Weather-proofing the bungalow workshop

With the roof and three out of the four walls on, and at least two days of rain forecast for the weekend, we had to act fast and get some plastic sheeting on, to protect the OSB3 outer skin.

The roof will eventually be clad in roofing felt (either charcoal mineral felt, or maybe strips of shingles - it depends on whether I can secure the shingles well enough to stop any driving wind from lifting them, as the roof pitch is only about 14 degrees).

But before any outer cladding goes on, we're covering the OSB3 with  DPM plastic, just to protect the wood, should any of the cladding come loose at any time in the future (plus, it buys me an extra couple of days to get  just the right sort of stuff delivered).

Under most of the beams secured to the floor, and at the bottom of every piece of OSB that comes into contact with any concrete structure (or anything that could possibly even potentially be a source of damp rising from the floor) we put strips of plastic. This plastic will be folded up and stapled to the walls, then an outer layer of plastic, from under the eaves to the bottom of the boards (covering the blue stuff) will be stapled on.

The roof took longer than expected to cover - making sure the plastic lay flat with no wrinkles or folds was a long, slow job; smoothing it out and stapling every half a metre or so. But the roof is now entirely water-tight.

Just got to make sure the walls are too.
Then when the (uPVC) cladding arrives, we'll be able to actually see it as a proper structure, not just large wooden box at the bottom of the garden!

Capacitive sensing using PIC microcontroller

We've been spending a lot of time with Arduinos in recent months. Which has been fun and quite interesting. But now we're back to prototyping some low-level ideas - and many of the libraries for Arduino either "work" or "don't work". There's little room for tweaking and customising - a lot of the functionality is encapsulated away - and once you get to the point where you're reading through someone else's (often quite poorly written) code, it's often quicker to write your own, from scratch!

One of the things we're looking at is detecting large bodies of liquid (ok, pint glasses filled with beer) for a fun experiment. It might turn into something, or it might just remain one on the ideas board. The first thing that springs to mind with detecting any kind of body of liquid is capacitive sensing.

We've done this in the past, using a "charge and count" digital input method (our touch-sensitive miniature guitars used this approach before we finally settled on the darlington multiplex idea instead, and very early prototypes of electronic Blood Bowl used the same, simple approach). The basic idea is to set a pin as an output, drive it high, turn it into an input and count how long it takes to drain; any addition capacitance added to the pin/pad will make it discharge more slowly and you can use this to detect the presence (or absence) of a finger.

In recent years Microchip have been promoting their mTouch technology but it's basically a variation on using two pins/ADC channels to create a voltage divider and using an analogue input to detect the presence (or absence) of an object. A great explanation of how it works can be found here.

The basic idea is:

• The design uses at least two i/o pins (even if only one sensor is required, although the second pin can also be used as a sensor, if queried in sequence, so three sensors would require three pins, not four - it is only for one sensor you require more pins than you're testing)
• Pin A is made a digital output and set high
• Point the ADC to PinA (this causes the ADC to charge up)
• Make Pin B (the sensor pin) a digital output and force low (set to ground)
• Turn Pin B (the sensor pin) into input (TRISx = 1)
• Point the ADC to PinB (the sensor line). While the ADC charge-and-hold capacitor retains charge, we've effectively created a voltage divider between Pin A and Pin B.
• Begin ADC conversion
• Read the values of ADRESH: ADRESL (10-bit analogue value)

This is why we need two pins for a single sensor (but there's no reason we can't swap them over and use PinA as the sensor and PinB as the ADC charge circuit if we need two sensors). When the ADC is pointing to PinA, it's held high and the internal charge-and-hold capacitor that makes up part of the analogue-to-digital circuitry is charged up to 5V (or whatever the supply voltage is).

The input pin is grounded (to put it into a known state) then made an input. When the ADC is pointed to the sensor pin, PinB, we now have the VC(hold) capacitor in parallel to the capacitance of the sensor. We also have a voltage divider - the VC(hold) capacitor is fully charged, and the sensor pad is at zero volts. The input pin reads an analogue value, somewhere between the two.

Any increase in capacitance at the "sensor side" of the voltage divider results in a lower analogue value on the input pin.

