Graphic panel overlays and light pipes

[P E Schoen]

I am designing a portable instrument which will have a 12.6" x 8.6" x 1/8"
thick panel. It will have two four-digit seven-segment red LED displays and
a 5x2 tactile keypad with a small red LED for each switch. The PC board with
the switches and LEDs will be mounted about 0.38" behind the panel.

The overall layout can be seen here:
http://www.pstech-inc.com/dwgs/PI-200_Layout.pdf

After talking to several companies about making the graphic overlay, I have
some questions, and since I have not designed a panel like this, I was
hoping someone may have experience or insight to offer.

I plan to use light pipes to transfer light from the LEDs to the surface of
the panel. But the ones I have found seem to have a lens that extends above
the panel, which requires the overlay to have holes for them. But it may be
preferable to have clear windows in the overlay, and light pipes which are
flush with the surface of the panel. I have not found any light pipes that
are designed this way, but perhaps I can use acrylic (Lucite) rods cut to
length, and then finish the ends properly for best light transmission.
Plastic rods are much cheaper than the light pipes, but if I need to polish
the ends it will drive the cost up. The light pipes I am considering are:

http://www.newark.com/bivar/plp2250/light-pipe-single-round-panel/dp/73H0910
http://www.bivar.com/images/cart/PLP2-XXX.pdf
and
http://www.newark.com/vcc-visual-communications-company/lmc020ctp/light-pipe-single-round-panel/dp/57P6754
http://www.newark.com/pdfs/datasheets/Visual_Comm/LMC_LITEPIPES.pdf
and
http://www.newark.com/vcc-visual-communications-company/lpc020ctp/light-pipe-single-round-panel/dp/57P6762
http://www.newark.com/pdfs/datasheets/Visual_Comm/LPC_Litepipe.pdf

The other question is how to construct the keypad. I am using tactile
switches which are mounted to the PCB and have a firm actuation force. But
the PC board will be mounted such that there is a gap of about 0.32" from
the surface of the panel to the surface of the switch. I plan to have holes
0.52" dia in the panel, and then glue a 0.50" dia x 0.31" long nylon spacer
to the overlay so that pushing the surface will transfer to the switch. I
may need to adjust the mounting so that there is either a slight preload or
a slight gap. The switches I am using are:

http://www.newark.com/omron-electronic-components/b3f-4005/switch-tactile-spst-no-50ma-thd/dp/52F3557

I made a rough prototype overlay by laminating a printed copy of the panel,
and it will probably be good enough to make a working prototype of the
instrument, but I am looking for the best way to design this for production
(20-100/year). BTW, I am getting the panels made by an on-line service that
has excellent quality, fast delivery, and very low cost. My panel will be
about $65 with 5 day turn.

http://www.frontpanelexpress.com/

Thanks!

Paul

 

[Don Y]

Hi Paul,

On 1/27/2012 12:32 PM, P E Schoen wrote:

I am designing a portable instrument which will have a 12.6" x 8.6" x
1/8" thick panel. It will have two four-digit seven-segment red LED
displays and a 5x2 tactile keypad with a small red LED for each switch.
The PC board with the switches and LEDs will be mounted about 0.38"
behind the panel.

The overall layout can be seen here:
http://www.pstech-inc.com/dwgs/PI-200_Layout.pdf

[why is the "Cont'y" (continuity?) indicator? off by its lonesome?]

After talking to several companies about making the graphic overlay, I
have some questions, and since I have not designed a panel like this, I
was hoping someone may have experience or insight to offer.

