Numeric LED Displays

ChooChooBarn · Post by **ChooChooBarn** » Sun Mar 30, 2008 4:29 pm

Greetings,

I want to put a gas price sign at a miniature gas station on a model railroad using LED displays. I can build the sign with no problem. I am going to make it show three different prices for gas, regular, premium and diesel, and I want to be able to change the prices as gas goes up and down. That way, my gas station will stay current. I need help with how to make it change prices easily. Was anything like this ever covered in Nuts & Volts over the past 5 or so years? I still have my back copies.

Jameco has a 3-digit display that will work. It is Jameco #335054. The price is right and the display itself is of the correct size for an O Gauge model railroad layout.

Oh, did I mention that I need to have this done by the end of April? I'm sure I can built the boards and such, but if it takes any programming for a controller, I'm lost.

Thanks in advance,
Tom

philba · Post by **philba** » Sun Mar 30, 2008 5:09 pm

This is doable although there are many questions....

What are you thinking of using to enter the prices? A PC? Notebook computer? switches?

What level of electronics expertise do you have? What have you built before. Do you have any programming experience?

What is your budget?

There are many ways to accomplish this project - a lot depends on your answers, though.

edit: 3 digits? Optimistic about the future?

ChooChooBarn · Post by **ChooChooBarn** » Sun Mar 30, 2008 6:01 pm

Thanks for getting back to me and asking the questions. I am a tinker, not an electronics expert. I can follow a schemetic. I'm not a programmer but I know my way around a computer, somewhat. I guess the easiet method would be to use a whole bunch of switches, seven for each digit and make a matrix that would tell me which of the seven switches have to be on to make each numeral. Since I want to have prices for all three types of fuel, I'd need 63 switches!. That's doable, but a little clumsy. I was thinking more on the line of PC control. I have a bunch of older, slower PCs that could be used for this purpose. I think it would be funny to be able to change the price of gas while someone is looking at the gas station.

I'll need 6, 3-digit7-segment LED displays, three for each side of the sign. The 9/10 will be back lit and obviously won't have to change, unless the gas industry makes some huge change to their pricing structure! The displays are $1.96 each in quantities of 10 from Jameco. That will allow for soldering mistakes. I can't imagine my sign will cost any more than $100, which sounds like an extremely large amount for such a small project.

You can check our my web site, www.choochoobarn.com to see what I do. I don't have any pics of the gas station on the site but will later, especially if I can get the sign to work.

If gas goes over $10.00/gal, my sign is the least of my worries.

smoshier · Post by **smoshier** » Sun Mar 30, 2008 6:36 pm

If you don't want to use a PC, all you need are some switches and a button. The button to manually change the price and the switches to pick what digit to change and/or to pick which price line to change. This would take more work to change the prices, but it would allow for a much smaller setup.

You would want to use an AND gate and Latch setup to hold the digits.

I believe that this would be better because you would not need to know how to program anything.

jaem · Post by **jaem** » Sun Mar 30, 2008 6:41 pm

This could be achieved in a much simpler manner: simply buy a few DIP switches (4 or 8 position), and run them through a 7447 BCD->7-segment decoder chip (will usually have a logic family code in the middle, e.g. 74LS47, 74HC47, etc.). You'll need a chip and a 4-position switch for each digit. This, IMHO, is the simplest way to do it, electronically speaking. You will have to memorize the numbers 0 to 9 in binary, and then just set the DIP switches accordingly, e.g. 1001 = 9 - the IC will take care of lighting the segments correctly. With a bit of practice, this will probably become quite easy to remember... and certainly easier than toggling all the segments on and off manually. If I get time, I can post back with a schematic.

EDIT: sorry smoshier, the first sentence wasn't referring to your post - I just wrote mine at the same time as you

philba · Post by **philba** » Sun Mar 30, 2008 6:50 pm

here's an even easier approach. Use a BCD encoded switch, a BCD to 7 seg decoder and the 7 segment display. Your choice of 7 segment display wont work for that as it is common anode and it is multiplexed. Better to use individual 7 seg displays. Jameco doesn't have a good supply but mouser has a ton that will work. Look for 10 mm (which is a hair less than .4").

Here is one digit's worth. Reproduce to your hearts content.

The bcd switch is on jameco http://www.jameco.com/webapp/wcs/stores ... tId=549407
The 4511 is a common part - jameco has them in surface mount. You probably want dip - mouser carries them.
The display is a single digit 7 segment display with right hand decimal point. Common Cathode. When you select the part, we can determine the resistor values.

Phil

philba · Post by **philba** » Sun Mar 30, 2008 7:08 pm

OK, more info. Mouser has cheaper BCD switches. here's one for $3
http://www.mouser.com/Search/ProductDet ... baaw%3d%3d
click on the datasheet link for more info

I've seen these switches cheaper yet on places like goldmine and all electronics. It would pay to cruise those sites.

Phil

edit: I'm goin' wild here. All has the following:

http://www.allelectronics.com/cgi-bin/i ... ITCH_.html
It will work but I'm not sure how professional it will look. 2 for a buck, though...

Engineer1138 · Post by **Engineer1138** » Sun Mar 30, 2008 7:29 pm

Did you guys forget the easiest and cheapest method?

A 7-segment display and a 8-position DIP switch for each digit (+ decimal point). Just turn each segment on individually to set the number. It's tedious, but since you won't be changing it frequently, and you can just make up a chart that shows what switch combinations result in what digit, it's easy.

