Mint Tin Bike Light

I started this project a little more than a year ago, but shelved it because it wasn’t working right and I didn’t have the correct components. It was a seasonal project that would have little use over the winter, so I sort of forgot about it.

This year I’ve been going on a lot of bike rides with friends, sometimes on public roadways, sometimes after dark. My bike has a nine watt 500 some odd lumen headlight, which makes it easy to see where I’m going, and definitely makes me visible head on. The tail of my bike however still has the stock reflector, plus the little reflector stripes in my shoes, not exactly high visibility. Not wanting to pale in comparison to the headlight, the taillight is a three watt 140 lumen beast powered by three AA rechargeable batteries.

The light is based on a boost converter from National Semiconductor, the LM3410. I’m using the 525kHz SOT-23 version, the LM3410Y. Originally I had trouble with the chip self destructing, as discussed on the Linear1 forums. It was hypothesized either the inductor was underrated or the diode was too slow. Ordering parts for another project later in 2008, I bought some better inductors and diodes, which more closely resembled the specs of parts used in National’s web bench simulator. So, lacking sufficient rear light, I rekindled this project and have a “working prototype” that’s gone on two rides with me so far.

bike taillight schematic small

The basic function is fairly simple. The 3410 is a constant current boost (step-up) driver. A small inductor is used to ramp up the input voltage, from 3.6vdc nominal to 15.4v at approximately 200mA. The current is monitored by a one ohm resistor. A pair of output capacitors help smooth out the ripple and an input capacitor helps the batteries cope with the high demand current (as high as 1.5a in some cases). I’m using nickle metal hydride batteries, which have a rather low internal resistance – they’re designed for high demand applications and when fresh, barely sag at all under the load.

bike taillight pcb layout small

Originally I had planned on carrying the batteries directly on the PCB, using some through-hole spring clip battery holders I found in the Sparkfun library. However, AA batteries must be bigger in Colorado than they are in Michigan, because using Sparkfun’s layout gave me about a quarter inch gap between the spring and the battery. The pads were also woefully undersized for physically mounting the clip and holding it securely enough to survive the stress of batter insertion and extraction. So I dropped their layout and drew my own that looks exactly like it, but is based on measurements from a real AA battery.

Along for the ride is a Microchip PIC microcontroller, the 12F683. It provides a bit of user interface for the light, creating different blink patterns as well as putting the light into a “stand by” mode, with the switcher shut down. I’ve programmed several blinking patterns, and somewhat organized them into “modes” which I can select using the little button.

A year ago, I didn’t have any sort of enclosure in mind. The led array was assembled on a ‘standard’ sized protoboard, so I probably thought about using a plastic or aluminum prototype enclosure. However, this year, I was thinking it would be a nice fit for a large mint tin. After printing out some mock-ups and messing around with battery configurations, I settled on using three batteries and having the electronics crammed into one side of the tin with the led array mounted in the lid of the tin. This setup might have worked, except for the battery snafu. I’m using a plastic three cell holder right now, and the extra thickness it adds is preventing the lid from completely closing. It closes enough that the light is easily held shut by some big rubberbands, and it survived bouncing around under my seat for two short rides. The next revision will have the battery situation resolved and I might have a better mounting solution by then too.

Overall I’m very pleased with the outcome of this project. I have more parts on order to make a few more lights for my other bikes and friends, and I want to experiment with other array configurations and colors. There are a two videos of the light on my youtube channel, but they’re nothing to get excited about.

Thanks for reading!

LED Cube

I was trying come up with something impressive to write about, after having the blog dormant since July. I tossed around a lot of different ideas… Although I haven’t been writing, I’ve been dabbling in a number of different projects. None of them are really in a state I’m ready to write about. That brings me to something I saw a few months ago on MAKE:. Bre and associates had constructed a simple 3x3x3 led cube, re-purposing a POV toy to drive the leds.

