Design Review

This blog documents the process of taking a prototype into low-level production. I'll start by going over the design of the full digit prototype. Below is the schematic, which is intentionally simple. Let's go into some of the design's details.

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LED Driver IC

When I started doing research for this project, I thought the LED driver IC would be a challenging part to find. As it turns out, there are many constant current (CC) LED driver ICs on the market with a wide variety of interfaces. I wanted a constant current LED driver with direct current instead of PWM because I wanted to dim the segments while avoiding ghosting issues. You can sometimes see this effect of PWM-based dimming if you glance back and forth across an LED source.

Instead of needing two chips, such as a shift register and a current source, the TLC5917 does it all and more. It works out of the box as a shift register with direct current drivers. The current is set with a single resistor. Additionally, the TLC5917 has special commands to dim the display based on a fraction of the set current. Beyond that, it can detect shorts and opens, and shifts out the results. If the microcontroller could also read in shift register data, it could do an incredible amount of diagnostics for less than $1 per board.

That's enough for now. In future posts, I plan to go over how I selected the LEDs and the connectors.

Optical Stackup

Today I'd like to go over the optical stackup I'm using for the full digit. The materials from top to bottom are

• Standard copy paper
• Two pieces of 1/8-inch-thick acrylic opaque white, laser cut to form segments
• One piece of 1/8-inch-thick 40% transparent white acrylic, laser cut to a size of 6.0 inches x 4.2 inches
• A PCB with reverse mount LEDs for a smooth top

I ordered the plastics from TAP Plastics. The files for the laser cutter will be posted to github.

Rev 2 of Full Digit Arrives

The Rev 2 boards in for the full digit recently arrived. I soldered up two of them and connected them together for a quick test. The major bug fix was in the connector pinout. Now the boards can butt up against each other like they were supposed to. I'm running this test using a Spark Core and a breadboard power supply from Adafruit. The colon is two decimal points. The digit on the right is upside down.

Welcome

I'm starting this blog to document the process of taking an open-source hardware project from concept to product. I'll be diving deep into matters that include design for manufacture, component selection, supply chain management, pricing, assembly, marketing, sales, and logistics. In my professional career, I've worked on many projects up through the production stage. However, these projects were hardware tools for my company’s internal use. This blog will follow the first project I am making available to the general public.

My personal goals for this project are threefold:

• Have a cool clock at the end.
• Learn about Internet-connected modules, such as the Spark Core.
• Gain experience with the process of taking a hardware project to market.

I can't guarantee I will make a successful product. Regardless, I intend to post updates on my progress here as I go through each of the challenges related to this project. Whether or not there is a successful product at the end, I know I will learn a great deal about the process. My hope is that you can learn with me.