Sawyer v2 schematic drawn in DipTrace

This schematic is basically the one drawn by Mike Ardai (N1ist) for his Color Stick project. I changed only three major things:

• He uses surface-mount components; I have changed this to a through-hole board.
• I’ve added an on-board power supply.
• Mike’s application requires three TLC5916 constant-current drivers, while in my project only one is needed. But the Star needs the addition of a 74HC595 and UDN2981 to source to the LEDs.

Through-hole vs. SMT: While I’m hoping to get some SMT experience in the coming months, I plan to build the first versions of this thing as through-hole because that’s where my comfort level is. From my readings in the DIY Christmas lights communities, it’s clear I’m not the only one who remains uncomfortable regarding SMT. I may redesign the PCB at some later point as SMT.

Power supply: I’ve been thinking about the power supply for the Sawyer Star for a long time. The originator, David Thorpe, uses a nine-volt battery, which he says runs his stars throughout the holiday season without needing a change; I think that with the RS485 chip and off-board PWM, a battery probably won’t hold up adequately.

After seeing the on-board power supplies of Robert Jordan and Robert Martin, I briefly considered bringing household current onto the board, but ultimately decided this would be a bad idea. I decided a so-called wall wart (a wall-plug transformer) would be a better idea.

My theory here is that many people have surplus wall warts hanging around the house, which would help reduce the cost of the completed circuit.

In my initial design of this circuit, I used a standard LM7805 linear regulator. The problem with this component is that it doesn’t adequately handle current above one amp. In testing, I found that the LM7805 got really, really hot without a heat-sink and even with one I was concerned about it’s ability to work properly.

I designed a second version of the power supply using an LM317-based circuits, which while typically used for adjustable supplies, also work in fixed-voltage situations (and handle 1½-amps of current). It’s a cheap part (50-to-60-cents) and only needs a couple of resistors and a diode to complement the design.

Unfortunately, the LM317 is, like the LM7805, a linear regulator and dissipates the difference between voltage-in and voltage-out as heat, so the switch from the 7805 to the 317 gains me nothing.

Based on work done by Robert Morgan, I stumbled across the LM2576, which is a switching regulator rather than a linear regulator. As a switching regulator, it lowers the voltage by literally turning it on and off (very fast), which is a technique called pulse-width modulation or PWM. This technique is also used as a method for dimming DC-based components such as LEDs (see below).

The LM2576 works up to three amps of current but it has a problem: it has a relatively high cost ($2) and requires an inductor, which is another uncheap part (40-cents). I’m awaiting these parts (hanging head in shame: ordered the wrong versions of both the regulator and the inductor previous) and will build them up on a breadboard to make sure they work.

I’ve also added a 40-cent bridge rectifier to the circuit so that virtually any wall wart – from 9-volt AC or DC to 24-volt AC or DC – can be used, as long as it is at least one amp. I am using a terminal block to bring in that input voltage rather than the coax connector typically used for a wall wart for two reasons: again, so that any scrounged wall wart can be used and because in many applications the wire will need to be longer than the standard length provided by manufacturers.

Driver chips: The choice of the ATTiny2313 means a relatively low number of pins and a certain constraint on internal memory are available (conversely, those compromises are rewarded with a relatively low chip price: ~$2). By necessity, then, the LEDs need to be handled through driver chips. I use two of those:

• The Texas Instruments TLC5916 is an eight-bit constant current, PWM serial sink driver with eight ports. That means it can control 256 levels (the eight-bit part) of dimming (PWM), provides the LEDs with the specific current they need (constant current), are driven by a serial signal and control the negative side of eight LED circuits. A serial signal allows you to drive a relatively unlimited number of ports with only two pins on the ATTiny2313. Multiple serial drivers can be “cascaded” together by merely taking the output of the first driver and wiring it to the input of the second.
• The commodity 74HC595 is a serial shift register chip that sinks eight ports. Like the TLC5916, it can drive eight LED circuits via a serial signal; unlike the TLC5916, it doesn’t have constant current or a PWM. An array of LEDs doesn’t need PWM or constant current on both the positive (rows) side of the circuit as well as the negative (columns) side – as long as they exist on one side or the other, they’re fine. But the 74HC595 is also like the TLC5916 in that it sinks – or handles the negative side of the – the output circuit. Two negative controls won’t work, so the 74HC595 has to be supplemented with another chip that takes the sink port and turn it into a source port – the UDN2981.

One last thought about this circuit: I drew it in DipTrace, which while not the easiest program I’ve ever used, did seem to become somewhat efficient by the time I finished. After I’ve built up this circuit on a breadboard, tested it and made sure it works, then I’ll move on the the application’s printed circuit board creation ability.

Life happens and my involvement in designing things that flashed LEDs wavered with my inability to get a working prototype for the Sawyer Star, a 64-LED array/matrix that is to be driven by Arduino. I was more than familiar in a general sense with this Open Source microprocessor platform, based on the ATMega 8 family (168s and 328s of late) and it was clear there was a huge community behind it.

After a few months of fiddling around with a board I had bought, I decided to investigate the Rainbowduino, a spinoff of the Arduino that is designed specifically to handle large numbers of LEDs. I bought one of those boards and played with it off and on for a few months. I finally decided that it was too powerful (and too expensive) for my needs.