I figured this would be a good easy project I could finish in a day, so I drilled out a piece of pine board and set to soldering up some 3mm leds. I wasn’t very careful, so the little matrices look kind of ugly, but it all works and you can’t see the wires in the dark!

3x3 led matrix building

It didn’t take very long to toss three of these nine led matrices together. Assembling them into a twenty-seven led cube was a bit trickier. I used some gator clips to hold parts of the cube while I soldered it. Eventually I finished all the connections and had a passable cube with fairly even spacing.

3x3x3 led cube finished

Assembling the matrix is a pretty straight forward task. All you really do is tie all the cathodes together. Each matrix will become one row in the finished cube. Electrically, the cube is built as a 3×9 array, three rows and nine columns. You could probably build it the other way around, anode rows and cathode columns, but it is easier to sink a large current than source it. I think the MAKE: software only lights one led at a time, since they’re relying on the microcontroller to both source and sink current. My design is a bit different. The mcu sources current to each anode column, and N channel fets sink current for the entire row. The N channel is easily able to sink a few amps, so the cube can light an entire row at once without having to multiplex the individual leds.

In order to keep the PCB layout simple, the connections are spread all over the place in terms of the registers inside the pic. It would have been cleaner to organize eight of the nine columns as a single 8bit register on the pic, leaving only one bit left over to deal with. Instead, I’ve created symbols for each column, and set them individually from 9bit numbers.

Each anode column is current limited by a 75 ohm resistor. The value chosen was rather arbitrary, since the leds have such a low duty cycle, a lower value would have afforded me more brightness when the cube is battery powered. I can tweak the brightness a bit in the software, changing the scan rate the rows are multiplexed at.

That’s pretty much it. I’ve found there’s not a lot you can do with only 3x3x3 and 1 color, but it’s still kind of fun. Trying to think in three dimensions while drawing the animation frames is kind of tricky. I started with excel, but that wasn’t very useful – I spent more time copy and pasting formulas than I did ‘drawing’. Luckily my buddy Dan helped me out with that. He whipped up an awesome little php script that lets you draw animations 27 leds at a time, and it formats the resulting numbers so I can copy and paste them right into the code.

There’s a few videos of the cube doing various things on my Youtube Channel. Here is perhaps the most interesting one so far.

Thanks for reading, and Happy New Year!

Capacitive Sensing Continued

Hello readers from MAKE: as well as all other readers 🙂

My prototype touch sensor worked so well, that it hasn’t needed much changing. I sent the design off to Custom PCB, and less than a week later, I had a pile of circuit boards waiting for me.

I changed the layout around a little, mostly adding a 2×8 header for accepting a ribbon cable style connection. The header combines power, ground and outputs into a single connection, making it easier to connect to the main board of my larger project (sprinkler controller). Each touch output is paired with a ground wire, which I suppose makes it more resistant to interference. The caps I used this time are polyester film 220 nF, doubling the amount of capacitance compared to what was used on the prototype.

Yes, the ugly piece of plexi is still ugly. Don’t worry, it will be hidden from view. In the final configuration, this board and its plexiglas spacer will be inside a plastic project box. I’ll have a laminated “keypad” overlay affixed to the outside of the box so I can see where the buttons are. The spacer will be flipped around, going on the solder side, giving me enough clearance to flush-mount the sensor with the wall of the box. Flush mount is very important, as even the slightest air-gap will ruin the proximity sensing effect.

Nothing much to see solder side… a few smt passives set options on the chip, as well as decouple and filter the incoming power. The big resistor limits current for the meager power led which no one will ever see once the board is in use.

I’m very close to finishing the larger irrigation control project, hopefully sometime this week! Thanks for reading!

Capacitive touch sensing

Presenting “6buttons”; a simple six button keypad based on the QProx QT160 charge transfer proximity sensor chip.