I plan to use light pipes to transfer light from the LEDs to the surface
of the panel. But the ones I have found seem to have a lens that extends
above the panel, which requires the overlay to have holes for them. But
it may be preferable to have clear windows in the overlay, and light
pipes which are flush with the surface of the panel. I have not found
any light pipes that are designed this way, but perhaps I can use
acrylic (Lucite) rods cut to length, and then finish the ends properly
for best light transmission. Plastic rods are much cheaper than the
light pipes, but if I need to polish the ends it will drive the cost up.
The light pipes I am considering are:

http://www.newark.com/bivar/plp2250/light-pipe-single-round-panel/dp/73H0910

http://www.bivar.com/images/cart/PLP2-XXX.pdf
and
http://www.newark.com/vcc-visual-communications-company/lmc020ctp/light-pipe-single-round-panel/dp/57P6754

http://www.newark.com/pdfs/datasheets/Visual_Comm/LMC_LITEPIPES.pdf
and
http://www.newark.com/vcc-visual-communications-company/lpc020ctp/light-pipe-single-round-panel/dp/57P6762

http://www.newark.com/pdfs/datasheets/Visual_Comm/LPC_Litepipe.pdf

I assume the prospect of having something *molded* isn't appropriate?
E.g., a single piece that implements *all* of the pipes? (see below)

The other question is how to construct the keypad. I am using tactile
switches which are mounted to the PCB and have a firm actuation force.
But the PC board will be mounted such that there is a gap of about 0.32"
from the surface of the panel to the surface of the switch. I plan to
have holes 0.52" dia in the panel, and then glue a 0.50" dia x 0.31"
long nylon spacer to the overlay so that pushing the surface will
transfer to the switch. I may need to adjust the mounting so that there
is either a slight preload or a slight gap. The switches I am using are:

Make sure the overlay is "flexible enough" in that 0.52" span to deflect
enough to actuate the switches. And, not *too* flexible (nor too
*brittle*!) that it won't fatigue at that point. You may also want to
consider how repeated use might impact the visual quality of the
overlay -- i.e., will you end up seeing the outline of the 0.52" hole
"embossed" into the underside of the overlay?

As with the light pipe, you could consider a molded piece that
presents *all* of these "plungers" so that the piece can be fitted
to the PCB (!) -- ensuring perfect alignment with the switches
(and LEDs, if you combine them all into one part).

Consider what will happen *when* one of those INDIVIDUAL actuator
snaps/falls off (rattling around inside the case AND making that
button inoperable)

[is it possible to float the button+LED indicator assembly closer
to the overlay panel to minimize the alignment problems and
possibiity of "bits" falling out of place?]

http://www.newark.com/omron-electronic-components/b3f-4005/switch-tactile-spst-no-50ma-thd/dp/52F3557

I made a rough prototype overlay by laminating a printed copy of the
panel, and it will probably be good enough to make a working prototype
of the instrument, but I am looking for the best way to design this for
production (20-100/year). BTW, I am getting the panels made by an
on-line service that has excellent quality, fast delivery, and very low
cost. My panel will be about $65 with 5 day turn.

http://www.frontpanelexpress.com/

Also consider (evaluate) the visual effect that mounting the displays
behind the panel will have. E.g., will you end up with "internal
reflections" when yiewing the displays off-axis?

And, of course, how all of these little "bits" will affect servicing
(if it is tedious to reassemble -- holding every little piece in JUST
the right spot as you cram it into its enclosure -- then your warranty
and service costs will have to reflect that)

 

[Martin Riddle]

"P E Schoen" <paul@peschoen.com> wrote in message
news:jfuu47$jdg$1@dont-email.me...

I am designing a portable instrument which will have a 12.6" x 8.6" x
1/8" thick panel. It will have two four-digit seven-segment red LED
displays and a 5x2 tactile keypad with a small red LED for each
switch. The PC board with the switches and LEDs will be mounted about
0.38" behind the panel.

The overall layout can be seen here:
http://www.pstech-inc.com/dwgs/PI-200_Layout.pdf

After talking to several companies about making the graphic overlay, I
have some questions, and since I have not designed a panel like this,
I was hoping someone may have experience or insight to offer.

