Building 2 digit up/down counter LED display

Thread Starter

izon

Joined Mar 17, 2013
217
Just for the heck of it, I'm going to check at my local glass shop for plastics.. I know they have the clear Plexiglass type but that won't do.
Also a neighbor mechanic has a full set of those type drill bits so he will let me borrow them.
 

elec_mech

Joined Nov 12, 2008
1,500
Okay, I think this is relatively complete. I've revised the schematic and BOM. Note there are two values for R12-R39, 200Ω and 130Ω. If my math and estimates are dead-on, then 200Ω will do it. If the LEDs require a slightly higher forward voltage than called out in the datasheet, then they may appear dim and the 130Ω can be used instead.

I've also included IC sockets, a stripboard PCB, power supply, jack you can add to box to connect power supply to and terminal strips so you can wire the display to the PCB and disconnect if needed. If you're trying to pinch pennies, you can skip the terminal strips and just solder wire directly to the PCB.

I've not taken into account items such as your box, standoffs, fasteners, wire, solder, or the like.

If I've missed something let me know. The stripboard is a little small, but the next largest one was over $20. :eek:

Values for R1 and R2 might change depending on the voltage requirement of the RC receiver. I've assumed 3VDC.

If you happen to have some of the parts, you certainly don't need to buy everything new. Prices given are for quantities of one. Price for things like capacitors and resistors go down considerably if you buy ten or more. While you don't need to go crazy ordering excess, I would suggest at least one spare IC of each type, ten or more spare LEDs, etc.
 

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Thread Starter

izon

Joined Mar 17, 2013
217
Regarding the plastics....with shipping it looks like that would run nearly $35 ...a bit spendy but I know it's tough to get around it. Suppose a piece of 1/8" pressed wood (used to call it Masonite) would not drill cleanly enough....could spray paint it black if it would work. Or what if I have a clear piece of the plastic & drill the holes then spray paint it black...no doubt the holes would grab a lot of the paint n need to be cleaned but if I just have a scrap piece around..cheap...save about $10 there. If its a possibility I assume flat paint rather than glossy. Your thoughts? Thanks for the BOM and suggestions again!
 

elec_mech

Joined Nov 12, 2008
1,500
Certainly, you can use just about any material you'd like and paint it flat black for the base. I've used some decent acrylic from Lowes that wasn't hard to drill. Acrylic is cheap, but you just have to be darn careful about drilling it. Lowes carries like three different types, but I don't recall which. Off the top of my head, I'd go for the mid-grade.

Make sure whatever you use is about 1/8" thick. Much less and it could be too flexible and not hold the LEDs well. Much more and you won't be able to see from the sides as well.

I'm not sure it would be any cheaper, but you could look at craft stores for a transparent green plastic on a roll then wrap that around one side of a clear piece of acrylic for the filter. You may need more than one layer to to darken it up a bit. You'd have to tape it to the edges, but you'll have a border of some sort anyways, so that would hide the tape. I've never tried this and it's extra work, but it is an option to consider.

Those rolls usually run ~$16, but if you have a Sunday paper or look online, you can probably find their almost weekly coupon for 20-50% off a single item and save more money from Michael's or AC Moore.
 

Thread Starter

izon

Joined Mar 17, 2013
217
All things considered, guess I might bite the bullet and order the plastic on line to get the right size and color... Digi Key order is on the way so then can begin breadboarding until I decide to order the plastic and build the enclosure.... may need some board layout thoughts or plans once I've proven things on the breadboard.
 

elec_mech

Joined Nov 12, 2008
1,500
While I know I've been beating the filter thing like a dead horse, if money is an issue, you can always leave it off. Yes, it will help make the display easier to see, but not having it won't make the display unreadable. Plus, you can add a cheap piece of clear acrylic in it's place and replace later if the office likes it and decides to pay you for an upgrade.

I've finished designing the PCB layout for your board. Note a few things:

1) The white rectangles on the bottom indicate where you need to remove the copper traces. This is extremely important. You can use a Dremel or just an X-acto knife.

2) C4 is the only electrolytic capacitor, so take care you install it correctly. Be sure the negative leads goes to GND.

3) Red wires denote +VDC, black denote GND, and blue are signal wires.

4) Be sure D1 is installed correctly. Note the location of the white line. This adds reverse voltage protection. I forgot to take its voltage drop of ~0.7V into account when I specificed the LED resistors, hence, you may want to see what the LED brightness is with and without D1 first.

5) I couldn't fit the LED resistors onto the board, so they'll either need to go onto another one or just get soldered to the end of the LED strings.

6) Note proper orientation of transistors and voltage regulators when installing.

