Shop OBEX P1 Docs P2 Docs Learn Events
Reducing a circuit as small as possible? — Parallax Forums

Reducing a circuit as small as possible?

MorrolanMorrolan Posts: 98
edited 2007-09-26 08:06 in General Discussion
Hi guys,
I've recently got back into electronics after a 10 year gap - I'm now trying to turn my attention towards a small project, and by small, I mean smallest physical size that I can manage...

Without building a stamp myself from a kit, (that can wait for version 2!) I've trying to create a circuit in the smallest physical package that I can.

The circuit contains:

1 BASIC Stamp 2
1 8-pin IC
6 resistors
1 PCB mount tactile switch
2x header pins for switch 1 (called CON in schematic)
2x header pins for switch 2 (called EXT in schematic)
2x header pins for 9v supply (battery)
3x header pins for servo

The attached image will make it clearer.

NOTE: The pins will ultimately be soldered in at 90 degrees to the board so that the pins stick out sideways, not upwards - I just can't find it in the Eagle library yet.


And now to my questions -

How small can I make my traces? I'm using eagle, and setting the autoroute routing grid to 10 (it was in an eagle tutorial) from the default 50, and the traces look miniscule?
Also, how much gap should I leave between traces?
How close should the pins be for a standard 1/4W resistor (2.54mm, 5 mm?) Is it possible to use smaller physical resistors in the circuit without going as far as SMT components?
How safe is it to solder a BS2 direct to a PCB without a socket?

The board will be professionally made by a company called Eurocircuits - they are excellent within the EU (RoHS compliance stops me importing most hobby-boards from the US) and they also do a service for hobbyists at www.thepcbshop.com - including full soldermask and silkscreening. Sorry for the plug, but they seem to be the best available within the UK, and I'm sure I'm not the only stamp user in the UK!

My questions might seem a bit newbish, but it's been so long that I've forgotten so much!


Many thanks in advance,

Morrolan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Flying is simple. You just throw yourself at the ground and miss.
Design_v1.PNG
614 x 783 - 24K

Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2007-09-24 16:24
    There are some surface mount 8-Pin IC options that would allow you to tuck that and all of your resistors underneath the BS2 module and make it really compact, where all you would have are the
    connectors and switches that could be placed in a nice·SIP style row alongside the BS2.
    ·
    One thing I noticed right off... you might want to double check your wiring.· ...the servo connections 1 & 2 in your schematic are both VSS.
    ·

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2007-09-24 16:27
    Well, you can make the traces quite small, but my experience is that this can be a waste. Two not so good things occur. One is that you have to use more etching solution to eat out all that copper. And even worse, it is far easier to over etch fine wires and have a useless board.

    Try approaching it as making the traces as large as possible and still fit the need to be compact. This will make manual construction much easier.

    Generally, this means I need a width that will just fit between two adjacent IC pads.
    Using #16 on the wire size seems to work well for me. Of course I usually make the power lines more hefty and I often flood unused areas with copper fill as ground plane.

    Regarding resistors, Eagle gives you dozens of options. It seems to be overkill.· I have 3 basic choices - 1/4 watt, 1/8 watt, and largest surface mount.· You don't really need 1/4 for 99% of IC work, 1/8 watt is fine.· But sometimes it is easier to get precise values in a 1/4 package.· You can either stand the resistor on end or lay it flat.· Surface mount is often more handy, but having to solder something the size of a pencil dot is crazy - so I use the largest ones.· I am getting old and my hands shake.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········
    ···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

    Post Edited (Kramer) : 9/24/2007 4:35:13 PM GMT
  • BeanBean Posts: 8,129
    edited 2007-09-24 16:41
    1206 resistors are easy to solder.
    And you can run a trace through them if you have to.

    Bean.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    I know what I know, don't confuse me with the facts...
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    www.hittconsulting.com
    ·
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-24 16:57
    Ditto to Beau's suggestion for putting stuff under the Stamp. Use a couple tall 12-pin SIP socket strips for the Stamp. I don't think you need to go the surface-mount route, unless you really want to. An 8-pin DIP will fit under the Stamp handily. And if you use 1/8W resistors, they can be bent transistor-radio style and soldered into holes on 0.075" spacing, if you don't need traces running between, or 0.1" if you do. These will also fit under the Stamp, assuming the socket strips are high enough.

