SMD multi-use boards

Steve2381

Steve2381

Senior Member
Hello all
OK, Starting a new thread on this one.

I work in special effects. Mainly Pyrotechnics, but quite frankly... anything and everything gets built.
It's always last minute, and the deadline is usually yesterday. You have to think outside the box in this field.

I need to design some pcbs that will be as 'multi-function' as possible.
Their use? Anything... and I mean anything. Usually basic timing setup's, or some kind of logic control.

Example... few weeks back, got asked to build a 'Banksy' style picture frame for the MTV awards in Bilbao, Spain..... here it is in action...


All that work for literally seconds in action! We bought a commercial A3 shredder.... threw away 99.4% of it and simply had the paper shredder mechanism with all its safety's removed/cheated. Then a push button operated an 8m2 to time it half way down.
Build a frame with some crap timber from Wickes. Then I took it to Spain!

This week.... 5x 6m flames to fire out of the UK tour War of the Worlds tripod. Love my job!!

Anyway... I digress.
This board ideally wants to be as small as I can get it. I am going to have 08m2, 18m2 and a 40x2 versions.

Lets start with an 08m2.

Supply is nearly always a clean 24v DC.

Input pins will be selectable with jumpers for either 10k multi-turn pots, or as direct signal inputs.
How best to achieve this? My inputs could be any voltage >9v and <30v, so I am thinking of using SOT223 5v regulators as buffers. Will that be OK?
Other option is opto-isolators, but then I need to work out how to use those with varying supplies.

Outputs usually switch SSR's, and they don't need to be on board, but I need to switch in the positive. So I am thinking of switching a BC848 SMD transistor (npn BC548 in SMD) which in turn, switches a MJD127 (pnp TIP127 in SMD).
Basically, SMD versions of what I usually use for my stripboard builds.

5v SOT223 regulator on the input. Breakout header for I2C if I need it.
Header for programming. Jumper to disable the serial input pin and make it useable.

But... not having much SMD knowledge, I am spending forever on Google and Farnell working out what is what.
I have an SMD workstation, and been having a practice.... seems OK.

Is there any hard and fast rules for SMD. There are SO many parts. I am using Diptrace, but the library is really hard to find parts in.
I am basically going down the route of 1206 sized capacitors and resistors for starters, as I don't know any different.

Any pointers.... gratefully received.
 
techElder

techElder

Well-known member
Steve, is there any chance you are going to share the results of all of this with the forum members?
 
lbenson

lbenson

Senior Member
If you're trying to go small on your board, I'd recommend a 3-pin header for programming--much smaller footprint than mini-stereo socket. It's what I always use. You can make an adaptor to go from your mini-stereo plug to the 3-pin header.
 
Steve2381

Steve2381

Senior Member
Already gone for the 3 pin header (keyed) for the programming port. I don't like the jack sockets.
Totally happy to share! I will no doubt mess up the first one or two, but I can upload any progress I make.

Not used Mosfets (well... not much). I purely need to switch a relay, so open to better suggestions than my usual BC548 and Tip127 route.

Are we happy with my plan to use a SOT223 5v regulator, maybe resistor divided down to say 4.2v as an input? That gives me a pretty wide range of input voltages.
 
Steve2381

Steve2381

Senior Member
Found these Mosfets. Not knowing what I am looking at really, but from what I understand from the specs... these will switch fully on using the 5v out of the Picaxe.
What is the advantage of a Mosfet? I get pretty reasonably switching through the TIP127. Plus, I can't find an equiv version of the Mosfet above in SMD.
Also, I think I am going to drop my resistor sizes physically down to 0805. The 1206 seem really large now I have started designing the board

http://www.farnell.com/datasheets/1690197.pdf
 
Hemi345

Hemi345

Senior Member
For an opto, check out the VOM617A (SOP-4 package)

For a load switch (high side switching), check out the DMC25D0UVT (small SOT23-6/TSOT26 package)
Or bigger DC loads or AC loads, check out the DIP-6 based ASSR-1611 or VO14642AT or VO14642AA (surface mount SO-4)

I don't know how good the libraries are for Diptrace, but Eagle has a ton of libraries (see my Sig for the PICAXE symbols and footprints). Sparkfun just redid their Eagle library which has a bunch of common parts too. Does SnapEDA have Diptrace footprints?