I plan to use light pipes to transfer light from the LEDs to the
surface of the panel. But the ones I have found seem to have a lens
that extends above the panel, which requires the overlay to have holes
for them. But it may be preferable to have clear windows in the
overlay, and light pipes which are flush with the surface of the
panel. I have not found any light pipes that are designed this way,
but perhaps I can use acrylic (Lucite) rods cut to length, and then
finish the ends properly for best light transmission. Plastic rods are
much cheaper than the light pipes, but if I need to polish the ends it
will drive the cost up. The light pipes I am considering are:

http://www.newark.com/bivar/plp2250/light-pipe-single-round-panel/dp/73H0910
http://www.bivar.com/images/cart/PLP2-XXX.pdf
and
http://www.newark.com/vcc-visual-communications-company/lmc020ctp/light-pipe-single-round-panel/dp/57P6754
http://www.newark.com/pdfs/datasheets/Visual_Comm/LMC_LITEPIPES.pdf
and
http://www.newark.com/vcc-visual-communications-company/lpc020ctp/light-pipe-single-round-panel/dp/57P6762
http://www.newark.com/pdfs/datasheets/Visual_Comm/LPC_Litepipe.pdf

The other question is how to construct the keypad. I am using tactile
switches which are mounted to the PCB and have a firm actuation force.
But the PC board will be mounted such that there is a gap of about
0.32" from the surface of the panel to the surface of the switch. I
plan to have holes 0.52" dia in the panel, and then glue a 0.50" dia x
0.31" long nylon spacer to the overlay so that pushing the surface
will transfer to the switch. I may need to adjust the mounting so that
there is either a slight preload or a slight gap. The switches I am
using are:

http://www.newark.com/omron-electronic-components/b3f-4005/switch-tactile-spst-no-50ma-thd/dp/52F3557

I made a rough prototype overlay by laminating a printed copy of the
panel, and it will probably be good enough to make a working prototype
of the instrument, but I am looking for the best way to design this
for production (20-100/year). BTW, I am getting the panels made by an
on-line service that has excellent quality, fast delivery, and very
low cost. My panel will be about $65 with 5 day turn.

http://www.frontpanelexpress.com/

Thanks!

Paul

If you make the LED windows clear, that will work fine. You can even
stand up the LED with spacers if required.
If your SMT, then there are short light pipes that could be used. I
would not worry about how close the pipe end is, just get one long
enough before going custom.
Since you are using tactile keys on the PCB, make sure they are as large
as the buttons otherwise the membrane overlay will wear in the area of a
smaller key. We use a membrane with keys designed into it, clear/red
windows for 7-segment displays and clear LED ports. THe membrane lays on
top of the 7-segment displays, and the LED ports are not a problem for
the T-1 3/4 led's we use.

Front panel express is fine for one-offs. They do come out neat.
But if you did the entire front in a membrane it would look spiffy.
Have a local sheet metal house cut a bare panel, clear alodine, and
stick on the membrane.

Another option is to just inlay the membrane in the small area you have
shown that has the keys and LED's.
Either way the cost of the membrane is going to run $2k or so for
tooling.

Cheers

 

[John Devereux]

"P E Schoen" <paul@peschoen.com> writes:

I am designing a portable instrument which will have a 12.6" x 8.6" x
1/8" thick panel. It will have two four-digit seven-segment red LED
displays and a 5x2 tactile keypad with a small red LED for each
switch. The PC board with the switches and LEDs will be mounted about
0.38" behind the panel.

The overall layout can be seen here:
http://www.pstech-inc.com/dwgs/PI-200_Layout.pdf

After talking to several companies about making the graphic overlay, I
have some questions, and since I have not designed a panel like this,
I was hoping someone may have experience or insight to offer.

I plan to use light pipes to transfer light from the LEDs to the
surface of the panel. But the ones I have found seem to have a lens
that extends above the panel, which requires the overlay to have holes
for them. But it may be preferable to have clear windows in the
overlay, and light pipes which are flush with the surface of the
panel. I have not found any light pipes that are designed this way,
but perhaps I can use acrylic (Lucite) rods cut to length, and then
finish the ends properly for best light transmission. Plastic rods are
much cheaper than the light pipes, but if I need to polish the ends it
will drive the cost up. The light pipes I am considering are:

http://www.newark.com/bivar/plp2250/light-pipe-single-round-panel/dp/73H0910
http://www.bivar.com/images/cart/PLP2-XXX.pdf
and
http://www.newark.com/vcc-visual-communications-company/lmc020ctp/light-pipe-single-round-panel/dp/57P6754
http://www.newark.com/pdfs/datasheets/Visual_Comm/LMC_LITEPIPES.pdf
and
http://www.newark.com/vcc-visual-communications-company/lpc020ctp/light-pipe-single-round-panel/dp/57P6762
http://www.newark.com/pdfs/datasheets/Visual_Comm/LPC_Litepipe.pdf