7) You may want to add some holes for standoffs. I suggest laying out the IC sockets and J2 and J3 first then making sure the hole locations will not adversely affect any used traces. Also be sure to cut off any traces connecting to the standoff holes so as not to introduce any shorts.
 

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Thread Starter

izon

Joined Mar 17, 2013
217
Awesome ! I am impressed with the layout on the board !

I have tried confirming the work on the board layout to the schematic
connections and I think I may have discovered an error.... haven't done
a total search but this one sorta popped out.

The schematic shows that pin 3 of U7 connects to pin 1 of U5. However the blue jumper wire goes from pin 3 of U7 to pin 2 of U5.

The case is the same with relationship of U6 pin 3 and U4 pin 1 vs. 2.

I'm assuming it only will require the jumper wire move to the appropriate
pin trace as I believe it is isolated from any other connection in both
scenarios.

Please let me know your thoughts when you have a moment.
Thanks.
 

Thread Starter

izon

Joined Mar 17, 2013
217
OK...
Now what about U3 pins 14 and 13 are tied together on schematic but don't appear to be on the board unless I've missed it somehow.
Regarding capacitors C2 - C7 they seem to be redundant. Each one
goes between ground and supply voltage. Yes they are at different spots
around the board and on the diagram they are "located" near the
different ICs but net result seems to be the same...

Your thoughts on these questions? Thank you!
 

elec_mech

Joined Nov 12, 2008
1,500
Well, this is why it is good to have someone check one's work.

Yes, pin 13 and 14 should be connected.

Yes, the caps are needed to help reduce noise, though it is better if the caps are placed as close as possible to the Vcc and GND pins. I've done a little shifting to help this.
 

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Thread Starter

izon

Joined Mar 17, 2013
217
Well we have assembled all the parts onto the PC board and all the jumper wires. So the power supply can be wired to any of the ground pads and to the "18 Volt Input" for the positive.... so are you regulating down
to 12vdc for the working voltage and that down to 3vdc for the RC receiver?

Had a bit of a puzzle on mounting U2... googled for pinout but first inserted it in same orientation of U1 which we had installed first. Upon
checking with schematic I finally discovered that U2 needs to be switched
180 degrees so Input and Adj were correct based on other items connected.

Have tried to go over all the connections visually and with continuity checks to somewhat confirm all is correct. Will have to deal with the "plastic" locating/buying and the resistor placement that tie to the LEDs
next so may not have a "post" here for a bit unless something comes up
as I move forward.
 

elec_mech

Joined Nov 12, 2008
1,500
.... so are you regulating down to 12vdc for the working voltage and that down to 3vdc for the RC receiver?
Yes, my theory being less heat will be generated dropping from 12V then to 3V vs. 18V to 3V. Grant it, the RC board shouldn't take much current once removed from the motors, but never hurts to be on the safe side.

Did you get an RC car? If yes, how many batteries does the car (receiver) take?

Had a bit of a puzzle on mounting U2...
Not sure if you saw it on the drawing, but I did include a small gray bar on U1 and U2 denoting orientation. Both should have the backs oriented toward the middle.

have to deal with the "plastic" locating/buying
Don't know if it will be any cheaper, but you could also go to glass store or maybe a craft store and get a dark green tinted piece of stained glass instead. I'd still suggest keeping it ~1/8" thick, but thinner would work too as long as it wasn't too lightly tinted.

and the resistor placement that tie to the LEDs
This would work nicely to mount the resistors to if you don't want to wire them in line. You can also tie the 18V to one of the bus bars and wire both the anode and cathodes back to this board if you'd like.
 

Thread Starter

izon

Joined Mar 17, 2013
217
Thanks for the info...
Well I'm going to have you look at this site for the pinout on U2:
www.onsemi.com/pub/Collateral/LM317-D.PDF

If that one doesn't work, I'm sure you can locate a pinout as well.
Please note that looking at the IC with the black main body toward you
and the silver metal heat sink in the back with pins numbered 1-3 from left
to right, PIN 3 is the Vin... 12 volt regulated supply coming from U1.

On the layout, pin 3 will go into ROW X which means the front of the IC
will face toward the inside of the PC board and the heat sink will face the
outside of the board.

Of course pin 2 in the center will, either way, become the output and
pin 1 will be the adjusted leg by resistor values to develop the 3 volts out.

Hopefully I've explained it properly and would be reassuring if you agree that I have it placed correctly.

Thanks for the suggestion on the glass again.. will see here after a get the opportunity to get back to it.... we have no "hobby" stores in this small town.. .just WalMart and the glass store may not carry stained glass.

