  3. With multi-meter black probe on negative power pad, place the red probe on the base "b" pin of the transistor (lower left, see image) to see if 5V signal is cycling.
    • If there is no signal, move the red probe to the base of the D12 to look for signal. If signal is there, the diode may be backward, or the transistor may be PNP (it happened to me). Short across D12 with wire or a short piece of solder. If the LED lights up, swap the orientation of the D12.
  4. If all else fails, don't give up. Take a close-up photo and post something in the comments to get some help.

Wow, you did it. Go back and finish up and be sure to click the "I Made It" button so I know this has helped someone out!

Step 6: A Bit More Ranting

You will notice that two of my blue LEDs blink at separate times, and that two of my ziener diodes are in backwards. I will illustrate a little bit more about "you get what you pay for". I spent nearly an hour diagnosing the transistor circuit because the LED was not blinking. I tried a couple of different resistor values R61 to see if that helped, even shorting it out completely to no avail. It was only when I shorted D12 out that the circuit started working! How could that be?

  • Change out D12 for another? "Negative function".
  • Check polarity on data sheet? "Negative function".
  • Put D12 in backwards? Works, but why?
  • Is Q1 an NPN, because it is behaving like a PNP resistor? "Yeee Haaa".

This is where another one of my cheap Chinese kits came in handy, an LCR meter, which confirmed it was indeed a PNP. I opened my other kit and it contained NPNs. Go figure. So I put two PNPs in with the diodes reversed, and two NPNs with the diodes correct, and bingo, I've got alternating lights. Lemonade!

Now, if you think Bangood customer service would have helped me out with that, good luck. Have trouble like that with one of my kits, you will get some help. That is unless it is the SMD Challenge. For that my friend, you are on your own. Just like a cheap Chinese kit.

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    Thorondor95

    4 months ago

    The Globe needs is more of people such like you do. :)