In the previous post, I discussed the main constraint of this project--the BOM cost should stay below $5 in reasonable quantity. Between the microcontroller, LED, connectors, and board, that means every cent matters. Before digging into the [quite simple] schematic, I figure a quick overview of the v0.4 bill of materials would be helpful.

Microcontroller

As mentioned in the video, we're using the ATTiny44A. Digi-Key has 'em at $0.855 each and the chip seems to have everything I need:

• 4K program memory
• 0-20 MHz clock speed (faster than I remembered!)
• 12 I/O lines
• Two timers

Actually, this was something of a crapshoot--I knew how many I/O lines I needed, but I pretty much ball-parked program memory. 4k seemed.. uh.. reasonable.

ProgrammerEasy decision--Adafruit's USBtinyISP kit. It's cheap, and [Ladyada] has a great guide that covers using this system in Linux with the free AVR GCC toolchain. This project added 'toolchain' to my vocabulary, so running through her instructions was an excellent learning experience. Technically not part of the BOM, but I needed it for the project.

Connectors

I was shocked by how much board-level connectors cost; I found some awesome Molex surface mount units that seemed perfect, but they were upwards of $0.50 each! I ended using the TE Connectivity HPI platform with 2mm pin spacing:

2mm pin spacing is a bit funky (2.54mm corresponds to 0.1"), but it keeps the connectors somewhat compact. Remember, I'm trying to cram 7 connectors on each Neuron--6 dendrites and one axon. In any case, the price made the decision easy: $0.02998/connector, $0.0562/header, and $0.01079/crimp terminal.

Crimp tool

Another tool. The ADS for the crimp connectors recommended a $2000+ press for making connections; the kindly customer service folks at Digi-Key recommended an Engineer PA-09 for $89.95:

My cousin [Curtis Layton] gave 'em a great review so this was an easy decision to make. If you do the same, I recommend purchasing them on Amazon for half the price :-/

RGB LED

RGB. Small. Cheap. Bright. I didn't give this much thought and ended up with a Lite-On model for $0.279 each:

What could go wrong? I'll tell you--leadless packages are a PITA to rework without a hot air setup, and 605 nm is NOT red--something I should have remembered from my days puttering about with HeNe lasers.

Other stuff

Very little beyond the board, which I'll cover separately along with the schematic. All inputs are pulled low using 10k external resistors (6 total), and the ATTiny44A has a 0.1 uF filtering cap. The LED has a few dropping resistors since I'm using 5VDC power. Not counting the board itself or any non-board-mounted connectors (or stuff like solder and my time), I'm at roughly $1.50 each if I make a hundred or so. So far so good.

More to come!