The other question is how to construct the keypad. I am using tactile
switches which are mounted to the PCB and have a firm actuation
force. But the PC board will be mounted such that there is a gap of
about 0.32" from the surface of the panel to the surface of the
switch. I plan to have holes 0.52" dia in the panel, and then glue a
0.50" dia x 0.31" long nylon spacer to the overlay so that pushing the
surface will transfer to the switch. I may need to adjust the mounting
so that there is either a slight preload or a slight gap. The switches
I am using are:

http://www.newark.com/omron-electronic-components/b3f-4005/switch-tactile-spst-no-50ma-thd/dp/52F3557

I made a rough prototype overlay by laminating a printed copy of the
panel, and it will probably be good enough to make a working prototype
of the instrument, but I am looking for the best way to design this
for production (20-100/year). BTW, I am getting the panels made by an
on-line service that has excellent quality, fast delivery, and very
low cost. My panel will be about $65 with 5 day turn.

http://www.frontpanelexpress.com/

Hi Paul,

You know you can get tactile switches in various heights? You can
normally make everything line up without little extension pieces.

Any you can definitely get PCB mounted light guides that can go behind a
panel, not used any that short though, can't you just use LEDs directly?


--

John Devereux

 

[P E Schoen]

"John Devereux" wrote in message news:871uqkbc5x.fsf@devereux.me.uk...

"P E Schoen" <paul@peschoen.com> writes:

I made a rough prototype overlay by laminating a printed copy of the
panel, and it will probably be good enough to make a working
prototype of the instrument, but I am looking for the best way to
design this for production (20-100/year). BTW, I am getting the
panels made by an on-line service that has excellent quality,
fast delivery, and very low cost. My panel will be about $65
with 5 day turn.

http://www.frontpanelexpress.com/

You know you can get tactile switches in various heights? You can
normally make everything line up without little extension pieces.

The switches I am using also come with an extension that is made to have
snap-on caps, but I want the keypad to be sealed with no holes. I made a
small panel to try various ways to mount the PCB and switches. What I did
was make a 1/8" thick aluminum sub-panel with 0.257" holes that I filled
with 0.25" nylon spacers that I already had. Then in the main panel I
drilled 0.50" diameter holes. When I mounted the PC board so the spacers
extended past the surface, the laminated overlay actuated the switches
without pressing them, and when I aligned them a little below the surface,
the overlay seemed to be too stiff to work well.

So, I used foam double-stick tape to mount the overlay above the extended
spacers, and the foam was soft enough to allow the switches to operate, but
the action is still stiffer than I would like. It's good enough for the demo
unit for now, but I'll need to look more closely at the overlay material
that will be supplied by the manufacturer. It's probably more flexible, and
thinner (6 to 10 mils), than the laminated overlay I made, which is 14 mils.
I think I need to get samples of the material to see if it's flexible
enough. They are proposing PET or polycarbonate (Lexan), but maybe I should
consider silicone.

Any you can definitely get PCB mounted light guides that can go behind a
panel, not used any that short though, can't you just use LEDs directly?

There are three T-1 LEDs that are thru-hole, and the others (for the
switches) are 1206 SMD. For the production unit, I could use T-1s and
extensions to mount them high enough to fit into holes in the panel, but
still below the overlay. If I use SMD, I'll need the light pipes. If I use
the sub-panel, the light pipes could be mounted through that, and the hole
in the panel could be larger to accommodate the diameter of the lenses. Or I
could use a thicker panel and have the holes counterbored, but that would
probably be more expensive than making the sub-panel.