We take multiple samples and look for a relatively low or high value on the analogue input to determine whether or not there is extra capacitance (and therefore a larger body of liquid) on the sensor. Relative means we keep a rolling average and look for values that are noticeably larger or smaller than this average, to indicate an object being placed or removed from the sensor.

The overall code looks something like this.

Define CONFIG1 = 0x0804
Define CONFIG2 = 0x1dff
Define CLOCK_FREQUENCY = 32

declarations:
   Symbol tx = PORTA.0
   Symbol rx = PORTA.1
   ConfigPin tx = Output
   ConfigPin rx = Input

   Symbol pin_a = PORTA.4
   Symbol pin_b = PORTA.2
   Dim adc_value As Word
   
   Symbol pin_led = PORTB.6
   ConfigPin pin_led = Output
   
   Dim i As Byte
   Dim last_val As Word
   Dim val_diff As Word
   Dim trigger_threshold As Word
   
   
initialise_chip:
   OSCCON = 11110000b 'oscillator = 32mhz
   WaitMs 10
   APFCON0 = 11000100b 'APFCON

initialise:
   trigger_threshold = 30

loop:
   
   Gosub readsensor_1
   'Serout tx, 9600, #adc_value, CrLf
      
   If adc_value > last_val Then
      val_diff = adc_value - last_val
   Else
      val_diff = last_val - adc_value
   Endif
   
   If val_diff > trigger_threshold Then
      If adc_value > last_val Then
         'this is a rising edge (remove glass)
         Low pin_led
      Else
         'this is a rising edge (glass added)
         If last_val > 0 Then
            High pin_led
         Endif
      Endif
      last_val = adc_value
   Endif
   
Goto loop
End

---

readsensor_1:

   'make pinA a digital output and set high
   ANSELA.4 = 0 'set to one for analogue, clear to zero for digital
   TRISA.4 = 0 'set to one for input, clear to zero for output
   High pin_a
   
   'point the ADC to pinA to allow the charge-and-hold capacitor
   'to start to charge up (RA4 is analogue channel 3)
   ADCON0 = 00001101b
      
   'make pinB a digital output and set low
   ANSELA.2 = 0 'set to one for analogue, clear to zero for digital
   TRISA.2 = 0 'set to one for input, clear to zero for output
   Low pin_b

   'wait for a bit
   WaitMs 1
         
   'turn pinB into an analogue input
   ANSELA.2 = 1 'set to one for analogue, clear to zero for digital
   TRISA.2 = 1 'set to one for input, clear to zero for output
   
   'point the ADC to pinB to complete the voltage divider effect
   '(RA2 is analogue channel 2)
   ADCON0 = 00001001b

   'read the ADC values
   Adcin 2, adc_value
   
Return

And the result works like this

The capacitive sensing pad is just some Bare Electric conductive paint - so it's got a relatively high resistance. The reason we made the "trigger threshold" a variable is so that different pad materials could be used, as well as catering for different glass thickness and even different fluid types that might affect the overall capacitance.

Do J20 and beer have the same trigger points? Would using copper instead of carbon-power-and-gum make it more or less sensistive? We'll have to find out....

But so far, it's all looking quite positive. It's only a simple test, but it's encouraging to see that the trigger stays set even after you release the glass - but it's not so sensitive that it triggers if you just wave your hand over the sensor.

Bungalow workshop roof and skin going on

With the carcass complete, it was with great excitement we received our B&Q order for 9mm and 18mm OSB sheets (the big 8' x 4' ones). Now, of course, everyone immediately screams "you DIY noob, only idiots use retail outlets for building materials!" But I've yet to find anywhere that sells OSB sheets for less than B&Q sells them for.

Jewsons charge more than £22+VAT per sheet. Stamco are about £20 a sheet (plus VAT if I remember correctly). Even our new favourite building supplies store, Chandlers (at Hove Lagoon) want £18/sheet (again, I think you have to add VAT to those prices).
But at B&Q a sheet of 18mm 8'x4' (2440mm x 1220mm) costs just over £16. Strangely a 9mm sheet costs only slightly less at just under £15 per sheet. But if you buy enough of them, they'll even deliver to the door for free (on a whacking great big palette which we've already got designs on to use as a vertical planter, but that's for another day).

