Alphanumeric LED Display

Alphanumeric LED Display
Originally uploaded by DanWake.

HPDL-1414 Alphanumeric Display

I recently acquired a little gem from an old piece of equipment we were throwing away at work - that gem is the HPDL-1414 four character alphanumeric display. It is a small chip manufactured by Hewlett Packard that contains four tiny, red LED alphanumeric displays behind glass magnifying lenses. The chip itself is CMOS, however it runs at TTL voltages, making it easy to interface with. Interfacing is very simple from a logic level as well - simply set the write enable pin high (disable writing), set the 2 bit character address via the address lines, set the 7 bit ASCII value of the character via the data lines, pull the write pin low (enable write), rinse, lather, and repeat for each character. The only disadvantages that I have seen with this display are that it does not support full ASCII (capital letters and some symbols only), the characters are not very bright (although this could be because I'm not giving enough voltage to VDD (I'm currently running it off of the 5v from the parallel port to save wires), and the letters themselves are fairly tiny. Otherwise, this is a great little display that is very easy to work with and tons of fun to write software for.

Currently I have a few functions written in C to put characters at individual addresses, scroll a text string at arbitrary speed (from super, mind-numbingly fast, to as slow as you want), and just for fun a function that switches all of the segments from '1' to '/' to '-' to '\' fairly fast, producing a rather interesting and attention-grabbing spinner effect.

The physical aspects of the construction were fairly straight-forward: I hacked up an old parallel printer cable for the computer connection by using trial and error with a multimeter to find which wire goes to which pin; the case is, fairly obviously, an old Altoids tin which I Dremelled out to make a window and lined with electrical tape. I initially tried to make my parallel cable using a 25-pin header that I tried soldering phone cable to. I would not advise trying this, as phone cable insulation tends to melt at fairly low temperature making it very not-fun to solder. And - after I eventually did get all the little wires soldered at each end, one of my lines was goofed up preventing all of the characters from appearing, and generally frustrating me to the point where hacking up an old printer cable was rather cathartic. Just a word of warning.

So - if you can get your hands on one of these, by all means go for it - I'm sure that it would be very useful in any project for which you can spare a few data lines and want to add a really neat retro looking display to get some data to the user.

Fun Little Project: Minty Mouse

First off - I'll admit - others have used the same name for their projects like this. And no, this really isn't directly related to electronics. But, it is related to fabrication which is usually the last, and most overlooked, step when doing a project. You've got a wonderful little circuit that amplified headphones, charges your iPod, and squeezes fresh orange juice, but how do you package it? Radio Shack sells some nice project enclosures at fairly reasonable prices ($2.29 - $6.99 right now) that give a nice finished look to a project.

You can also use items that aren't necessarily designed for the hobbyist, but which work very well. As in this project, the Altoids and Altoids-like tins are fairly popular for their small size, durability, and cheapness; you also get mints as a bonus! Gladware, Ziploc, and other similar containers are relatively cheap and can provide an assortment of sizes for various projects. They can also be made water-tight fairly easily and are made from thin plastic which is fairly easy to work with. For my Christmas light project this year, I'm building everything into a cheap Stanley tool box that I picked up for around $10 at Lowes.


With all of that out of the way, here are the details on the construction of my version of the Minty Mouse. I started with a pseudo-Altoids tin and an old mouse that I found at work. I opened the mouse, discarded the top piece, removed the circuit board, and cut the sides off of the bottom piece to make it slightly smaller than the circuit board. I then sanded what I thought would be the thickness of the tin from the bottom of the bottom piece (later, it would turn out to be too much...) I then drilled a hole through the tin at the location where the mouse ball would need to protrude and smoothed the edges with sandpaper. The circuit board just fit inside the case without wiggle room, but the enclosure for the ball was a touch too tall - sandpaper took care of this and still left everything workable. The buttons were probably the trickiest part of the entire project - both in adding them to the top of the tin and in getting them to interface properly with the switches on the board. I cut three notches in the top of the tin using a Dremel (a tool which I would highly recommend!) with a grinding wheel. I also had to cut a notch out of the top edge of the bottom of the tin so that the buttons would have a place to move downward when they are pressed. Once I had finagled the buttons so that they operated clenaly, I used the trial and error method of determining how far above the switches the bottoms of the buttons were and bent two 'U'-shaped pieces of aluminum which I super-glued to the buttons.