More details to come later, wanted to get some pictures and video online tonight.

simple schematic – i plan a “backpack” pcb which will provide some visual feedback, a clicking noise and translate the six outputs into an i2c bus device.

the brains of the operation, this chip does all the work. special mylar capacitors are required for it to work properly. i tried cheap-o ceramics with terrible results. the orange thing is a 10mhz resonator.

the “buttons” are printed out on plain paper, using the silkscreen layer from my pcb layout program. the capacitive dielectric is provided by the FR4 pcb material, the paper and a 1/8th inch thick layer of plexiglas; I guess you call that a multilayer capacitor!

the sensors are simple copper rings, which radiate the electrostatic field this chip uses to sense proximity. a ground plane pour around the IC helps to minimize cross-talk between sensor channels and prevent stray fields from detecting proximity around the chip itself. the capacitors near the chip also sense proximity and will need to be shielded with aluminum foil or something.

forgive the craptastic music in the video!

edit: also read Capacitive Sensing Continued

Random picture

I found this digging around some old forgotten directories on my web server.  It appears to be four dimly lit RGB leds.

5mm rgb led

LED Clock; Firmware taking shape

The firmware for my LED Sign / Clock project is taking shape.  I’ve worked out my initial wishlist of features, and put together a basic menu structure.  I figured a menu is the  easiest way to access a lot of options, while only relying on two buttons.

The basic menu consists of options for setting the time and date; minute, hour, day, date, month, year.   The basic options are pretty self explanatory.  I have also planned some more advanced options:

1. Adjustable display brightness – Varying the duty cycle and refresh rate of the matrix can effect its brightness.  I need to make a simple scale for these variances to allow for two or three brightness levels.

2. Adjustable scrolling speed – The program draws the same information several hundred times per ‘scroll’, changing this counter affects how fast the display appears to scroll.

3. Selectable time format – User can choose between 12 hour AM/PM time and 24 hours ‘Military’ formats

4. Message Mode – User can enable / disable scrolling messages as well as display a single message, a random message or messages in sequence.

Other than that, the code is still pretty basic.  I’ve completed a bunch of internal fixes, like the scrolling code now handles messages of variable lengths. up to sixteen characters.  I’ve also created i2c eeprom reading routines to extract menu prompts and other text strings I’ve stored in the eeprom to save flash space on the chip.

LED Sign has a purpose!

The single character LED sign I had been playing with now has a purpose! Shortly after discarding several ideas of having it as a serial display for PC/Server status, or hooking it up to the internet and a webcam, I came up with an actual useful purpose. The sign can be a clock! I have two ‘modes’ planned; traditional numbers and binary. All geeks love binary clocks, but most of us are lazy and would rather read regular ‘ol numbers.

To facilitate the role as an clock, I had to redesign the circuit quite a bit. Two new ICs were added, allowing the LED Sign to keep time, and providing some much needed storage.

led matrix sign clock rtc microcontroller schematic

The first of the new ICs is a Real Time Clock. Because I’m cheap, I chose the M41T80 from ST Micro. It’s an inexpensive clock, similar to the Dallas DS1307 but lacking a few minor features. First, the clock has no power-on reset detection. It just starts up as soon as power is supplied. The Dall 1307 has a stop bit which gets set if the clock experiences a POR, so the firmware can test if the clock needs to be initialized or not. The T80 datasheet mentions some registers may get set to default values on power up, so I’ll have to read it a few more times to see if there is a way I can check for a POR. Second, the T80 has no support for a separate backup battery. Instead, ST recommends you place a diode in series with the clock, and use a large capacitor to provide backup power. Last, there is no automatic leap year / leap second correction, oh well!

The second IC is a 16 kilobit serial eeprom, similar to the Microchip 24C16, I chose one from Catalyst semiconductor due to lower costs. The eeprom is arranged as eight banks of 256 bytes each. The chip contains a 16 byte write-buffer, I’m not sure if it can cross a bank boundary or not, I’ll program my firmware assuming it can not. The eeprom will be storing character strings related to operation of the LED Sign as a clock, as well as user programmed messages and possibly simple graphics.