Now let's forget the cock-up they made with delivery dates (a single order of 9mm and 18mm boards was delivered over two days). Or, at least, forgive it. Because no sooner had the sheets been delivered, than we were dragging them through the house, to the back garden and throwing them up on top of the bungalow.

The roof has plenty of rafters at 300mm and 400mm centres, under the 18mm OSB roof, making it super-sturdy for walking on (this is going to be really useful to get at the apples on our enormous apple tree!)

The 18mm boards are for the roof, and the 9mm sheets for the walls. While the rain held off and the fog lifted this morning, it's been bitterly cold - making holding, lifting and carrying 8ft sheets around really hard work. Nevertheless, it only took a few hours to make serious progress on the outer skin.

Steve's matra might be "measure twice cut once".
I measure three times, cut once, cut it again, throw it out, start again, cut twice then patch up the gaps with any left over bits.

The "twin width" roof proved trickier than I expected. But we did manage to measure and cut the last remaining bit of roof before the sun went in and it got too cold to carry on (I just got a bit click-happy, taking photos before we had actually finished!).

With the roof and two sides done (two of the trickier sides too, with all that cutting and shaping around the top) the remaining two sides should be finished and ready by the weekend. Hopefully the rain will hold off until we've managed to get at least the first layer of waterproofing in place (before the final cladding goes on). Then we can move inside, put some insulation up and hopefully it'll be a bit easier working "indoors".

Light up Lego droids

Having a Lego R2-D2 and not making it light up - after getting Luke and Darth Vader to hold their own light-up lightsabres - seemed like a crime. So I ordered some tiny 0805 LEDs in red and blue. At first it was tempting to just make R2's eye lens light up. But that's not how he works. There's a little light just below the eye lens, that - in the movies - flashes red and blue.

So I figured I'd do that.

(the little black dot immediately under the eye lens was originally a red dot painted onto the head, which I drilled out with a 0.8mm PCB drill bit)

(the back of the head showing where the tiny hole was drilled - you can just about see if on the right hand side of the inside of the head)

But soldering two 0805 LEDs without bridging the terminals proved to be really really tricky. So I got some twin LEDs in a single 0805 package. If truth be told, these are probably more like 0806 or 0807 sized packages - they're ever-so-slightly wider than a regular 0805 component. But not by much. Not much at all!

This turned out to be an exercise in hand-soldering really little components. And not even doing it the easy way, with a pcb and a hotplate and solder paste (where you can just heat the whole lot up and let the surface tension of the solder simply pull everything into place).

This was spot-soldering onto pads half the usual width of an 0805 LED. In short, it was tricky. But, after a few goes (and uncrossing my eyes a couple of times) I managed to do it!

the whole assembly is really tiny - smaller than it appears in the photo because I had to place the LED closer to the camera lens than my fingers, in order to get it to focus

So with a single LED with three trailing wires, I can make either the red or the blue side light up (or both for a reddy-purpley colour). Which means that next time I'll be looking to put the smallest PIC I have available into R2D2's tummy to toggle between the two colours.

For now though, I need to do something that doesn't involve keeping my nose just an inch or so away from the end of my soldering iron!

(the components will be held in place with translucent hot glue, acting as a light tunnel, so both the red and blue sides of the LED will be allowed to shine out through the drilled hole)

Bungalow workshop carcass complete

It was with great joy and an oversized mug of tea we celebrated completing the bungalow carcass today. We lost a couple of days due to poor weather, but made up for it when the sun finally came out.

It was, of course, still icy cold - but not cold enough to stall progress. On the north-facing wall, our 1.4m wide double-opening window is now securely in place (and it still opens!)

The triumph of the day was getting the massive, heavy doors hung on the frame.

We were expecting all kinds of problems, from the frame not being square, to the doors not closing properly or maybe swinging open on their own (from the frame being installed at an angle). But none of it came to pass. The doors behave just like uPVC doors should. The open easily, close properly and when you turn the handles, lock together and make nice, tight seal against the frame.