Another possibility is to use a relatively thick foam adhesive backing that
will give flexibility to the keys and a soft feel to the panel, almost like
a floating floor system with a thin foam underlayment. Maybe I should allow
the overlay manufacturer to select the PCB components and determine the best
way to make the panel. I'm locked in to the present basic design only for
the initial prototype to be used as a demo. I will be redoing the board, and
I need to align the LEDs a little better, so the production unit will be a
bit different. Hopefully I can just get this unit working reasonably well
and in a presentable form so we can get input from potential customers at
the show, and maybe there will be even more changes.

Thanks to all,

Paul

 

[John Devereux]

"P E Schoen" <paul@peschoen.com> writes:

"John Devereux" wrote in message news:871uqkbc5x.fsf@devereux.me.uk...

"P E Schoen" <paul@peschoen.com> writes:

I made a rough prototype overlay by laminating a printed copy of the
panel, and it will probably be good enough to make a working
prototype of the instrument, but I am looking for the best way to
design this for production (20-100/year). BTW, I am getting the
panels made by an on-line service that has excellent quality,
fast delivery, and very low cost. My panel will be about $65
with 5 day turn.

http://www.frontpanelexpress.com/

You know you can get tactile switches in various heights? You can
normally make everything line up without little extension pieces.

The switches I am using also come with an extension that is made to
have snap-on caps, but I want the keypad to be sealed with no holes. I
made a small panel to try various ways to mount the PCB and
switches. What I did was make a 1/8" thick aluminum sub-panel with
0.257" holes that I filled with 0.25" nylon spacers that I already
had. Then in the main panel I drilled 0.50" diameter holes. When I
mounted the PC board so the spacers extended past the surface, the
laminated overlay actuated the switches without pressing them, and
when I aligned them a little below the surface, the overlay seemed to
be too stiff to work well.

So, I used foam double-stick tape to mount the overlay above the
extended spacers, and the foam was soft enough to allow the switches
to operate, but the action is still stiffer than I would like. It's
good enough for the demo unit for now, but I'll need to look more
closely at the overlay material that will be supplied by the
manufacturer. It's probably more flexible, and thinner (6 to 10 mils),
than the laminated overlay I made, which is 14 mils. I think I need to
get samples of the material to see if it's flexible enough. They are
proposing PET or polycarbonate (Lexan), but maybe I should consider
silicone.

Yes getting enough flexibility is a problem. We usually use the thinner
overlay material and the spacing needs to be quite closely
controlled. There is usually some item in the stackup that can be fine
tuned like the PCB spacers or the panel thickness. I never thought of
the foam tape idea; that might work. By the way the $2000 someone
mentioned sounds high for a plain flat overlay. We pay more like $400
for a self-adhesive overlay with a few spot colours. (Even that always
seems high to me, why is it a thin piece of printed plastic sheet costs
more than a PTH PCB?)

I would expect $2000 to get me the keyswitches integrated too, which is
probably the way to go for production. (But we have made 1kpcs of a
6-key unit with the separate tac switches with no real issues).

Any you can definitely get PCB mounted light guides that can go behind a
panel, not used any that short though, can't you just use LEDs directly?

There are three T-1 LEDs that are thru-hole, and the others (for the
switches) are 1206 SMD. For the production unit, I could use T-1s and
extensions to mount them high enough to fit into holes in the panel,
but still below the overlay. If I use SMD, I'll need the light
pipes. If I use the sub-panel, the light pipes could be mounted
through that, and the hole in the panel could be larger to accommodate
the diameter of the lenses. Or I could use a thicker panel and have
the holes counterbored, but that would probably be more expensive than
making the sub-panel.

Another possibility is to use a relatively thick foam adhesive backing
that will give flexibility to the keys and a soft feel to the panel,
almost like a floating floor system with a thin foam
underlayment. Maybe I should allow the overlay manufacturer to select
the PCB components and determine the best way to make the panel. I'm
locked in to the present basic design only for the initial prototype
to be used as a demo. I will be redoing the board, and I need to align
the LEDs a little better, so the production unit will be a bit
different. Hopefully I can just get this unit working reasonably well
and in a presentable form so we can get input from potential customers
at the show, and maybe there will be even more changes.

