justDIY Project Log

I don’t have much to say on these, other than I scored a bunch from Future for a seemingly great price.

These appear to be HEAVY DUTY leds, destined for the automotive market. They’re discontinued now, as Lumileds is pushing the all mighty rebel for every application under the sun.

Apparently lumileds marketed these leds strictly as automotive indicator grade leds. Their design guide shows a stop light made of six of these leds, spot welded in a 2 x 3 array to heavy solid aluminum buss bars instead of a typical PCB mounting. I won’t be doing any of that, but I did draw up a layout in Eagle and came up with a 2 x 8 array for my mint tin bike light.

This board is etched and waiting to be cleaned and assembled, more pics to follow!

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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.

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.

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!

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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!

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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

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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.

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.

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!)

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.

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.

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

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