I built up quite the supply of 1205 caps and resistors because I was soldering everything with an iron. Then I discovered the skillet/frying pan method and have been using 0805 parts whenever possible. I solder the top side using the frying pan, then solder the bottom side by hand (if needed) using 1205-sized parts. Skillet method allows me to solder 0.65mm+ spaced pads pretty easily.
 
Steve2381

Steve2381

Senior Member
Skillet/Frying pan method? I shall have to investigate that.
Trouble with SMD pcbs (I have found already)... single sided boards are almost impossible to design, as you can't use components as bridges (well.. you can't get much under a component).
Either a few jumper links, or we will have to go dual sided.
 
Hemi345

Hemi345

Senior Member
I should clarify. What I meant to say is I try to place most components all on one side. All my PCBs are two layer utilizing vias. Check out some of the pics at the bottom of my Humidistat thread in the finished miscellaneous projects section for my latest PCB.
 
Steve2381

Steve2381

Senior Member
Yes... I saw those Hippy. Quite expensive however. Its 4 times the cost of the chip, and I don't need the jack socket.
Might get one to play around with however
 
Buzby

Buzby

Senior Member
Steve2381 said:
Yes... I saw those Hippy. Quite expensive however. ...
Click to expand...
1) Is it more expensive than the cost of your time developing and troubleshooting a replacement ?

2) Does an extra £10 add much to the cost of a Martian Tripod ?

If most of your I/O devices ( terminal strips, SSRs etc ) are non-SMD then the space saving you will gain by 'rolling your own' will be negligible compared to the space used for a AXE201, even with it's jack socket.

If you can accept using a little more space, the AXE401 with off-the-shelf A*******o shields, or an AXE405 for custom stuff, would give you rapid results for 90% of your projects.

Although it would be cool to design a generic board, I suspect it will soon need bodging to fit the requirements of a job you didn't see coming !.

Cheers,

Buzby
 
Steve2381

Steve2381

Senior Member
I totally see where you are coming from.
If I got an AXE041..... surely I may as well go with an Arduino Uno of the same physical size? (and without being tight.... I can get two for the cost of an Axe401 assembled).

The reason for my 'tinkering' with our own design was so that we did not have to add external transistors or Mosfets onto the pins to give us the switching power for out SSR's, valves etc.

Same goes for inputs. It was so that I could simply insert a jumper, or bridge a link to enable an onboard pot or digital input.

Plus... gives me something to do on a winters night!

Minor update...... just found these..
http://www.farnell.com/datasheets/1874730.pdf

Do you think they could be configured to be the controller for the input/output pins? I could then select whether they are a digital in, analogue in or a digital out. Hmm
 
E

eclectic

Moderator
Steve2381 said:
I totally see where you are coming from.
If I got an AXE041..... surely I may as well go with an Arduino Uno of the same physical size? (and without being tight.... I can get two for the cost of an Axe401 assembled).
Click to expand...
A naïve question.

Do you fluently "speak" the A** language?
Or can you get "translation" help fast?

Lots of folks here, (not me),
"speak" extremely fluent PICAXE.

e
 
Steve2381

Steve2381

Senior Member
A** ? That could be short for lots of things!
If you mean A******s... then yes, use them all the time. Just like Picaxe for the small footprint of an 8 pin DIL for instance.
 
techElder

techElder

Well-known member
Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino Arduino
Arduino Arduino

Will anything happen to me if I put "that word" in a post 32 times? :D
 
Buzby

Buzby

Senior Member
Steve2381 said:
Minor update...... just found these..
http://www.farnell.com/datasheets/1874730.pdf

Do you think they could be configured to be the controller for the input/output pins?
Click to expand...
That's a neat chip, but is it really what you need ?. It has two selection pins to select three different signals.