I’ve also added some micro switches for adjusting the clock and changing settings, also a 32.768kHz crystal was added to providing the timing source for the RTC.

pcb layout led sign clock

At this point, the layout of the printed circuit board has become pretty complex. I tried making one of these at home, but didn’t have the patience to exactly align the top and bottom layers of my press and peel sandwich. So, I decided to try a pcb prototyping house. There are a lot of board houses to choose from, many of which cost an arm and a leg. All of the domestic board houses are ruled out, I’m sure they do a fine job, but they cost too darn much. I settled on Spark Fun’s BatchPCB service. They’re not the cheapest board house out there, but their cost is fair. They include a lot of features most other board houses charge extra for, like double sided silkscreen and solder mask, 8 mil pitch and spacing, 20 mil holes, etc. I placed my order on the 6th, and had the PCBs by the 22nd. All the time in between, by mind set to wandering, and I made some POV toys. Once the pcbs showed up, I incurred another delay. Turns out I hadn’t ordered my RTC chips yet! So, another few days wait brought goodies from Mouser (man they are quick, and inexpensive!)

batchpcb led clock circuit board

The boards from batchpcb look awesome. Nice bright green solder mask, tinned pads and holes, smooth clean edges. This is the ‘top’ side of my led sign. There are a few passives on this side, along with the two new ICs. This side is covered by the LED matrix once the board is fully assembled. Don’t mind the flux smeared everywhere – I did clean it off before soldering the matrix down.

led clock circuit board

The bottom of the board contains the PIC processor, mosfet column drivers, a crystal for the clock and some microswitches. Pin headers for power and programming the microcontroller have also been installed. The module is a bit thick at this point, thanks to the socketed IC and the pin headers. On the finished version, I’ll solder the IC straight to the circuit board, and probably use wires instead of a header to supply power.

led sign clock done

The firmware is in the early stages right now, so my next post on this subject will try to cover whatever features I’ve decided on. Right now I know I want a few things:

1. Scroll the time
2. Occasionally scroll the date
3. Occasionally scroll short messages, either randomly or programmaticlly
4. Support some sort of software brightness control

I’ve made some revisions to the design since I had these boards fabricated. One big oops I made was forgetting the pull-up resistors for the i2c bus! Luckily there’s plenty of room on the board, and both i2c lines ran near the Vcc rail. So a little quick scraping action to peel back the solder mask and presto, new lands for 0603 sides resistors. I’ve also added a diode and big capacitor for the RTC’s backup power.

I hope to work on the firmware more this week, so I should have more details about how the clock works next time!

EDIT: Added a quick video of the time scrolling


I recently had the urge to create some “eye candy”; Thinking along the lines of my LED Sign (it’s not dead, just waiting on parts!), I chose to create a similar effect, using only a single line of leds, instead of a 5×7 array.

Persistence of Vision is some sort of effect, either psychological or biological in nature, that allows our eyes and brain to ‘see’ motion and patterns in a sequence of rapidly stills (hence a movie projection). This effect can be exploited using basic digital electronics to create a virtual LED sign, which the viewer will ‘see’ when the pov toy is put in motion.

Commercial POV toys are usually some sort of led matrix attached to a spinning contraption, and display either a fixed or scrolling message. Those are fine, but the technical aspects of motorizing the display make it beyond the scope of a simple project. Therefore, my project relies entirely on elbow grease – swing the wand and watch the messages appear.

