Project: Smart Watch – Making PCB and Successful Soldering :)

Taking on board what I learnt yesterday, I repeated the procedure of making the PCB. This time the PCB came out better than before, with no bad connections šŸ™‚

Soldering went well at first; all LEDs, resistors and switches went on with little problem and all work as expected. The trouble came when I was soldering the jumpers. I wish I had thought this through a little bit more before-hand and used through-the-hole, but nonetheless it is now working. I used Sellotape and printer paper to insulateĀ the underlying tracks and then soldered the jumpers. After a number of worrying failures and a little bit of lost copper track (fixed with solder), it was done.

The jumpers are messy, but I couldn't be happier with the other passive components.

The jumpers are messy, but I couldn’t be happier with the other passive components.

The only concern I have now is that when I solder the bluetooth module to the pads on the right-hand end, the jumper will unsolder and/or break. Fingers crossed.

Yay, the LEDs work.

Yay, the LEDs work.

Project: Smart Watch – Making PCB + Destroying PCB (oops)

Today was the day I etched the (first) PCB. I began by printing the design onto paper and ironing the traceĀ onto aĀ copper clad board (remembering to mirror the image because it is SMD).



I then etched the board in Ferric Chloride, washed it, cut it to shape and drilled the necessary holes.


After multiple rounds of checking connections and scraping off incorrect connections it was time to ATTEMPT to solder the SMD components.

I began with the resistors. What I had done before was to pre-solder the pads before placing the components down. Massive mistake. This deposited loads of solder on each pad meaning I had to reheat and “solder-suck” repeatedly resulting in traces melting off the board and ruining it šŸ˜¦

Accepting that this was a lost cause I chose to practice different methods of soldering.

I finally came upon the perfect method: Flux on the component, a tiny bit of solder on the component, flux on the pads, hold the component down with a sewing needle and heat eachĀ pad. This resulted in really neat joints which somewhat made up for the disaster.


Sorry for the poor image quality

Perhaps you can just make out some very neat resistors to the left and right and a complete massacre in the centre. I think this speaks for itself.

I will repeat this process again tomorrow, hopefully with happier results.

Project: Smart Watch – First Thoughts on Case

Perhaps a bit prematurely, I wanted to start designing the case for the watch.

The main principlesĀ of this case will be:



Attachable/Removable to/from a watch strap

Two contacts to allow for power input.

Ideally I would like to 3D-print the case, but since I don’t have a 3D-printer I need to make sure I get it right first time because it’s pretty costly to buy individual items online. (Or perhaps I could get access to one at uni?)

Therefore I have decided to first make a prototype of the case by layering up 1mm thick cross-sections of plastic.

I used PTC Creo Parametric 2.0 to design a first draft of the case and this is what I came up with:

PTC Creo design. Left to right: Hidden line, shaded, 3rd angle drawing

PTC Creo design. Left to right: Hidden line, shaded, 3rd angle drawing

So, this design includes:

Floor/short-edge panels at each end for a large metal contact set into the body. A small screw will pass through the body to connect GND and VCC to the outside of the strap.

Outer edge panels (each with a small screw) to connect the transparentĀ top cover. Having not actually designed the cover yet, this part may well change.

I’m pretty pleased with the design. Possible modifications include thinner walls, thinner pannels… and everything.

Project: Smart Watch – Circuit Board

The watch has to be skinny, and compact. I wanted as many LEDs as possible and a minimum of 2 switches, but 4 would be best.

In this vein I delved for the first time into surface mount (SMD) components. In the end I ordered 5 red, yellow, green and blue SMD LEDs from ebay as well as some nice SMD 680Ī©Ā resistors and some titchy little SPST switches.

Tiny SMD Switch

…Talk about tiny…

I hadĀ toĀ check that I can even solder these, so I grabbed some strip board and my soldering iron.

I have to say it wasn’t as bad as I thought! Although I have no idea how to tell the polarity of an SMD LED. Google? So that includes a switch, an LED and a resistor – nuts. It runs at about 4mA according to my ammeter, which is on the boundary of acceptable, and if anything its too bright!

So time to design the PCB. I’ve etched a number of circuit boards at home before, but it’s never gone well. I always set my sights too high and make it too compact for home etching. I wish I was still at school with proper etching equipment.

My PCBs sometimes work out alright, after a few attempts

My PCBs sometimes work out alright, after a few attempts

Hopefully, being a small circuit board, it wont be too much of a gamble as to how well it etches.

Before I have always used Express-PCB to design my boards but since it isn’t installed on my new PC, I took that as an excuse to try out Eagle PCB.

My (hopefully final) design is as follows:


So this design basically includes 20 DIL Through-the-Hole pins to allow the arduino to connect by its header rails and 4 SIL TTH pins to allow the bluetooth module to connect by a header. Everything else is surface mount. There are 4 switches, 6 LEDs and 6 accompanying resistors. Begrudgingly I had to use jumpers as far as I can see in order to get the power to the bluetooth module without increasing the area of the board. On the plus side, the arduino has internal pull-up resistors meaning I don’t need extra space for them. The rectangle represents the area I will remove to make room for the micro-usb port on the arduino.

I hope to etch it next week.

Project: Smart Watch – Basic Layout

So I had two basic options as to how to lay out the internals of the watch:

Stacked or adjacent.

A stacked layout will provide a skinny appearance but will be taller and potentially more annoying.

An adjacent layout will be wide and flat which could be equally annoying.

After playing with some cardboard models etc I decided that it would be best to stack the design.

watch structure

This is the design I settled on. I reckon I can get it down to around 7mm thick which could be ok.

The battery will be mounted on the underside of the wrist with wires coming through the straps. (Probably won’t work).

Projects: Smart Watch – Power

So, I spent a LONG time looking around the internet for suitable batteries. They had to be small and have decent capacity.

I decided 150mAh was the minimum. And in the end I settled on these little 3.7v Lithium-Polymer cells with 250mAh.

250mAh LiPo Battery

Since these are only 3.7v each I either have to put two in series OR use a voltage booster to achieve the 5.5-12V range.

A nice little voltage booster like this should do.

The batteries haven’t actually arrived yet – I ordered them from China so it could be a week or two still before they turn up. There’s no point buying a voltage booster until then.

But I actually had very little idea how much my circuit used. So I tested it using my ammeter and as it turns out, the Bluetooth module uses 5mA when connected and idle, ~20mA when connected and transceiving, and ~35mA when unconnected.

5mA is acceptable and so I will do my best to keep data transmission to a minimum in order to minimize power usage.

I was pretty shocked to find that the arduino uses around 30mA when running even the simplest of loops. And so I spent a couple of days sorting this out.

I found that if I put the arduino into “sleep mode” the power consumption dropped to about 0.5mA which is much more up my street. Luckily, the serial pin on the arduino could be used as an interrupt to wake the module up from sleep, so all I have to do is send some junk data to the arduino so it wakes up in time to catch the real data.

However, in the periods when the arduino was awake, ie when flashing LEDs, transmitting data etc it still used about 30mA which is pretty unacceptable.

By reducing the clock frequency from 16MHz to 1MHz, this can be reduced to around 16mA which is just about acceptable as hopefully it will be asleep most of the time. I daren’t reduce it any more.

So, as it turns out there are approximately 3 general states the watch can be in:

Completely idol using approximately 5mA

Idol but with the bluetooth module awake using approximately 16mA

and fully going for it at 32mA.

Overall I estimate that the average usage will be around 8mA which should give a battery life of well over 20 hours. Yay.

On top of this will be the LEDs, but by using largish resistors I hope to keep the current down to 2-4mA each and they shouldn’t be on too long anyway.