I’ve been trying to make electronic fireflies for ages now, and most of my previous attempts involved RGB LEDs, and one of them per ATtiny13 with code to flash a random colour now and again. This was always going to be a pretty expensive method, but after seeing http://www.instructables.com/id/Jar-of-Fireflies/ I realised it would be a much better idea to have one ATtiny13 control /many/ LEDs.

I wanted the first one to be in a jar, but in future I plan to have much longer wires so that I get one controller PCB and 12 fireflies extending off to cover a corner of a room, or part of a ceiling, etc. I could even add an LDR so it can detect dark, possibly by using pin 1 (so the chips couldn’t easily be reprogrammed).

The circuit schematic is kind of odd: I took a normal 2×3 matrix, where PB0 and PB1 control the two columns (as they can do PWM, so all LEDs can be PWM controlled), and added another LED for each position but in reverse. Since all the ouputs can tri-state (where effectively they act as though they were not connected), I can light up any LED I want individually.

While I could easily extend this to a full charlieplexing scheme, that would mean losing the hardware PWM for every LED. I could easily add another two LEDs between PB0 and PB1, but it’s really not worth the added complexity – 12 is plenty!

Each LED is an 0603 green LED soldered to two very thin wires, which run to a home made PCB at the top of the jar. The battery holder is a standard kind of cell holder from Rapid, and on the other side of the PCB is the ATtiny13 (soldered to the solder-side directly) and two 180 ohm resistors. The entire thing is through-the-hole because I don’t have any surface mounted ATtiny13s lying around and did have loads of 180 ohm through the hole resistors.

The code is fairly simple: in an infinite loop it chooses one LED at random, lights it up following a rough sine wave (actually modeled on a real firefly flash!) and then might repeat it once or twice, then waits a random amount of time before doing the whole thing again.

Each time the thing is turned on, a value is read from 0×00 in the internal EEPROM memory and used as the seed for the PRNG, then incremented and stored – giving 255 different patterns, more than enough that you can’t see any repetition!

By far the most difficult part of this was soldering all the tiny LEDs – if it wasn’t for that, this would be a particularly easy project to pull off. Using normal through-the-hole LEDs is an option, or even LED holders which would solder to the PCB and an LED just slots in. Surface mount ones are small enough to be less noticable and look better when lit up, though.

Eagle schematic and PCB file, C source code and compiled hex file available at: http://randomskk.net/projects/fireflies_in_jar all files are released under CC BY-SA-NC license

I know I got them months and months ago, but I’ve only just had time to actually do anything besides work. While I do have a big project coming up (but not really at the point where I can write about it), I did finally make the PCB for the nixie power supply I designed a while back.

While I did realise I’d forgotten to put in a 56k resistor when I designed the circuit, I was able to add this in with a through-the-hole resistor I had lying around that was just the right size.

I powered up the circuit with a 9V battery and held my breath as the voltage reading on the multimeter jumped to 246! A quick turn of the pot on the back and the voltage hit 170V, ideal for the nixies.

I connected one up and lo and behold, it lit up! The glow really is ethereal – cameras cannot capture this, you have to see it in person. That didn’t stop me from trying, though!

The next step is to make a PCB to control all 12 nixies (probably with three of the power supply modules) and maybe link it into an RTC (though that is a bit boring – I might try doing something else, like GPS position?).

Schematic – Board (the board isn’t great, with some pretty fine tracks that run pretty close to each other, but it does have the 56k resistor I forgot in my version) (both files under CC-BY-SA-NC).

This device analyses the music it picks up via the electret microphone, then flashes the LEDs in time to the music. It’s encased in the box SparkFun sent me the microphones in, since the box was just begging to be used as a case for something! I’m sure that was intentionally designed.

Getting a bit more technical:

The microphone picks up the noise and sends this to the LM386 amp, which amplifies it about 200x before it’s read by the ATtiny13′s ADC at 8-bit resolution. The ATtiny13 then keeps a running average of the noise level, and flashes the LEDs if the current volume exceeds the average by a scalar amount.

As a result, the LEDs flash on when the music hits a peak, and are off otherwise – no matter what volume.
The brightness of the LEDs is also somewhat correlated to the loudness of the peak, since a louder peak will generally keep the LEDs on for longer.

Check out the video of it in action:

Download the schematic, PCB layout, code:
https://randomskk.net/projects/lightstrip/ (all files released under Creative Commons BY-SA-NC 3.0).

So I created a PCB for the Space Experiment mockup.

It’s functionally identical to the breadboarded version but is its own PCB, looks a lot better and has some text on it.