[edit] I forgot to mention what I did to fix the fact that I had sanded too much off of the bottom of the mouse. This fix was accomplished by cover the bottom of the tin with a packing label (also procured from work...) and building up a bar at each end of the bottom with thin strips of labels. The other advantage provided by this is that it makes the bottom look uniform (and covers up the lousy hole that I cut in the tin...) and helps to elevate the ball so that it rolls better. [/edit]

All in all, this was not a terribly difficult project, just time-consuming, but I think the results are neat, and it's always interesting to see the looks on people's faces when the see you mousing with a small tin - especially if you open it up and offer them a stashed mint!

Computer Setup

When building electronic projects, it's sometimes nice to have a PC around - for looking up datasheets online, pulling up schematics, or even for providing I/O while testing a circuit. My recommendation for a machine would be to use an older computer with a Pentium II 233 or so processor and as much RAM as you can cram into it. The older machine is nice in that it is fast enough for most simple things that it will be used for on a bench, but old enough that it can support all sorts of old cards and should have most of the "legacy" ports built into it.

These "legacy" (I hate that word - these are such good ports, they're just old - call them classics or something) ports are the standard parallel, serial, and MIDI. The parallel port, which long-time readers of this blog should know is my favorite, provides 12 digital output lines and 5 digital input lines. The serial port is another handy one which provides a TX (transmit) and RX (receive) line which can be used for programming, data-acquisition, and other similar uses. The unsung hero in my opinion of these classics is the MIDI port which provides 4 analog inputs which are each converted to an 8-bit value and 4 digital inputs.

In addition to ensuring that these ports are present, the machine should have a network interface card (NIC) if you are planning to get online, or at the least a modem and a phone line. It might also be wise to look for some of the older ISA cards that are available that provide extra ports and connectivity - check eBay, local auctions, or old computers that people might be willing to part with for these.

For software, I would suggest running either Linux or a version of Windows prior to Windows 2000 - Windows 98se is a good choice as it is the latest version before 2000 that isn't ME (I would advise no one to run ME...). The old versions are suggested because the versions of Windows that are NT based have security to prevent low-level access to the hardware ports. Also - it's nice to run real DOS sometimes for older applications and command-line based programs that you might write. Linux is also a very nice choice since it is so open, easy to program for, and lots of people who do hardware work run it.

Programmable Integrated Circuits

A highly useful component for all sorts of projects is the Microchip brand PIC (programmable integrated circuit). The chips contain a microcontroller, some RAM, and some storage space (quantities and speeds depend on the particular model). Basically - one chip can act as a tiny stand-alone computer with inputs and outputs for controlling other circuits. Most are programmable in-circuit allowing for updates to the code that is run. To program one, you will need to build or buy some sort of PIC programmer (specifics again vary by model of chip to be programmed). You will also need some software for compiling code (unless you want to write assembly - your choice!) and sending it to the PIC. Microchip itself provides a free integrated devolpment kit (IDE) called MPLAB that works very well.

If you are having troubles coming up with uses for this highly versatile devices, there are many ideas and projects out there. Good luck!

Etching a Circuit Board

You've got a circuit that you've prototyped on a breadboard, built using perfboard, and now you want something a little more professional to sell/give away/use/whatever. Or maybe you don't. Either way - you might want to take the next step with a good circuit and etch your own circuit board. This makes construction easier, the circuit potentially more compact, and it looks a lot nicer to boot. There are a few different ways to go about making the board - the most common is to apply some sort of a mask to a copper-clad board and then soak the board in a solution that dissolves the non-masked copper. Laying out the mask can be done by hand with a resist-ink pen (RadioShack sells a decent kit for doing this), ironing on a printout from a laser printer, or by using a special photo-resist board.

