A "sometimes I amaze myself" soldering repair

I have two TP-LINK powerline extenders (TL-WPA4220) which allow me to have WiFi in the far reaches of my empire. Well in my big barn, at least, one for each end as its 50m long with walls of 0.75m granite. Anyway, voila:

So far so good until a recent flood (about knee deep) and one was fitted low down and got drenched. It was unpowered at the time, so with a little patience and perhaps a hair drier - in theory, at least - all should have been well, because they ain't cheap! The problem with that last statement was the wod "patience" - I have little and thought it was dried out when it obviously wasn't....

Buzz click pop, bad smell and I let the blue smoke out. Because I'm in remote rural France I have to get everything online and delivery is - er, well - "Gallic" let's say. So as well as the cost, it's the inconvenience, and so I thought I'd open 'er up and see if it was maybe just a fuse ha ha. This is what a good one looks like inside - there is a LOT of stuff in there, but pay attention to the expanded section, becuase that's where the problem lay.

That problem was fairly obvious: in between the big 4.7uF cap on the left and the pale green vertical inductor on the right - and under the tough white rubbery goo keeping some of the high voltage parts safe - something (who knows what?) had gone "pop" in a fairly big way. When I say "fairly big", I mean a) so that it wasn't actually there anymore b) it has taken out the green solder mask underneath and with it, a fair part of the copper track linking it ot the 4.7uF cap. c) the smoked remains of whatever it had been were all over the cap and inductor and inside the case. Even the rubbery goo was blackened. The inductor and cap were so blackened that I didn't know at first what had popped. It was only when I removed them to see if they were still viable that I spotted the disaster zone*. Ironically, both were fine and fully "in spec" and cleaned up fine.

*Spotted the disater zone only after I had taken a scalpel to the white rubber goo to cut away enough to see "what lies beneath". Answer: an empty pad marked "FB1" and the aformentioned bare board and missing copper. All in all, not good, but I hate being beaten. I mean I didn't even know what an FB might be, but I knew I didn't have one "in stock". Turns out its a ferrite bead and it looked as if it was probably an 0603 size...

I'm okay with 0805s but 0603s strike fear into my heart. I had had some recent success with SOD-323 diodes on some blown Wemos D1s and they aren't much diffferent from an 0603, so...

I trawled ebay to try to find some 0603 ferrite beads...and then I thought "Maybe some of my old "junk" boards might have something...long story short - whole house search until I removed a modem card (!) from an ancient PC. It held not just the one bead I'd need but four - which was good news as anyone who has ever dropped / pinged / burned / lost anything that small. So - with the obligatory cocktail stick tip as reference - we have the blank spaces where the FBs used to be and my calipers showing each to be 2.3x1.3mm - actualy slightly(!) larger than the SOD-323, but this has to go into a space I can only just get into!

First, I cleaned off all the soot residue, removed some "erupted" copper trace, flattened the remainder, polished and fluxed and tinned it...I was only going to get one go at this...

Taking a clear photo of the result is pretty much impossible without an industrial microscope, as the scale is so minute, but my best offering follows. Think back to the "good" shot above, remind yourself of this size of this thing and where it has to go back into:

...and it's a marvel I managed at all! So even though it's a terrible shot, it allows us to see several things:

First, you can see some soot still on the white rubber goo and where I had to scalpel it out...Secondly (despite vigorous scrubbing with isopropyl alcohol) the white FB1 legend is still burn-stained. Third, how close I had to get to the 4.7uF cap with my (huge) track repair...

As I look back at this and the good "before" shot, I relaise the original was probably an 0402 size. I'm glad I never saw it - I'd never have dared. Also using the bigger(!) 0603 helps bridge the gap in the blown-out copper track...

No-one, dear reader, was more surprised than moi to find an instant power on, lights cycle, reconnect to the "other end" of the power line adapter and my SSID being broadcast strongly...all as I stood by with fire extinguisher at the ready...

Testbed 6 - the "Mothership"

The last piece in the jigsaw is of course the Wemos D1 mini, which runs everything. So here, she is: the "starship enterprise" ready to boldly go...well, onto my landing to be precise.

Once in place, I can do a final full systems test and get the raspberry Pi NODE-RED controller ironed out.

Before that happens I'm going to build a "production" wall switch using the ESP-01S + touch controller and wire it up to the mains.

Finally, I will put together some videos showing it all working "without its clothes on" including the NODE-RED controller and the Esparto live web interface built-in to each device.

Then...phew!...I may will release the long-overdue Esparto v2.0. Anyone can then download and use it on Sonoff, Wemos D1, NodeMCU, ESP-01S etc which will reduce the 6months+ lead-time its taken me to about 15 minutes to get a fully functional MQTT-controlled IOT box up and running.

My next post will be a "sneak preview" of Esparto v2.0 as running on the very piece of "Heath Robinson" junk you see above...

Testbed 4 - more sub-assemblies: temp and rf433

This one has the (very "bouncy") tact button that all my IOT devices have on GPIO0

• quick press: switch on / off
• medium: reboot
• long: factory reset

It also has the TMP36 temperature sensor and the output module to transmit RF codes to my "el cheapo" sockets: These are what I bought for the 300+ year old farmhouse before I had the (insane) idea to replace everything with a fully-fledged WiFi IOT Home automation system.

Being one who doesn't have a lot of cash to splash, I wondered if I could re-use them...Then I found the wonderful rcswitch library https://github.com/sui77/rc-switch and I found some ridiculously cheap RX/TX pairs on ebay (buy similar here) 5 sets for eu2.79 or 56c a pair. So the TX you see here set me back - what - 28c? I love cheap.

Setting it up was easy: The rcswitch lib has an example that uses the RX to read the codes of the handsets. I was short of Wemos D1s at the time, so I dragged a dusty old Arduino UNO out of a drawer and had the code running in about five minutes flat. I captured all the codes from my four handsets for on and off. I then knocked a little transmit library for the Wemos with a few "hard-coded" values and no-one was more surprised than me to hear the happy sound of clicking less than an hour or so after I started! Tweaking the code, removing the hard-coded values and setting up the automatic re-configuration then occupied me for the next few days, of course...

Now my system uses these as "slaves". Each "truc" or Wemos-controlled "big box of sensors" reads MQTT configuration data at startup to see which RF switches are linked to it. When it next switches on or off, it automatically sends the RF codes to make all its "slaves" match. In the real world, the main box would be wired in with the overhead light in each room, and the el-cheapos would be floor lamps, or corner lamps. When commanded either by touch or remote MQTT, all go on/off at once.

Testbed part 1

Here is the testbed I built to cope with both ESP-01S and Wemos D1 Mini devices.

It has mains input which is fed to a relay board and then on to the actual appliance (usually a lamp) - this is used for final testing, which is all remote (via MQTT) as I don't like going near bare mains wires - and neither should you!

Most of the "bench" testing is done with 12v input from a PSU which is dropped to 5v by an L7805. That's fine for the Wemos D1, as it has an oboard 3.3v LDO, but when I'm testing ESP-01S, it doesn't, so there's a socket to take an external 3.3v LDO (an AMS1117) on a 3-pin breakout board.

The baseboard itself is from a company that doesn't seem to make them anymore, which is a shame as they are solid, rigid and very well-made.

Next comes the Wemos "tripler" base - a lovely piece of kit, but with a serious "gotcha" (see later post)

And then the separate sub-assemblies each of which I will detail in a separate post