After a wide variety of false starts on circuit designs, I went back to my inspiration for the Sawyer Star — David Thorpe’s “Animated Christmas Star Project” — and reviewed his design. It is dead simple: an 18F2620 controller PIC from Microchip Technology, some resistors, a ULN2803 Darlington transistor array and a traditional five-volt power supply.

Why not, I wondered, just substitute an Arduino for the PIC?

Which got me back on track, got me back working with microprocessors and — ultimately — interested in this blog again.

I had moved this web site (along with a dozen others) over to an Apple Mac Mini last year (another reason focus was lost here) and during the transition lost the ability for permalinks to work the right way and also lost the ability to upload images. I fixed those problems this morning.

Further, there was a written but unpublished post here, which I will fix up shortly and get up for you to see just how far I got down the wrong road last year.

You’ll be seeing more stuff here soon, I’m sure.

Let us address the second point first: I had always wondered why, in a schematics application (such as Eagle), when you placed an integrated circuit, for example, the pins were not displayed in sequence. Pins 12-18 might be, for example, in the upper-left-hand corner, while pins 1, 2, 8 and 11 might be in the lower left-hand corner, with pins 3-7, 9 and 10, might be scattered to what seemed like the wind.

I have lost the actual link, but somewhere recently I read a forum posting where it was patiently explained to we non-electrical engineers, that the pins are grouped by internal logical function rather than arranged physically. Also, many integrated circuits are available in a variety of formats – through-hole and surface mount and sometimes in a variety of surface-mount formats as well, where the pin numbers are not the same.

So, what I have been drawing have been diagrams that make it easier for me to breadboard (or perf-board) circuits. While those are beneficial, rarely can they be used for anything else.

My other major gripe with schematics programs (aside from the lack of choices in the Macintosh world) was the difficulty I had in making the wires actually attach. I can’t remember any specific applications, but I definitely remember that the large part of what turned me off to schematics applications was the actual difficulty I had in making wires connect.

Further, I had deluded myself into believing that I could create PCB files that could be sent to board manufacturing companies (called “Gerber files”) by using Illustrator. I had run across a set of instructions for converting Illustrator files to Gerber files and believed they would work.

And, after extensive experimentation, I found that they did … almost. The one part that doesn’t work is the ability to put text onto the silkscreen layer. If I were building PCBs exclusively for myself, that wouldn’t be a problem, but I have envisioned that the DMX/LED matrix circuit I hope to one day design will be something that I can give back to the community, and for those types of boards, you really need to indicate component positions with silkscreened text.

So, that was out.

A few weeks back I stumbled across a conversation on a forum where folks were talking about a schematics and PCB application called DipTrace. Most of the comments were positive; there were plenty of people who said it was easy to use; it had a free version for non-profit use that topped out at 300 pins (about twice what I’d need) and it created Gerber files.

The only downside was that it ran under Windows (patooie!), not on the Mac. I do have an old Compaq sitting here that I use to test web sites out on and to run the Christmas lights, so I thought I could actually break down and try to use DipTrace on it.

I was able to go through the tutorial in about an hour and had both a schematic and a Gerber file (albeit of only seven components, but nonetheless …) to show for it.

I’ve recently begun work on a new matrix circuit and rather than draw it in Illustrator, I decided to use DipTrace. I’ll post when I have something I feel comfortable showing.

Since we last spoke I threw the LED matrix project out to the Do It Yourself Christmas community to see if I could get some others interested and willing to help. I got a couple of bites and have spent the last few months working with them, trying to further my goal of building an LED star that has 60 lamps, three colors and can be driven by DMX-512. You can see the discussions.

The current circuit is based on the Atmel ATTiny2313, two 74HC595 shift registers and a ULN2803 Darlington array to handle sink current (plus an RS485 chip to handle DMX signals and associated resistors and capacitors).

Unfortunately, I haven’t been able to make as much progress as I would have liked. I have a breadboarded circuit that works, but unfortunately, it works backwards. That is, when the DMX application sends a signal to an LED to light, it is dark; when there is a DMX signal but sent to have the LED dark, it lights.

Big brains in the DIYC community haven’t been able to figure this one out, so for now I remain mired in my own ignorance.

That’s all I know right now. Stay tuned for more information.

That said, I also focused a bit in recent months on my Christmas lights show and have documented there a few of the DIY efforts not previously chronicled.

Take a gander at:

• The Lynx DMX SSR4 controller
• The Lynx MR16 controller
• The LED-Triks electronic sign

Have a happy new year and expect to hear more from the workshop later this winter.

A few months back it became clear that this electronics thing wasn’t going to go away. I was, in fact, going to be building more circuits over time and perhaps even designing them. I had chronicled the lighting of the backyard railroad at 45mm.com, while the Christmas lights project was at PacificaLights.info.

But that scattered stuff all over the map and I didn’t have a good place to put things that fell in between. Plus, I had never installed WordPress and was interested in giving it a try (we won’t talk about the ill-fated, three-day attempt to get Movable Type to work).

Plus, I had all these iterations of the domain name dmcole — so why not use one for its own content?

I don’t expect to post here often; I don’t expect the material to be earth-shattering. I do expect it to be of mild interest to those who are interested in do-it-yourself digital lighting solutions and those who are microcontroller hobbyists.

Now, I’ve set up by blog and I’m ready to go.

Onward.

\dmc