Thanks for now.
 

Thread Starter

izon

Joined Mar 17, 2013
217
Ooops ... forgot. I just bought an R/C car at WalMart and it uses 2 AAA batteries so, yes, the 3 volt regulator should be perfect to supply the
receiver in place of the 2 batteries. Haven't torn it apart just yet, ha.
 

elec_mech

Joined Nov 12, 2008
1,500
Doh, I don't know how I missed that, but you are correct, U2 should be in reverse with the front facing the center of the board. Attached is a corrected drawing.

For future reference, note that Digikey (as well as Mouser and Jameco) normally has the datasheet for their parts, so if you order the parts from them, you can look up the part number and get the right data sheet.
 

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Thread Starter

izon

Joined Mar 17, 2013
217
Yes, I would have known the suppliers have that info but googling actually worked quite quickly with a variety of views and confirming multiple posts.

I'm a little disappointed in the R/C car I got... tried it out the way it was meant to be used and the range was quoted as up to 80 feet.. ha...set it out on my driveway and it would be pushing it to say it gets 20 feet before the action is very intermittent..also, unless you have both antennas supported in an upright posistion with some vinyl tubing, the range is less with the antennas (just lengths of wire) hanging down and at odd angles.
If you touch the antenna it kills the signal... no surprise there... but wondering if I put this transmitter board in a "project box" I may have to find a small telescoping antenna to use which will make it a bit awkward to handle. I suppose both antenna wires work best when both are in the same plane as each other. So then if I build all this display into a wooden box, wondering if I can get by with keeping the wire antenna inside and taping it against the inside of the wood case in a vertical position.

Wish there was a way to "boost" the output of the transmitter but when working with RF at 27Mhz, it isn't a simple step. Wonder if the unit could tolerate a greater voltage like another cell or two without burning anything up and yet give a stronger output.

Thanks for your thougts on this info....
 

elec_mech

Joined Nov 12, 2008
1,500
Hmm, I'd try a few things:

1) Make a simple wooden box with the same or thicker wood than you plan to use for the finished product, place the box over the car and see if it works.

2) Mount said box vertically and tape wire antenna vertically and check again.

3) Replace the receiver wire with a longer wire and see if it improves anything. Not sure how sensitive 27MHz is to wire length, but no harm in trying.

4) If receiver does not work inside box, simply route wire to one side of box on outside. See if it will work better mounted horizontally on bottom or top.

5) If longer wire works, you can try hanging it down along with power cord - not ideal, but this gives you a longer antenna without running a wire to the ceiling which may not be well received in an office environment.

6) If all else fails, look for an RC hobby shop in your area and get some advice or a better TX/RX combo.

Not sure about 'boosting'. RF is black magic to me, but I'd worry about damaging the circuit if you increased the voltage.

The receptionist will be the one controlling the counter correct? How far away is that person from where the counter is going?

If this becomes an issue, let me know and I'll talk to my local hobby shop.

Okay, on the filter front, I think I've found a less expensive alternative.

Went to A.C. Moore (big, nationwide craft store) and found green cellophane for $4 for a 30" x 25' roll. Took it home slapped it on one side of a piece of acrylic and viola, had myself a decent green filter. Pictures attached. It has a slight blur to it you're looking up close, but for an LED filter, it's fine.

Went to Lowes and found a 18" x 24" of acrylic for $9 or $9.40 depending on if you want 10x or 50x stronger than glass version. I think the latter would be easier to machine and less prone to cracking, but either should work. This would provide you with enough for a base to mount the LEDs to as well as a filter. They also carry a scoring knife for $4, but you can get away with a sharp utility knife. Unfortunately, the acrylic was only 0.080" thick, but this will still work work - just not quite as rigid as I'd like.

So for under $15, you can get the display materials. Beats $30+.

Link to A.C. Moore store locator and 25% coupon.
 

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Thread Starter

izon

Joined Mar 17, 2013
217
Thanks or all the suggestions...won't respond to all the points now but wanted to let you know I came up with a perfect piece of black plastic for the LEDs....it is ABS type and salvage from a large flat screen TV cabinet back cover. It is 1/8" thick and rigid so seems just right and FREE !

Trying to upload a photo or two to attach.
Later....
 

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elec_mech

Joined Nov 12, 2008
1,500
....it is ABS type and salvage from a large flat screen TV cabinet back cover. It is 1/8" thick and rigid so seems just right and FREE !
That is perfect. Perhaps you can find a scrap piece of acrylic or glass from something else as well? I can cut off some of the green cellophane, roll it up and mail it to you if you'd like. If interested, just PM me your mailing address.
 
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