(ok, in these photos, the uprights under the double windows haven't been fully installed - but they're in place now and everything is good and sturdy!)

After installing the doors, we finished putting the noggins in the back and side wall(s) and left the structure ready for cladding with it's first outer skin.

(more over-enthusiastic photo-taking, as the diagonal braces are not in on the "high" noggins in this photo)

Despite the freezing weather, it was a good days work (ok, allowing for brew breaks, a late start because of the overnight frost, and freezing fog in the morning, and finishing before it got dark around 5pm, it was more like a good four hours work).

The OSB sheets should be here soon and we can get the outer skin on!

SD cards for microcontrollers

Getting SD cards that work consistently well with microcontrollers is pretty tricky these days. As users demand larger and larger volumes, the smaller, FAT16-compatible drives are becoming increasingly difficult to source.

Not that they're not out there.
Just that it's a bit of a minefield getting ones that work with a microcontroller - especially if it's for a pre-built module with slightly flaky firmware (I'm looking at you, WTV020 ad4 audio playback device!)

A few years ago, we built our own audio playback module. We documented quite extensively how to make SD cards work with a microcontroller. And just about every SD card pre-built module seems to work within the same parameters as ours:

FAT16 support means cards not bigger than 4Gb
Some Windows machines won't format 4Gb into less than 32kb clusters, so the actual maximum size is more like 2Gb.
A lot of 2Gb cards simply won't initialise in SPI mode, so stick to 1Gb max.
Most unbranded SD cards simply won't initialise in SPI mode - it's part of the SD card specification, but cheap suppliers sometimes don't support this backwards compatibility

We managed to snag a load of assorted SD cards off eBay - they worked out at less than a quid a go, so were pretty good value for money.

We freshly formatted each card, to use FAT16 and 16kb allocation units.

Except, of course, that not all of them worked in our audio playing device

(our device is not much more than an Arduino Pro Mini with a WTV020 sound module mounted directly on top of it)

Strangely, manufacturers that worked at smaller capacities didn't work at 512Mb. For example, when we first built the devices, we tried them out with an assortment of 128Mb and 256Mb SD cards. The only cards that consistently worked were Nokia and/or Sandisk branded.

Unbranded cards simply wouldn't initialise.
So we were surprised to find today that neither our Nokia 512Mb nor our Sandisk TransFlash 512Mb SD cards would boot up. But - strangely enough - unbranded cards (which ordinarily we'd avoid as they almost always fail to initialise) work perfectly.

Which goes to show - there's no rhyme or reason to which cards you should choose for your microcontroller projects. Don't just stick to one brand and assume the same manufacturers cards will work in a variety of different sizes/capacities. Similarly, don't assume that all unbranded disks will fail.

In short, you'll have to take a punt and hope you get lucky!

Bungalow workshop windows in

A few months back, we took the big bouncy van over to Kent and picked up some white pvc double patio doors and a large(ish) window for about a hundred quid. It was only when we got there,we were told the sale also included two upright windows (which had been fitted by the previous owner either side of their door frame).

So - after much deliberation - we decided to use them on the east-facing wall of the bungalow. It might make it look a bit weird from outside (as there'll be a worktop running along the inside, effectively cutting each window in half) but it also means plenty of natural daylight.

And, being east-facing, it'll get the early morning daylight but, in the summer months, shouldn't create too much of a greenhouse effect (since the sun is at it's hottest between around midday and 3pm by which time it won't be shining directly in through these windows).

We fixed the wooden framework around the sides of the window before placing it in the frame and screwing it to the existing structure. Neither of us had any idea if this is how your "supposed to do it" but it's a method that worked well for us. And the window feels good and solid, in it's new location.

We also got on with the back wall of the building, placing the necessary uprights (before it went dark, we managed to get a few diagonal support pieces in too). And the second window went in relatively painlessly too.

With the noggins in place, we're almost finished with the carcass of the structure. There are a couple of long rafters that need finalising and fixing in place, but we'll do those when we've the OSB3 sheets to put on the roof - which should allow us to position them to support any joins between the boards as necessary.

We've the large window to put in the north-facing wall, then the whole thing should be ready for the outer skin. Exciting indeed!