justDIY Project Log

I had purchased some huge (2″) 5×7 matrix a while ago, as part of my led sensor research. They’ve basically been banging around the lab since, getting pins bent and such. So this past weekend, I decided to put them to some other use. Their pin layout is sort of weird, it doesn’t match up with a breadboard at all (one of the reasons they never made it into the led research). So, I decided to make up some back-packs for them, or is it a carrier board? Anyway, the board features one 5×7 matrix, one pic 16f737, a few transistors and some data connectors. The board provides two means of serial communication; asynchronous rs232 at 19200 bps, or synchronous i2c at 100kbps. A second connector provides power and ICSP pins.

This project has no practical application as of yet. The main reason I made it was to improve on my double-sided pcb fab techniques. This time I found using point to point traces instead of a large “pour” made things work a lot smoother. I used the ‘sandwich’ method with press ‘n’ peel blue. Roughly 1.5 min per side.

One thing I had to keep in mind while doing this layout was accessibility to solder both sides, since I can’t through plate my own vias. So things had to be laid down in specific order. I soldered the vias first, using some cheap resistors with very fine leads as my conductors. The method involved sticking the resistor into the via, with just a bit poking out the other side, then bending the resistor 90 degrees and holding it to the board. Then a quick dab of solder onto each joint set them in place. Next, straighten out the resistor leads, and trim them off. Another quick few dabs with the soldering iron and each one was fully connected. Next came the smt parts. The switches were rather easy, but those blasted little 0603 capacitors always give me grief. I tin both pads, then apply a bit more flux as “glue”, then try to reheat one of the pads, to reflow the solder onto the part. It works great with 0805 and larger parts, but the little 0603 usually gets sucked onto the tip of the iron by the surface tension of the solder.

One hard choice I had to make was whether to solder the chip straight into the board, or use a socket. I opted for a socket, which meant a harder time soldering the “top” layer. Luckily, I was able to dig up some 14 pin machine pin sockets, and thanks to the machine pin itself, they stand proud of the board a little, just enough to sneak in with the soldering iron.

For whatever reason, I decided to solder the led display next, leaving the connectors for last. I partly wanted to see which side of the board the connectors would look better on… I think next revision, they’re going on the bottom. During assembly of the connectors, I nicked the display a couple times with the iron, oh well!

Right now, the display is flashing my initials. Oh, here is the schematic, nothing exciting really!

quick video:

The huge size of my previous pcb layout kept bugging me … it was about 1.3×1.2″ and would have consumed more than half of the mint tin space! So, I spent few hours coming up with something a little more compact.

This second layout is more compact, but still a bit on the large size; measuring 1.2×0.9″. I won’t be able to make it much smaller, without going double sided, and thats just not something that’s easy to do at home.

I’m not sure what happened to the weekend, but it’s almost over, and I haven’t even dusted off a breadboard yet. Oh well!

MintLite – The Luxeon Powered Mint Tin Flashlight!

This idea has been rattling around in my head for more than a month now, and I finally have thought it out enough to do some doodling in Eagle. The basic idea is built around a six watt Luxeon K2. I plan to use a pair of 2.5aH lithium batteries to provide approximately eighteen watt-hours of power. The Luxeon will be controlled by a microcontroller, providing different brightness levels, as well as protecting the luxeon from excessive current when the batteries are fully charged. The microcontroller will also monitor the battery voltage; dimming the light as needed and eventually shutting down completely to prevent over-discharge. The light will contain it’s own battery charger, powered by USB using the MAX1811. The MAX1811 will charge a single lithium cell (or cells in parallel) at up to 500mA off a self-powered USB port. The 1811 allows charging from a bus-powered port as well, but for sake of simplicity, I will ignore that option.

The circuit keeps things fairly simple. Switch Q1 provides pwm control of the luxeon. Header SW1 will connect to some sort of switch, for turning the light on and off, and changing brightness. The MAX1811, IC2, takes 4.3 to 6.5 volts as input, and regulates it to 4.2 volts for charging the lithium cells. Charging status is indicated by LED2, which will light when the charger is in bulk charging mode (current mode).The microcontroller, IC1, is a PIC12F683. The 683 provides a lot of bells and whistles for such a small chip. I will be using analog input 0 to monitor the battery voltage. General purpose input 2 will monitor the charging status, perhaps to disable charging when the batteries are in bulk charging mode. General purpose input 4 will use an internal pull-up resistor to monitor the switch. General purpose output 5 is controlling a mosfet transistor responsible for PWM of the led.

The pcb layout is in it’s early stages, and designed mainly around parts I have on hand. I don’t think I’ll actually prototype this PCB, since it’s much too large, and the wrong shape. Whether it gets printed or not, it was fun to draw. There are two main things I want to change. First the FET (Q1) in the TO252 package is rated at something like sixty amps – way more than I need for this project. ON Semiconductor has some nice SOT23 fets rated at 4 amps that should fit the bill nicely, and save a lot of space. Secondly, I need to find a smt version of the usb connector, perhaps a mini usb instead.

Hopefully this weekend I’ll be able to breadboard this circuit and see how it all goes together. Stay tuned for “Part II”.

If necessity is the mother of invention, being cheap must be like a step-father or uncle or something?

I needed some level shifters / line drivers that I could easily use on the breadboard for microcontroller projects. Rather than give some other entrepreneur $10 for their version, I made my own.

Nothing particularly special here, aside from the cool factor added by the two LEDs which light on RX/TX events. The chip is a standard MAX232 clone, and I’ve got five 1uF ceramic caps mounted on the back side. The leds are driven by some SOT-23 transistors. To make things simple, I stuck the pin header through the board the “wrong way” and forced the pins almost all the way through their plastic spacer / retainer. This way the unit plugs right into the breadboard, and lays there real nice, even with a heavy serial cable attached.

So far so good, it works well with my bootloader at 115kbps, so I figure that’s good enough!

EDIT: Eagle SCH and BRD files available here (7-zip format).

The “core” building-block I mentioned in my last post is nearing completion, at least, the software.

I have my microcontroller handling “switch emulation” tasks. It can emulate either group of momentary switches or a group of toggle switches. Response time is real good in a dimly lit room, and it works decently well even with the overhead fluorescents turned on.

Right now I’m working on a basic keypad PCB I can throw together, for a ‘proof of concept’ prototype. The first keypads will likely only support 6 keys, and I’ll build from there. Six keys requires twelve LEDs, six of them need direct and discrete anode and cathode connection to the microcontroller. The other six LEDs are providing bias light for the sensors to “see”.

I should have a video up tomorrow of the breadboard in action, and hopefully some pcb’s by next weekend.