Another method which is somewhat popular (but much more difficult) is to make (or buy) a computer numerically controlled (CNC) mill. This is basically a drill press that has some method of either moving in two dimensions or moving the board in two dimensions that is controlled by a computer. This allows the piece to move past the drill bit, which is controlled vertically by the computer. If the bit is just below the surface of the piece as it is moved, a line will be cut out of the copper cladding. If the piece stops and the drill bit goes down, a hole is drilled through the board where a component will go. This whole setup, while complicated, allows for nearly complete automation of the process and can produce very nice boards.

Computerized Christmas Lights

Yup, you heard (read?) right... computerized Christmas Lights! Why? Why not?! The last two Christmas seasons I have gone to see a very interesting light display in Winona Lakes that was put together by one of their Computer Science professors. It consists of a lot of strands of christmas lights - some on trees, but most on a large conical "Christmas Tree" that is against his garage. All of the lights are controlled by a computer running Linux and some custom software (actually, custom programming language - the guy is a CS prof...) that controls when the lights are on and off, which allows the tree to change colors and display patterns. All of this is set to music that is broadcast via a small FM transmitter, so you can tune in on your car stereo to hear it.

This got me to thinking about how cool it would be to build my own version, on a much smaller scale at first. I looked around and came across a website that dedicates itself to and calls itself Computer Christmas. The layout leaves a bit to be desired, but there are plenty of circuits and great ideas available. This circuit provided the inspiration for me to begin putting my own together using twelve of this circuit and the twelve output lines of the (wait for it...) parallel port. I've had all of the parts ordered since summer and have just been working on getting around to putting them all together. This is where Monday's post comes into play - the boards that they sell work with the parts I already have, making assembly very very easy.

So - be watching for further updates on this project, and hopefully around Christmas time some pictures/videos of my light display.

Great Site with (sadly) Only One Product

I was researching some equipment for work and came across www.simpleio.com which has an opto-isolated triac board that they sell in various different forms. First - there is a version with eight triacs and another with four. They also sell the same boards without terminal connectors for a substantial saving, kits that can be assembled (with and without terminal connectors), and just the bare boards that can be populated with your own components. As if this weren't enough - they also have the schematics and board layouts available to download for free! But wait, there's more! There's also a section with details about how the circuit and the individual components work. The whole feeling of the site can be summed up quite well in this from the schematic page: "I hope you like this enough to buy the boards (the triac output bare boards are fun!), but even if you don't buy from us, you may know someone who will." This looks to be an excellent site and I plan to purchase some of their boards for my own use - more on that later.

Add Another Display to your Computer

Want a way to show a little more information on your computer without taking up any of that precious screen real estate and without shelling out the big bucks for another video card and monitor? A simple (and cheap!) HD44780-based (or compatible) character display can provide all this with a little money and a little effort. Connections are fairly simple - there are eight data inputs to the chips which connect to the eight output lines of (you guessed it...) the PC parallel port. The next bit is a little tricky, depending on your display. Some displays run off of +5 volts which is quite easy to get - it is quite common on computers and is also easy to generate using an LM7805 voltage regulator. Others, like the one that I used, require -5v to run which isn't quite so easy to find or make. I found a handy little circuit that uses the very common and quite useful 555 timer to convert an input between +6v and +35v to -5v.

There is a lot of software out there for sending text to the display - from a Linux kernel driver to a WinAmp plugin to all sorts of programs for displaying system information.

Circuit simulators

A good way to test a circuit design without actually going to the trouble of buying the parts, putting it together on a breadboard, and then trying to troubleshoot it/buy new parts when it (almost inevitably) fails is to use a circuit simulator. One that I've used and like fairly well is a java applet that is simply called "Circuit Simulator". It has quite a few different components that it simulates and since it is java, it should work on any platform that supports java - MacOS, Linux, even Windows.

I've used other simulators, but I can't remember what the names of them are at the moment, but I will check with some of my friends who have used them as well to see if they can remember specifics. Searching at the moment is proving fruitless... more to come later, hopefully...