Manufacturing an instrument - have a couple of questions

[darrenb]

I have pretty much finished the circuitry and programming for a new project and I'm now at the case design stage.  And I have never designed a case before.

The case design is simple - just an aluminium extrusion with the switches, connectors etc. on the front and back panels.  A picture of it is below.



My questions are;

1.  The large cutout at the front is for a 16x2 lcd display.  What is the best way to attach the display behind and to the front panel?  I assume it will require standoffs but I don't want to be able to see the connectors on the front.  Is there another way?

2.  The small hole in the back panel is to simply hold a 3 terminal screw terminal.  Can someone steer me in the right direction to find something that will give me the screw terminals on the outside and some kind of pcb to pcb connector on the inside, or just 3 pins that I can solder to?

Thanks.

[david77]

1. I take it the round holes underneath the LCD cut-out are for buttons. So you could mount the LCD on the same board your button are on using a few stand-offs.
In that case I'd make the cut-out for the LCD the same size or very slightly smaller than the actual size of the visible LCD screen. That way you would not see the bezel or the "innards" of the display.

If I interpret your picture correctly the front and the top and bottom side panels are one bent piece of sheet metal. In that case I'd mount all the PCB's inside as a sandwiched stack on the back panel.

2. That sounds like you're looking for something like this http://www.ptr.eu/en/produkte/terminal-blocks/. These wire terminals are made by many different companies.

[darrenb]

David.  Thanks so much for this reply.  The idea of having the pcb's sandwiched from front to back is perfect.  It solves both problems - how to attach the screen and buttons to the front and how to attach the screw terminals to the back.

Now I just have to find some long standoffs.

[Magicmushroom666]

Do you already have a source of extruded aluminium in that shape?

[darrenb]

Yes I've been able to find a number of possible suppliers.  A couple include;

http://www.china-mould.com/enindex.asp
http://www.everestcase.com/

[daedalus]

just a couple of remarks, as david has covered most things. You probably want to add a small radius fillet to the corner of your lcd cutout, and the rear cutout. These two sheets will likely be milled in small quantities, so doing so will make production easier.

As for the rear panel pass-through, i would use a socketed screw terminal, ideally one that can be fixed in place. Its a nice feature to be able to plug/unplug, and it neatly places the screw terminal bits outside the case.

weidmuller have some suitable parts, check 2077163 on farnell. There are several other suitable brands, its a pretty common part.

[darrenb]

I'm stuck with the mechanical engineering of this project, but first here are a couple of pictures of the extrusion that I think I'll be using..





An explanation of the product is as follows;

- First, it'll be black anodised aluminium not the silver that you see with these samples.
- On the from there will be a 16 x 2 LCD display and below it four push buttons, the same as you see in my original post at the start of this thread.
- At the back there just needs to be a three connector screw terminal for +ve, gnd and data.
- Out of interest the product will be a lead acid battery monitor, much more accurate than the ones that currently exist.
- I intend to customise the front and rear panels with screen printing to label the connectors and buttons and also to make the front panel slightly larger so that the unit can be panel mounted.

I am stuck with the mechanical engineering aspects of the design.  The thing is already breadboarded and works great.  I have no problem with the electronics design or the programming and I have made pcb's before.  But I am not sure whether to arrange the circuit boards vertically and have standoffs between them, or if I should have a large horizontal pcb from the front to the rear and if I do, how do I attach the LCD to the front panel.  Are there such things as right angle joins for pcb's.  Is it ok to use long standoffs running from the front to the rear of the unit, and if I do will it rattle if I use screws to attach the standoffs to the pcb's at each end, will nylon fittings solve this.  If I design it and the measurements or the extrusion are 0.3 mm out will it be loose or not go together.  If I look on Digikey for board to board connectors there are thousands of them but which ones are the most commonly used?

Ideally if someone with experience in these areas can help that may be the best way to get through this phase.  I would be happy to pay, within reason.  If you are local that would be even better - I live in Castle Hill NSW Australia which is only a few kilometres from where Dave lives as far as I can tell.  Please PM me if you think you can help.

[FJV]

Do you mind seeing a screw head in the front panel? Then you could drill 4 small holes around the square cutout and mount the LCD on 4 spacers. There might already be pre-drilled holes on the display for that.



Also there are bezels for lcd displays available.



As for the buttons, are panel mount types that fit directly in front panel holes an option. (preferably one with a nice bezel)

[darrenb]

For the sake of completeness I thought I'd update this thread what I finally came up with.



This picture contains one of the three boards that will be required for this design.  The board on the bottom is just a button board to hold the four push buttons on the front and to attach to the LCD display.  It also mechanically matches both the height and width of the enclosure to ensure the lcd and the buttons cannot move.



This shows the LCD and button board as well as the serial LCD controller that I made months ago to interface to the LCD.  The LCD controller is my own design, programming and board design.  It's much better than anything I can buy and I can make them in volumes of 25 and up for $4.31 each.



This is the top spacer board.  There are two slots for PCB's in the aluminium extrusion and this one slides in the top.  It has no electrical components on it and just supports the top of the LCD / button board assembly.  All of the boards in this first hand made prototype are just cut out from prototype board.



This is the third and main board which goes in the bottom slot of the extrusion.  It contains the electronics and supports the bottom of the LCD / button board assembly.



Here's a picture of the boards roughly in place without the top half of the extrusion attached.



This shows the assembled unit without the front plate attached.



And this is with the front plate attached.  In the production version all of the aluminium will be black anodised and the buttons will the about 7mm diameter instead of the 3mm on these longish tactile switches.



This shows the unit assembled and running with a printed paper mock up of the from panel design sitting in front.

I'm more than happy with the overall design so far and the only issue I can think of is that it may rattle if the tolerances are not accurate.  To solve this however I'm planning on things being up to 1/2mm out and may use a few dabs of silicone or polyurethane to secure the boards in place.

I'm committed to this thing now as I have ordered and am having manufactured two hundred and fifty of the aluminium extrusions including customised front and rear panels and two colour screen printing.  Unit cost at this volume around $7.2 each.

I also have 100 16x2 LCD displays on the way ($1.88 each) and 100 x 500 amp 75 millivolt shunts for about $6.50 each.

The rest of the components are all readily available through DigiKey and a few other suppliers.

These first 250 units are not perfect.  I'd love to have a custom moulded enclosure made, but as it is it's solid and heavy and the unit feels valuable.

The plan is to make 25 testing units initially and then make and sell this initial batch of 250 units.  If I can make $50 on each one then I'll have made $12,500 from them and then it'll be time for a better and much smaller enclosure.

[david77]

It's hard to decide about these things without having the parts to play around with.

Here's my idea: Could you imagine gluing the LCD directly to the frontpanel? There must be a suitable glue to do that.
Then you could use 90° angled push buttons that mount directly on your main PCB. This way you could eliminate the empty spacer board and some wires.
And can't you integrate the small PCB on the back of the display onto the main PCB, too?

The main PCB would then cover the whole area of the case, about 0,3mm smaller than the depth of the case maybe so you don't run into tolerance issues.

I'd use standard 0,1" headers for the interconnects, no need to go smaller as far as I can see. There are types available that can not be connected the wrong way round. I like this type: http://de.farnell.com/wurth-elektronik/61900511621/gehaeuse-2-54mm-5kont/dp/1841383