[Karl Lunt] wrote in to share his LED firefly project. His goals for the project were to develop a low-power, low parts count module that can sense when it’s dark and then mimic the blinking patterns you’d associate with its biological namesake.
We like his design which uses a coin cell battery holder as the chassis for the project. The ATtiny13 driving the hardware is held in place by the two power wires. This lets him flash new firmware by rotating the chip and plugging in a little adapter he build. The LED connection might look a bit peculiar to you. It has a resistor in parallel, which doesn’t satisfy the normal role of a current limiting resistor. That’s by design. [Karl] is driving the LED without any current limiting, which should be just fine with the 3V battery and short illumination time of the diode. The resistor comes into play when he uses the LED as a light sensor. Past firefly projects included light dependent resistors to detect light and synchronize multiple units. [Karl] is foregoing the LDR, using the LED with a resistor in parallel to combat the capacitive qualities of the diode. As we mentioned, this senses ambient light, but we’d love to see an update that also uses the LED to synchronize a set of the devices.
[Dr. Iguana’s] experience moving from projects powered by disposable Alkaline cells and linear regulators to recycled Lithium Ion cells using the buck regulators seen above might serve as an inspiration to make the transition in your own projects.
The recycled cells he’s talking about are pulled out of larger battery packs. As we’ve seen in the past, dead battery packs for rechargeable tools, laptops, etc., are often plagued by a few bad apples. A small number of dead cells can bork the entire battery even though many perfectly usable cells remain. Once he decided to make the switch it was time to consider power regulation. He first looked at whether to use the cells in parallel or series. Parallel are easier to charge, but boosting the voltage to the desired level ends up costing more. He decided to go with cells in series, which can be regulated with the a less expensive buck converter. In this case he made a board for the RT8289 chip. The drawback of this method requires that you monitor each cell individually during charging to ensure you don’t have the same problem that killed the battery from which you pulled these good cells.
How many times can you put two LEGO pieces together and take them apart again before they wear out? The answer is 37,112. At least that’s the number established by one test case. [Phillipe Cantin] was interested in this peculiar question so he built the test rig above to measure a LEGO’s lifespan.
The hacked together apparatus is pretty ingenious. It uses two servo motors for testing, each driven by the Arduino which is logging the count on an SD card. One of the two white LEGO parts has been screwed onto an arm of the upper servo. That servo presses down onto the mating piece which is sitting inside that yellow band. Look close and you’ll realize the yellow is the handle end of an IC puller. When the post on the lower servo is moved toward one arm of the puller it grips the lower LEGO piece tightly so that the upper servo can pull the two apart. In addition to the assembly and disassembly step there’s a verification step which raises the mated parts so that a reflectance sensor can verify that they’re holding together. [Phillipe] let the rig run for ten days straight before the pieces failed.
Don’t miss his video description of the project after the break.
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