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Board Overview and Status
The DL is a first prototype of a project with a couple of purposes:
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The DL is an easy, cheap first board for beginners to assemble and make that does something interesting. It features near-foolproof frying-pan surface-mount construction with only a couple of through-hole components to mess with (see Speaker below). The project will be used in tutorial classes on electronic construction.
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The DL is the heart of a piece of e-jewelry that, with the right programming and a pretty case, should be a lot of fun to play with and wear. The intent is to put a play-dough lizard case on it and wear it as an interactive pendant.
The DL is built around the Silicon Labs EFM8BB1 microcontroller. This 8051-API SoC is a little over 50 cents quantity one, and has a lot of nice peripherals and general purpose pins. The 8BB1 also happens to come in a nice "large" 16-pin SOIC package for ease of soldering. The processor will run nicely on less than 3 volts, which is great for battery operation.
The DL is battery-powered, currently with a CR2032 primary button battery. This battery is too large, and primary is not a good thing here, but it keeps the cost low and is readily accessible.
Passives are 1206 SMD. This is kind of expensive even compared to 0805, but gives the possibility of a person with normal vision and coordination being able to drop parts without a magnifier.
The fun of the DL is its interactivity. The current incarnation has one input and two outputs.
LEDs are used to light up the eyes and body of the lizard when desired.
Because there is only about 2.4V of headroom, there are no cheap blue LEDs available. Thus, discrete red and green LEDs are be placed near each other on the board, giving some color control. It is expected that these LEDs will be PWMed for brightness. The current LEDs can generate 54mcd of light at 20mA.
A speaker is used so that the lizard can make noises when desired.
It turns out that I could find exactly one tiny magnetic speaker at a reasonable price. Sadly, it's through-hole, but I'll live with that for now.
Capacitive touch sensors allow the lizard to respond to touches without moving parts.
The current incarnation of the DL has two capacitive touch pads intended for a short sensing wire each. This allows partial differential operation to improve noise rejection, or simply having more than one input. The pins have been chosen such that the internal comparator of the 8BB1 can be used for RC-mode sensing.
This board was designed with a recent release (2015-06-09 BZR 5726) of KiCAD. Because it uses some new KiCAD features, it is likely that building from source will be necessary. The latest Debian version of KiCAD is not new enough.
This board is currently undergoing final review before being sent to OSHPark for prototype production. Project costs are about $1.47 for the parts and $1.40 for the 5x1 cm board through OSH Park. The cost target was $2, so not perfect but in the neighborhood.
It is expected that most of the passives on the current board are actually unnecessary:
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The 51 ohm current limiting resistors for the LEDs might be avoidable with careful PWM.
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There may be enough leakage current to avoid the drain resistor in the RC touch system.
At this point these savings won't make a huge difference.
Some primitive software has been written for HW validation. The speaker emits a tone. The LEDs light up. THe touch wire is touch sensitive. More work is needed here.
The hardware design and source code for this project is available under the MIT License. See the file COPYING in this distribution for license terms.