Thanks to all,

Paul

--

John Devereux

 

[Martin Riddle]

"John Devereux" <john@devereux.me.uk> wrote in message
news:87obto9toz.fsf@devereux.me.uk...

"P E Schoen" <paul@peschoen.com> writes:

"John Devereux" wrote in message
news:871uqkbc5x.fsf@devereux.me.uk...

"P E Schoen" <paul@peschoen.com> writes:

I made a rough prototype overlay by laminating a printed copy of
the
panel, and it will probably be good enough to make a working
prototype of the instrument, but I am looking for the best way to
design this for production (20-100/year). BTW, I am getting the
panels made by an on-line service that has excellent quality,
fast delivery, and very low cost. My panel will be about $65
with 5 day turn.

http://www.frontpanelexpress.com/

You know you can get tactile switches in various heights? You can
normally make everything line up without little extension pieces.

The switches I am using also come with an extension that is made to
have snap-on caps, but I want the keypad to be sealed with no holes.
I
made a small panel to try various ways to mount the PCB and
switches. What I did was make a 1/8" thick aluminum sub-panel with
0.257" holes that I filled with 0.25" nylon spacers that I already
had. Then in the main panel I drilled 0.50" diameter holes. When I
mounted the PC board so the spacers extended past the surface, the
laminated overlay actuated the switches without pressing them, and
when I aligned them a little below the surface, the overlay seemed to
be too stiff to work well.

So, I used foam double-stick tape to mount the overlay above the
extended spacers, and the foam was soft enough to allow the switches
to operate, but the action is still stiffer than I would like. It's
good enough for the demo unit for now, but I'll need to look more
closely at the overlay material that will be supplied by the
manufacturer. It's probably more flexible, and thinner (6 to 10
mils),
than the laminated overlay I made, which is 14 mils. I think I need
to
get samples of the material to see if it's flexible enough. They are
proposing PET or polycarbonate (Lexan), but maybe I should consider
silicone.

Yes getting enough flexibility is a problem. We usually use the
thinner
overlay material and the spacing needs to be quite closely
controlled. There is usually some item in the stackup that can be fine
tuned like the PCB spacers or the panel thickness. I never thought of
the foam tape idea; that might work. By the way the $2000 someone
mentioned sounds high for a plain flat overlay. We pay more like $400
for a self-adhesive overlay with a few spot colours. (Even that always
seems high to me, why is it a thin piece of printed plastic sheet
costs
more than a PTH PCB?)

I would expect $2000 to get me the keyswitches integrated too, which
is
probably the way to go for production. (But we have made 1kpcs of a
6-key unit with the separate tac switches with no real issues).


Any you can definitely get PCB mounted light guides that can go
behind a
panel, not used any that short though, can't you just use LEDs
directly?

There are three T-1 LEDs that are thru-hole, and the others (for the
switches) are 1206 SMD. For the production unit, I could use T-1s and
extensions to mount them high enough to fit into holes in the panel,
but still below the overlay. If I use SMD, I'll need the light
pipes. If I use the sub-panel, the light pipes could be mounted
through that, and the hole in the panel could be larger to
accommodate
the diameter of the lenses. Or I could use a thicker panel and have
the holes counterbored, but that would probably be more expensive
than
making the sub-panel.

Another possibility is to use a relatively thick foam adhesive
backing
that will give flexibility to the keys and a soft feel to the panel,
almost like a floating floor system with a thin foam
underlayment. Maybe I should allow the overlay manufacturer to select
the PCB components and determine the best way to make the panel. I'm
locked in to the present basic design only for the initial prototype
to be used as a demo. I will be redoing the board, and I need to
align
the LEDs a little better, so the production unit will be a bit
different. Hopefully I can just get this unit working reasonably well
and in a presentable form so we can get input from potential
customers
at the show, and maybe there will be even more changes.

Thanks to all,

Paul


--

John Devereux

You can also get the keys 'domed' or with a domed boarder. Might make
flexibility a little better.
I used this approach with the standard 5mm round push button thru round
holes under square domed keys.

Cheers