Tuesday, 10 December 2013

Best Case for the Job

Prototype Follow Up

I love the range of cases from Hammond Manufacturing. The only draw back with these cases is that in one off quantities, they are expensive.

What I love about the Hammond cases is that Hammond supply excellent 3D cad models of all their products, a you can use these to help created great looking project mock ups before you spend a cent.

Hammond 1593NBK Step Model in Altium

Wednesday, 27 November 2013

Dangerous Assumptions

When You Assume....

Back in the start of my Alarm interface project, I ran into problems with my current limiting circuit.  My little 3A FET would go up in smoke with limited to 500mA and I just put this down to a need to heatsink the FET.  The thinking here is that passing 500mA when switched on was too much for the little SOT23 package.

I was wrong!

But before I knew that, I decided to beef up my circuit, switching the PMV48XP for a TD2955PT4G which comes in a D-PAK package, and rated to run with a Heavy Duty 12A steady state current.  500mA should be nothing for this beast, and the bigger case might compensate for the lack of heatsink (I was trying to get a physically small solution).

However, the only thing I knew was that I was going on guesswork here.  I didn't have any hard maths that proved what I was thinking was correct, and rather than commit to winging it on my next run of Seeed studio PCB's I built a test circuit to check it out.

Rather glad I did that now!

High Side Limit Schematic

High Side Limit PCB

Sunday, 8 September 2013

Finishing Up

Final Tests and Code

Listening to Others

I've made no secret that I took my inspiration from Lior's Alarmino project, and I was quite pleasantly surprised when he contacted me on one of my posts.  He suggested that I take a look at the value of the filter cap I'm using on the 4V rail supplying the SIM900.

In his design, the bulk storage cap was charged by a linear regulator that delivered a maximum of 1A, but as my design uses a switch mode that's (nominally) rated to 3A I figured I could go smaller with my filter cap as my circuit can deliver a higher rate of charge - even with the substitution of the eBay sourced LM2596 circuit.

Thank You eBay!

Tuesday, 6 August 2013

Design and Test, The Third

Filter Testing and Options


In my previous posts, I'd debugged my power supply, and got my Arduino Leonardo clone up and running.

I'm getting closer to finishing but there's still one functional block that I'd not yet tested and it's my active filter.  Again, a big hat's off to Lior for the inspiration behind my build, but I have done some things my own way.

For example I used a Sallen-Key low pass active filter to get a 1.5kHz Low Pass Filter.

Thanks to the internet, rather than hand crank the numbers, I used this on-line calculator from OKAWA Electric Design. From that design I whipped up this LT-Spice File.

LT-Spice Circuit (Active Filter)

Design and Test, Part Two

Testing Day Two

As mentioned before, I'd tested and debugged my hardware to the point where it's time to start integrating the systems.  Or in other words get the micro talking to the rest of the board.
Notice Something Strange?

Friday, 26 July 2013

Design and Test

Testing, Day One.

It's been a while (a large part was waiting on parts - I'm looking at you Futurelec!) since I posted about my Alarm Monitoring system but as I've finally been able to get some build and test underway I thought I'd share the progress.  Which is a nice way of saying I'm not finished yet.

Step By Step

With this design I've incorporated a fair few new circuit blocks I haven't tried before, being:

  1. FET Current Limiter Circuit
  2. 4V, 3A (TWELVE WATTS BABY!) Switch Mode power supply
  3. AVR microcontroller (yes, never used 'em before)
So to start, I loaded only the power supply section before placing other parts.  This is a good practice when prototyping as if you stuffed something up, you don't blow up the rest of your board.  I've actually designed in zero ohm links on the power supply rails which could also prevent the same destruction but through experience I prefer to do it his way.

Zero ohm links are good for when you are setting up production boards.  You can adjust anything that needs adjusting in isolation before installing the links.

But for prototypes - Why load everything on your first prototype when a SNAFU in any area could make the whole exercise pointless?

Partially Loaded

Thursday, 25 July 2013

Polyswitch Current Limiting

Polyswitch Testing

Just a quick little video I took while testing my Raspberry-Pi protection circuits.

To recap, I was testing the current drawn when a 5V1 zener clamp is protecting an input from an over volt condition.

Monday, 22 July 2013

iPhone 4 Screen Teardown

Reduce. Reuse. Recycle!

In an effort to get fit, I'd been going for long walks.  Anyway, I enjoy listening to podcasts while doing so, and I did this on my trusty iPhone 4.

Or should I say 'rusty'  The sweat in my pocket payed havoc with the end connector and when some obvious (non-photographed) corrosion set in I couldn't charge my phone.  Eeep.

So out with the contact cleaner and toothbrush and I could soon charge my phone again, but the contact cleaner seeped up into the phone and left cloudy dark spots on my screen, just like this. Again I'd taken no photos of my own at this stage.

But at the end I got some sweet LEDs to play with!

Power Supply Protection for Raspberry-Pi

Defining the Problem, Again

The power rails of the Raspberry-Pi are bought out on the GPIO header as well as the IO pins.  This is both a blessing and a curse.

The advantage of this is that you have 5V and 3V3 rails available to power your circuitry that you are hooking up to your GPIO pins, but again, as the Raspberry-Pi doesn't have any protection circuitry you run the risk of killing your Raspberry-Pi through:

  • Short Circuiting the Rail
  • Reverse Biasing the Rail
  • Over-Volting the Rail

Friday, 19 July 2013

GPIO Protection For Raspberry-Pi

Or How Not to Let the magic Smoke Out

Previously I've written about my love / hate relationship with Polyswitches, and discussed some input and output pin protection methods.  I was looking into this so I could make a breakout board / protection device for experimenting with my Raspberry-Pi and not let out the magic smoke.