Revision one of my POV toy is compact. It measures approximately 2.25″ length wise by 1.1″ width wise. The circuit consists of an inexpensive Microchip PIC 12F683 microcontroller, and a 74HC595 serial shift register. The display consists of eight 3mm leds, I chose blue for the first unit, but I’ll probably make a few more with different colors, I especially want to see green and amber – those leds don’t see a lot of action in the world of consumer electronics.

pov led microcontroller shift register schematic

The circuit is very simple. A tiny microcontroller provides the brains, a simple and inexpensive shift register provides the brawn, and a switch and hall effect sensor provide some control. My first unit has no hall sensor, so power is controlled with the push button switch. When “off”, the pic is in a ultra-low power sleep state. Come to think of it, I could have put the shift register into a tri-state mode when “off”, but for now, it holds its gates at Vdd, which provides no bias across the leds, so there shouldn’t be much current going anywhere. The hall effect sensor will eventually control the power to the display, as well as provide some synchronization. Right now when swinging the wand, the message is legible in one direction, and backward in the next. I plan to try putting a small round magnet in a tube, and gluing it behind the hall sensor. When swung in one direction, the magnet will be forced over the sensor by g-force. When the wand is swung back, the magnet will be forced away from the sensor. This should let the microcontroller know not only to blank the leds during the ‘backward’ swing, but also how much time it has to render the entire message, speeding up the display for a quick swing, or slowing it for a long swing.

The circuit board is a single layer layout with two jumpers. I went with as many SMT components as I could afford, since I hate drilling holes in my home-made circuit boards. Power is currently provided by a 3.7 volt 1600mAh lithium ion cell.

I tried making some video of the unit in operation, but my camera wouldn’t play ball. Next weekend I may have more time to fiddle with photography and try to get some ‘action shots’.

Although version one has been a great success, and was easy, I was unsatisfied. The eye candy just isn’t sweet enough. Version two takes the flash factor up a notch, with RGB leds. I’m working on a ‘full color’ POV wand toy. Version 2.0 has been built, but it has some flaws – and is the subject of my next blog entry. Here is a video of “2.0” running a test pattern on its 24 led array.

LED Sign; Now With Scrolling!

This morning I hammered out a quick and dirty method to scroll a message on my single character LED sign. The scrolling is based around a few subroutines. A text message of 12 characters is passed to the scroll subroutine, along with the number of loops to make. The scrolling routine breaks the text message down into individual bytes, and loads up a static 60 byte message buffer based on data from the font table. Each “scroll” iteration causes the program to copy data from the message buffer into a 5 byte display buffer. The message buffer address is offset by one byte each iteration. Since one byte equals one column on the display, this action cause the display to appear to scroll. Another sub routine watches the address pointer, and wraps it around to the beginning when appropriate.

Single Character LED SIGN

We’ve all seen the various sites where someone put a massive led sign online, and they let people abuse it. What I’ve managed to build is a “single character” led sign. I don’t know if I’ll be able to get my sign online, but I’ll take a stab at it.

After wasting a lot of time to convert a font table I borrowed from somewhere, my little sign finally had a built in character generator. It can generate any of the printable original ascii characters (dec 32 to 127), and display them on the matrix. Data feeds into the sign at 4800 baud (I think i mentioned 19200 earlier – way too fast), and is displayed on the matrix for a few seconds.

If the display sits idle for too long, a basic screen saver (screen waster?) kicks in, and does a simple animation. Any received characters replace the animation and reset the screen saver timeout.

Although exposed, I haven’t implemented the i2c interface yet. This pc board has some issues, and is too large. I don’t like the connectors sticking out to the side like that, so it’s back to the cad package to try and re-arrange things a bit. I think I will fork this design into two separate paths. One async serial (rs232), and one sync serial (i2c / spi). The sync version would be used as a display for other projects. The async version would be hooked up to a PC, and programmed with a simple message. Once detached, the display would just repeat the message over and over. One problem I ran into is storage. I had hoped to store text strings and the font table in eeprom, but the processor I chose, the 16f737 has no on-board eeprom. So instead, the font table is stored in the flash program space along with a few short text strings. The next revision(s) of the board will include a spot for a so8 ‘seeprom’ for extra storage.