It actually took three tries to get it right. The first time, it all worked nicely and I etched it perfectly, only to realise I’d messed up the design!

I’m making the PCB as a shield for an Arduino, so it plugs into the top. I had the layout fine and was initially assuming I’d have the copper on the bottom (since the LEDs have to be on top), but later decided I wanted the copper on top. I quickly did a change layer to top for everything, without realising that I’d left the text mirrored! What printed, if made properly, should have resulted in mirrored text on top. Instead I made it so the text was readable – which means the copper had to go on the bottom for it to connect to the Arduino!

The next revision fixed this but was overetched and would have taken a whole load of fixing up, which is a pity.

Finally, I had it all perfect! All the text is really readable, I have a right angle socket for the LCD connector, and it all worked first time. Lovely!

I did have one new trick in the process this time:

This awesome plastic scourer thing has a nice, easy to use handle and really really tough plastic bristles, which are absolutely ideal for PCBs. Not only can I clean them at the start (and get the copper really nicely scratched up too), but it also strips the toner off like no one’s business. It’s taken about twenty minutes out of the process, especially when the transfer didn’t work and I have to take the toner off.

This was also the first PCB made with my new laser printer, which absolutely rocks – for PCBs, I print at 1200dpi, extra toner, high contrast, onto transparency, from the manual feed tray. I’ve set it up so I can just select the “Samsung_ML2510_PCB” printer on the list and all those options are used, so it’s just a case of hitting print and it does it. All the PCBs transferred really, really well with it! I can definitely recommend the Samsung ML-2510 mono laser for making PCBs at home.

• I have far more through-the-hole RGB LEDs than surface mounted, and they’re brighter to boot
• I also have more ATtiny13s in through-the-hole packages
• The chips are easier to change out
• It’s easier for others to assemble!

However, the new prototype isn’t without problems. It seems the photocopier merely flipped my image instead of mirroring it, so the ATtiny13 has to be inserted backwards. This means I can’t really program it more than once, and bending the pins more than once breaks them. Oops! The other components are more lenient about which way they go in.

Second, I actually made it without ordering any 12mm batteries, so I can’t power them via battery. I did solder a 2*AA battery box to one, which is nice but takes up a fair bit of space. I’d like to work out the best way to power these things.

I tried silkscreening the PCBs this time, but this wasn’t too successful. It seems the toner sticks to copper a _lot_ better than to plain FR2. Additionally the silkscreen was not flipped right either, so I had to either put it on the copper side or have it backwards on the top! I went for a bit of both:

However, the basics do work right now – although my code still doesn’t do light detection, which I need to work on – and it looks really good when encapsulated in some foam.

I think the next step will be to finish off the code and then get these things produced properly for some nice PCBs.

Put simply, this is a tank containing etchant, some sort of aeration device, usually a heater of some description and some plastic to suspend the PCBs.
In my case, I went for a plastic tank from Tescos that looked a useful size, a fishtank air pump and heater on the cheap from eBay, and some of this “clear etchant” from Rapid.

About ten minutes later with a hot glue gun, I’d put together this:

It has a piece of plastic with large holes in from a 99p peg basket from Tescos, and some more holes drilled in that. The blue stone at the bottom is an airstone, it’s connected up to the fish tank air pump and the heater is held in place by tape on the side. Right now it’s just full of water.

I got some PCBs ready for it the next day and tried out etching them.

This is where I ran into some difficulties. First, the plastic didn’t work perfectly to keep things from slipping to the bottom, a problem that should be easy to remedy with some bits of plastic at the edges. Second, the holes really don’t work out when the PCB is lying flat – you end up with circles where it is etched and the rest of it is still copper. A slanted piece of plastic may help with this, or a bent piece of flexible plastic so that the entire PCB is covered in the etchant.

The heater wasn’t really the right tool for the job – it didn’t seem to measure the temperature right, possibly because of how much it was submerged, which meant that it kept turning off. Even when on, however, it did not manage to warm the etchant up much. A small immersion heater may do a better job of this, since the hotter the etchant is, the better.

The air pump did work fairly well, although a more poweful one may have gotten more bubbles over the entire length of the air stone.

The whole affair took rather a long time, but I did manage to etch 5 LED Firefly prototype boards, and a few ICSP-to-breadboard adapter PCBs:

They came out fairly nicely, so no complaints there. The clear etchant has since turned a nice blue, which is somewhat interesting.

I’ll be sure to update when I’ve got the new design working, because it still has the potential to be a lot better than doing it the old way.