    If you're having your boards fabbed professionally, check with the fab house to see what the minimum conductor size is. Typically, for through-hole stuff, 0.012" for the traces, 0.062" for the pads, and 0.035" for most holes (0.042" for header holes) will get you where you want to go. If the board company allows it, 0.008" traces on 0.008" spacings is not uncommon and can help with the tight spots.

    BTW, I think you will need pullups on your switch inputs.

    -Phil
  • MorrolanMorrolan Posts: 98
    edited 2007-09-25 09:04
    Thanks for the great response, I'll see if I can go through things point by point over everyones suggestions!

    1. Beau

    8-Pin IC's.
    The problem I have is that I the IC I'm using I am buying through a third party company, and they only sell an 8-Pin DIP version, no SMT. However, it may be possible to solder this directly into the board and mount the Stamp above this in a socket.

    SIP-style row
    Edit: Google image search is your friend! I was considering using these style sockets, but is it possible to get hold of 90 degree PCB versions so that wires/plugs stick out sideways, not upwards?

    Servo Connections
    Thanks for this, I've realised that in my schematic I've linked to VSS instead of VDD for a 5v supply!




    2. Kramer

    1/8W Resistors/SMT
    Noted, and thanks!

    Ground Planes
    I'm trying to do this in Eagle at the moment, but for some reason it doesn't connect GND to the copper-filled plane, even though I'm doing it as suggestion in the tutorial from the Cadsoft website?
    If anyone could help me with this it would be greatly appreciated.


    3. Phil Pilgrim
    Traces
    Thanks for the help with the spaces and tolerances, although I'm a metric man, so I need to get my conversion calculator out now smilewinkgrin.gif

    Pullups on Switches
    I'm curious why you think I need pullups on the 2 switches? Is it because I screwed up the schematic and have linked everything to VSS instead of VDD? tongue.gif



    Thanks everyone, your advice is proving to be invaluable!

    Morrolan

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Flying is simple. You just throw yourself at the ground and miss.

    Post Edited (Morrolan) : 9/25/2007 1:09:03 PM GMT
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-25 15:05
    As to the switches: when the switch is closed, the input pin will be at ground potential and will read as "0". But what happens when you open the switch? Without a pullup resistor, the input will float and could read as either "1" or "0". A pullup will guarantee a "1" reading.

    -Phil
  • MorrolanMorrolan Posts: 98
    edited 2007-09-25 15:29
    Ahhh, that's a drawing fault - the switches are connected to VDD giving me 5v, not ground.

    Would pull-ups still be required in this case? If so, what values would you recommend?

    Cheers,
    Morrolan

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Flying is simple. You just throw yourself at the ground and miss.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-25 15:39
    In that case, you'll need pulldowns to Vss. 4.7K - 10K should work fine. If the switch is far from the board in an especially noisy environment, use 1K - 2.2K.

    -Phil
  • MorrolanMorrolan Posts: 98
    edited 2007-09-25 19:34
    Call me thick (it's been too long) but exactly what would these resistors do?


    Thanks,
    Morrolan

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Flying is simple. You just throw yourself at the ground and miss.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-09-25 19:37
    They establish the default state of the input when the switch is open. Otherwise, the input will float and could read either high or low.

    -Phil
  • MorrolanMorrolan Posts: 98
    edited 2007-09-26 08:06
    Ahhh right, so the pull-down resistor will essentially keep the signal line at 0 and stop the pin from floating?


    Attached is a modified schematic (without the pull-downs) showing Beau's suggestion for a SIP row of connectors, and also VSS/VDD mix up has been corrected.
    Not the best or cleanest layout, but I understand it!



    Thanks,
    Morrolan

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Flying is simple. You just throw yourself at the ground and miss.

    Post Edited (Morrolan) : 9/26/2007 11:52:01 AM GMT
    Orthv1.1.JPG
    1002 x 592 - 63K
Sign In or Register to comment.