Stuff like this I would head on over to BG Micro to get it fast & cheap. No affiliation, just a happy customer for many years (I'm playing with their serial VFD disply next to me right now!!)

philba · Post by **philba** » Sun Mar 30, 2008 7:36 pm

the bcd switch is a lot more user friendly but your idea would probably be cheaper (though not dramatically). I prefer the BCD approach since it can be made to look pretty professional and the model RRers I know have a lot of pride in the appearance of their gear.

MrAl · Post by **MrAl** » Sun Mar 30, 2008 7:53 pm

Hi there,

Lots of good ideas.

My idea is to use two pushbuttons and a multiposition switch, or
a set of N pushbuttons switches, and counters for each digit.
The counters are the decimal counters and wired in groups of
three digits. One pushbutton cause the count to increment,
the other pushbutton to decrement.
This would take two pushbuttons and three counter ics per display.
The drawback is that some sort of power has to be applied or
the display has to be reset at turn on time.

Dip switches should work ok too, and will maintain the display
settings even if the power goes off.

The PIC solution would be to drive the display with a PIC and do
everything with two pushbuttons, one for up and one for down,
and store the settings in the EEPROM. This would take two
pushbuttons and one PIC chip per display.
Alternately, use one pin of each PIC for the enable pin, and use
only two pushbuttons for as many displays as you like, selected
each PIC with a rotory multiposition switch instead.

Robert Reed · Post by **Robert Reed** » Sun Mar 30, 2008 9:10 pm

Nothing to add to these great suggestions, but I just had to say congratulations on a fantastic layout! How many acres does it consume?

And how long did it take to complete to its present level? Looks like it might take a whole team of operators to keep things running smoothly.

Bigglez · Post by **Bigglez** » Sun Mar 30, 2008 10:22 pm

Greeting Tom,

That's an interesting project, and I see you have been given
several variations from others.

ChooChooBarn wrote: I'll need 6, 3-digit7-segment LED displays, three for each side of the sign. The 9/10 will be back lit.

It occured to me that the data on each of three signs
is duplicated by the other three signs, and you could
drive both sets of three from the same electronics,
thus saving some time, effort, and space.

It also occured to me that driving the sign from a PC
(as you suggested) would overcome the tedium of
a simple switch arrangement and the need to reset
the display if the power is lost.

A uC (microcontroller) such as AVR or PIC would
offer all of these features and with a serial data
link it could be driven from a PC easily. It would
involve writting simple code for the uC, and running
a terminal emulation program on the PC (hyperterm
or similar). There would be no switches in the project,
and it would recall the last message (display) when
power is applied.

Your $100 budget would cover this project and the cost
to buy an ISP (In System Programmer) that 'burns'
the blank uC chips. No doubt you would soon find other
applications around your project for uC support,
and although I'm not involved with the hobby I see
other vendors offering uC controlled pieces for the
RR enthusiast.

BTW, most medium range uC chips sell for under five
bucks in hobby quantities!

If you decide to invest your time into learning how
to do this I'm sure you will get lots of support from
this forum (or just PM me).

Your project would probably interest N and V if you'd
like to write it up later.

Comments Welcome!

ChooChooBarn · Post by **ChooChooBarn** » Mon Mar 31, 2008 7:01 am

First of all, thanks a million to all who have posted remarks and suggestions.

To Robert Reed:
This layout covers 1700 square feet. There are 22 trains and 150 moving figures and vehicles (animations). It got its roots in 1945 with one train going around a Christmas tree. My father opened the business at its present location in 1961 with 6 trains and 6 animations and it's grown to its present size since then. The display is not finished, as is 99.9% of the model railroad layouts in existence. We improve on it all the time. We close Jan. - March every year and clean it and work on it. I don't have a huge staff of people working on it, it's basically me and a part time guy the runs the display during our open season. It's a labor of love and more of a work of art than a model railroad. 99% of the buildings are scratch built by either my father (he passed away in 2001 at 84) or me. I've been doing this full time pretty much my whole life and I'm 57. Everything is completely automatic. I have multiple trains on loops that do the same thing day in and day out. I use a lot of reed switches, relays, timers, flip-flops boards, LEDs, etc. The display has a fire scene that happens every 5 minutes and nighttime every 20. I accomplish the automation of these using drums and micro switches, archaic, but functional. I have the hardware and software to do all this using a PC, I just don't have the guts to change.

To all other posters:
Thanks again. I will probably go with the DIP switches for now, but will try some of the ideas posted. I love to learn new things, it's just that I don't know enough about electronics, etc., to do stuff on my own. I know enough to get myself into trouble and enough to go out and spend $$$ on chips and componenets I never get around to experimenting with. As we all know, there aren't enough hours in a day to do what we really want to do.

dacflyer · Post by **dacflyer** » Mon Mar 31, 2008 9:06 am

just out of currioucity,,is there something simple like this for
alpha-numerics displays or no ?

muntron · Post by **muntron** » Mon Mar 31, 2008 10:10 am

dacflyer wrote:just out of currioucity,,is there something simple like this for
alpha-numerics displays or no ?

No, it won't be quite as simple. There are two kinds of alpha numeric displays that could be used. The 16 segment display might lend itself to this solution with twice as many dip switches.

The other type is either a 5x7 or 8x8 matrix. This would require a uC or other controller chip (Maxim makes some).

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