I could say it's to help kids / beginners protect themselves from basic mistakes, but even after practicing Engineering for nearly 20 years I have learn that Murphy still bites, so it's also a lesson in self preservation :)


Friday, 12 July 2013

I've Seen The Light

Unintended Consequences

Around 4 years ago, we built our house.  The builder delightfully quoted an extra $1500.00 to convert our standard garage roller doors to remote controlled units.  While recovering form the shock of that addition I was strolling though our local Bunnings when I came across these DIY kits.  

If I recall correctly, they were even cheaper back then, and I managed to get a pair for under $500.00

So a little DIY later, I was up $1000.00.  Win.

Installed Roller Door Remote Controlled Motors.

Cashed up, with remote controlled roller doors.  Life was good.  But something was missing....

Yep - no lights were fitted to the roller door motors.  So when I parked in the garage at night, and closed the door, it was a bit hard to see.

Tuesday, 11 June 2013

Stencil Success!

So I'd managed to knock up a pair of solder paste stencils, and now it's time for the moment of truth:  can I actually manage to lay down pastes and successfully solder some parts on?

Plastic Not-So Fantastic

In my last past, I made mention that I'd only intended to test the plastic stencil if the metal one failed.  Well, I got in-touch with my inner feminist and decided to start with the plastic stencil after all.  Why?  Well, the plastic sat flush with the boards, and I was thinking I'd probably not need the top frame in my Jig.

Sitting Pretty

Tuesday, 4 June 2013

Stencil Mania

Living in Australia....

...has a whole load of advantages, but one thing that sucks is that we are far from everyone else.  So once I've ordered my parts, I have to wait a while for the bits to arrive.  

And that usually gets me thinking.  Well, reading articles on the internet is kind of like thinking....


There are some pretty talented and smart people out there, and I've recently read about some examples of DIY Solder Paste stencils.  Both of these guys have achieved some pretty impressive results, and with my upcoming projects, I started to think that the use of solder stencils might just save me some time.  

While I can hand solder SMD parts, and when set up I've been able to knock off over 100 parts an hour, it's bloody fatiguing.  Plus I have this hot air tool I picked up from eBay that I haven't tried yet either... so I began to hatch a plan.

However, rather than muck around with etching and all the drama that comes with using chemicals (e.g. negative WAF) I thought I'd give it a shot milling out a stencil at work.

Sunday, 2 June 2013

Double or Nothing!

Double or Nothing!

As I talked about last time, in my last Seeed order I organised multiple designs in the one PCB service.  In this blog I want to let you know how I went from this:

Two for One

To This!


Monday, 20 May 2013

Seeed-ing. And Not the Bit-torrent Kind.


That's my opinion regards Seeed studio and their PCB Fusion service.

As I mentioned in a past post I've been working on a new project, and I've ordered the boards from Seeed. In fact, as I'm a tightarse (is it an just an Aussie thing?), I waited until I had a few projects to order.  You see, if you just order one $25 project, it costs you an extra $40 in shipping, but with Seeed you get *free* shipping for orders over $50 - and it's registered Air-mail shipping too.

So that's the first big plus.

Tuesday, 30 April 2013

Why We Test

The Magic of FETs

I've never had a lot to do with FETs before so I've been keen to experiment with and learn about FETs on my home projects.

As I wrote before in Basic Protection I like the simplicity of using a P-FET to provide reverse bias protection, and with my Alarm Interface I was planning on combining this reverse bias protection with current limiting.

As usual, I got it wrong the first time....

Thursday, 4 April 2013

Cutting out the Middle Man

Home Alarm Monitoring

I have a home alarm system, and it's reliable and the monitoring company provides a good service.  For a bit less that a dollar a day, if the alarm goes off, they give me a call.  

Each and every time it goes off.  

My system has multiple zones, and my thinking is that if only the one zone is triggered, it's probably not a burglar in the house.  And I don't want my long weekend away spoiled by my wife continuously worrying about the single zone alarm that went off 2 hours ago and we are not going to be home for two more days.

I've requested with the monitoring company to *not* call me when it's only one zone, but policy is policy and they need to call.  Fair enough.

Saturday, 30 March 2013

Basic Protection


For a recent project I'm working on, I wanted to add some basic IO protection.  The requirements for this were:

  • As usual, it had to be cheap
  • It had to be bi-directional.  The pin to protect was configurable as either input or output.
  • Provide short circuit protection
  • Provide over-volt protection
  • Provide reverse bias protection
  • It also has to be cheap

Monday, 18 March 2013

Missed it by *that* much...

I've always loved Get Smart.  I always felt just a little bit sorry for Max, when he'd try so hard and *just* miss out... but then you'd cheer when it all came together for him in the end.

It's a bit like how I feel about Tag Connect.

Sunday, 17 March 2013

Awesome Coffee

So my mate, Mark, wanted to roast some coffee.

Did I mention he own's his own Cafe and wants to roast 10kg's at a time?

Funny thing is, whenever he got into a conversation about making his own roaster, just about every time someone would say 'That's Awesome'.  So the mighty roaster was named.


Told you it's Awesome!

The Not So Obvious Telephone

Yep, after practising engineering for 18 odd years, I'm still (too) often dumbfounded about how simple things work. Like the telephone. Been around since 1876, and didn't require electronics as we know it to work - just batteries, wires and coils.