NFC Login 2.1


This should be the final revision. Just a few minor tweaks. The regulator is now a TPS77533 rather than an incorrectly footprinted TPS77333. Corrected the bi-colour LED wiring. Move to a smaller 4MHz crystal for the USB side of things. Shrunk it a bit as I’m no longer expecting to find any PCB bugs. Dropped the 0.1″ pin header as the TagConnect worked fine.

The most obvious change is the small capacitive touch sensor PCB on the front. The previous version searches for a tag a couple of times a second. This works fine but I don’t like the idea of all the 13.56MHz EM noise pollution. This will poll for touch frequently and if it senses a touch it will poll for tags rapidly for a few seconds. In theory it could be more responsive too. Not had a chance to code this yet though.

There’s is also a Login NFC the way to another implantee. I’d recommend you check out Hans Peter’s blog.

Laser improvements – coolant monitoring

One of the quickest ways to destroy a CO2 laser tube is to let it overheat. My laser has a very simple system – just a plastic breakfast cereal container of water with a small aquarium pump. It works well enough, but is easy to forget.

My first modification was to power the pump from the laser so that it’s running when the laser is switched on at the mains. That gets around the “oops, I forgot” problem, but doesn’t cover a pipe coming loose or the temperature creeping up. I really wanted to actually check cold(ish) water was flowing through the glass laser tube.

The water and temp sensor

The water and temp sensor

To check the flow, I started with a LM35DZ temperature sensor and a cheap water flow sensor from eBay. For neatness I carefully milled the flow sensor to embed the temperature sensor in it. This was then attached to the output of the laser. I intend to check that enough water is coming out and that it’s below a temperature threshold.

I etched a board with a MSP430G2533 microcontroller that counted the pulses from the flow sensor over a fixed timer period and used the onboard ADC to read the temperature. I set the threshold at 40C and 75% of the normal flow.

PCB with logic error

First PCB with logic error

My initial attempt used a MOSFET to pull the last signal down to GND if things were awry. Unfortunately this fired rather than disabled the laser! This was replaced with a 74LSxxx AND gate so I could force it to 5V and disable the active low signal. Some connectors to match the controller board meant I could drop it in with no rewiring.

The finshed PCB - with a fix for a lifted trace

The finshed PCB – with a fix for a lifted trace

So far it has been working well. (I actually finished this months ago but didn’t document it.) It’s not actually been needed to save my tube yet, but it’s nice to know it’s there. I might later and another sensor on the input and also flag if the difference between the two is too high. I could also add a cheap LCD display but that seems a little over the top.

Coolant monitor in the laser

Coolant monitor in the laser

As it’s a single sided board with some through hole pin headers, when it’s in place you only see the “boring” side of it with not tracks or components. Oh well – there’s no need for it to look pretty. Here it is in place. It’s a drop-in addition on the 6 pin cable to the controller board. It’s powered from the existing 5V line and simply forces the signal to fire the laser high (as it’s active low) when it senses trouble. From teh top you can see:
  • The connector to the sensor
  • A debugging / programming header
  • A currently unused connector for a screen
  • The connectors to patch into existing power and signals

If anyone wants a copy of the PCB layout or code, just ask.

Further PCB improvements

I’m glad to report I’m getting even better result with my PCB etching. Results do seem a bit up and down each time I get back to it but I feel I’m narrowing it down and getting more polished.

One. I’ve started using Ziplock vacuum bags to evenly press the transparency against the PCB. A really nice cheap substitute for a vacuum light box! I can thoroughly recommend them.

Etching underway

Etching underway

Two. I’m now able to get even better exposure with 3.5 minutes under the UV box rather than using the fluorescent tube. Maybe my developer solution used to be too strong. Maybe ditching the glass in favour of the Ziplock helped. Regardless, I’m getting a really good result. Here’s the partially etched board showing great definition.

Three. I got some Tin plating solution. No idea why I didn’t do this before, as it’s really easy and gives a great finish. The instruction say to carefully clean the copper with an abrasive. However, I assumed that as I’d just removed the etch resist from the copper with acetone and it had to be just about as clean as it could be. I popped it straight in the tinning solution before I got fingerprints all over it.

Etched and soldermasked board

Etched and soldermasked board

Milled, drilled and populated

Milled, drilled and populated

I had to mess something up and I removed the protective layer on the soldermask before exposing. Some of it dulled a little on contact with the transparency. Oh well. Otherwise I’m very happy. The ridiculously small negative text even came out a little. Here’s the board before drilling and populating. It a simple MSP430 based alarm for when my two year old son opens the front door. He can now just reach the handle.

When I’ve got it all sorted (and have tried the via rivets too) I think I might do a proper write-up.

First etched surface mount PCB

Well, after experimenting with CNC milled PCBs I decided to give etched boards a ago. As I have a “UV exposure box” (i.e. a cheap eBay thing for drying dragons’ fingernails) for the soldermask, I decided to try photoresist board. Odd then, that I found this was far too stong for the PCBs. Even a minute would appear to give a good exposure when I started developing the, but it turned out to be so overexposed I stripped all the resist off the boards. An expensive way to end up with plain copper boards.

Etching the boards was also not as simple as I thought. Sodium Persulphate seemed useless. Using Freeic Chloride and a sponge seemed too rough for the delicate photoresist. Sloshing Ferric Chloride didn’t do much until I heated it up – by putting the containing (including PCB) in the microwave. Anyway, enough of my woes and failed attempts.


PCB after etching

When I eventuallly got a combination that worked this is what I came up with. This is the board post-etch. It’s not quite as good as I hoped but definitely good enough for the MPS430 microcontroller (TSSOP) and the 0805 LEDs, resistors and capactiors.

ImageAnd this is the end result after applying the Dynamask 500 solder mask that I’ve used before.

I’m hoping I don’t ruin it when I come to try surface mount soldering…