If you drive these selects from your Picaxe then you are using two outputs to define the purpose of a single input. You will run out of pins real quick !.

If you drive them from two sets of jumper pins then your required board space is 'two jumpers and a chip', whereas if you just use jumpers your board space would be 'three jumpers', which is probably smaller.

My take on your problem would maybe a board with the Picaxe and it's support components, with an area like a 'sea of holes' tracked for different optional components. Then as each project comes along, you populate the relevant holes with the required components. No need for jumpers.

My gut feeling is that if you want a single 'good for all' board, then you are going to trade off size for flexibility.

Cheers,

Buzby
 
Steve2381

Steve2381

Senior Member
I am continuing my research!
Ideally.... I am trying to find that chip, but I2C controlled and addressable (found one I think). Then I am heading in the right direction, as I can control a lot more options one just the I2C bus.
 
techElder

techElder

Well-known member
hippy said:
If you do it enough times it could. The forum isn't here to promote other people's products or to facilitate disruptive behaviour.
Click to expand...
So, is it any better to put “A******s” 32 times? Don’t we really all know it means the same thing? :D
 
Steve2381

Steve2381

Senior Member
I have (what is a probably stupid) question...

These digital/analogue switches I have found in various formats have different 'on' resistances.
Is this going to affect their performance as logic switches or outputs?
I can understand that the analogue versions are going to produce different results than expected due to this resistance, but is it really going to be a problem?
 
W

westaust55

Moderator
Based on the digital/analog switches you earlier gave a link to,
The max Ron resistance is 15 Ohms and typically half that.

For an input circuit which will only draw around 1 mA (maybe microamps) that represents a very small volt drop across the switch and would have negligible/ no effect upon input operation.
 
Steve2381

Steve2381

Senior Member
Thanks Westaust.
I have found some tiny 8 way switches that support analogue signals. They are madly small, and this is now totally a 'to see if I can do it' operation.
But.... how the heck do you design pcb's with these small (or in fact, any sized) TQFN package IC's? (8 pins on each of the 4 sides of the chip)
No matter how you orientate the chip, you just cannot wire to all the pins!
 
Buzby

Buzby

Senior Member
To make it easy you need a 4 ( or more ) layer board, that's how the big boys do it.

Most PCB layout packages support multi-layer, and most fabricators can supply, but the cost is a bit higher.

Personally, I've never done anything smaller than 0.05" DIP spacing, and I've never really needed 4 layer boards, so I can't help you with the details.

However, I would have thought a standard 2-sided layout would suffice, especially as the package doesn't need pins through the board, thus leaving a lot of space on layer 2.

Cheers,

Buzby
 
Steve2381

Steve2381

Senior Member
Well.... it's a learning curve. Almost a relaxing chill puzzle kinda thing now. Sit back and try and work out the routing.
I don't really understand how you determine the width of the tracks for current carrying. I am basically going for 'as wide as possible'.
The widest you can have with the TQFN package is 0.2mm. That seems madly small, but I suppose it's switching limit is 200mA.
 
rq3

rq3

Senior Member
Steve2381 said:
Well.... it's a learning curve. Almost a relaxing chill puzzle kinda thing now. Sit back and try and work out the routing.
I don't really understand how you determine the width of the tracks for current carrying. I am basically going for 'as wide as possible'.
The widest you can have with the TQFN package is 0.2mm. That seems madly small, but I suppose it's switching limit is 200mA.
Click to expand...
A trace on 1 ounce copper carrying 0.2 amps would need to be about 0.002 inches wide. Most board houses won't accept anything under 0.006 inches; some call it quits at 0.012 inches. 0.2mm is about 0.008 inches.

Do a web search for "trace width calculator". There are billions of them on line, and they will all get you in the ball park.
 
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