![\"flw1\"](\"https:\/\/howiem.org\/projects\/wp-content\/uploads\/2014\/05\/flw1.jpg\")
<\/a><\/p>\nDesigned and built by Jeff Thomas, Peter Hand, and Juergen Grau. More information here<\/a>.<\/p>\n I wanted to make something quick and simple as a birthday present for a friend, and I had four little LED starburst displays sitting in a box, so some sort of random word dispenser seemed like a good idea. Designed a quick circuit in Eagle:<\/p>\n Download schematic and layout: FourLetterWord<\/a><\/p>\n The schematic in Eagle is pretty messy and tangled. I often find that if I’ve got loads of connections to make to a microcontroller, it’s not until I’m laying out the board design that I can see the way things should be connected.<\/p>\n This is the perfect example: I’m connecting two identical starburst displays to the microcontroller in a multiplexed fashion. In an ideal world you’d connect the anodes of the displays together, segment A to segment A, seg B to seg B and so on, leaving just the cathodes of each digit to be connected separately. This makes the software a little simpler – pin D3 (say) on the microcontroller controls the same segment on all the digits: On a single sided PCB, though, routing the connections like that gets really tricky – you end up with loads of connections that have to jump over others so they end up on the right pins.<\/p>\n The alternative is to design the circuit so it’s easy to route (even if that means segment A on one display is connected to segment F on the other etc) and then sort it all out in the software.<\/p>\n Hence the relatively neat looking PCB design:<\/p>\n Note that there’s an error in the design \/ layout – I didn’t realise until too late that two of the microcontroller pins I wanted to use (A6 and A7) wouldn’t work as digital pins, so I had to use some wires to connect them to some free pins on the other side of the controller. Live and learn.<\/p>\n With the PCB milled out and populated, I hacked a rectangular hole out of a random wooden box I had knocking about, and stuck a switch and a button on it, and squished a bit of foam and a CR123A battery holder in.<\/p>\n CR123A batteries are great for this sort of project – they only run at 3 volts, but that’s enough to drive LED displays without needing to add current limiting resistors. Helps that the display is multiplexed, too; though it looks like all the displays are lit up simultaneously, they’re actually taking turns, one digit at a time. Stops the LEDs from burning out – at most they’re on for 25% of the time.<\/p>\n The software’s pretty simple – there’s a list of about 45 swear words chosen at random. Proper randomness isn’t easy for a microcontroller (or a computer) to do on its own, so I measure how long the user has pressed the button, in microseconds, and use that value to help choose a word.<\/p>\n Press the button and an animation sweeps across the display, with each letter of the word coming up one at a time like a fruit machine.<\/p>\n<\/a><\/p>\n
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