LEDs Part 1


In this post, I'll go over the LEDs I selected for the seven-segment display.

While designing these initial prototypes, I have been keeping in mind that the design needs to be straightforward to manufacture. I've also been considering the overall system design, especially the mechanical assembly of the final clock. The LED selection had a lot to do with that. Digikey has 19,037 options for discrete LEDs. Here's how I narrowed it down to one.

Let me start by mentioning that RGB LEDs, like the Neopixel, are cool. However, they are overkill for this product. They are also expensive and less available than single color LEDs.

The first way I narrowed the search was selecting only reverse mount, surface mount (SMD) LEDs, which Digikey calls "Surface Mount, Bottom Entry." This choice was important to the system design in two ways. Firstly, keeping all parts on the same side of the board is much cheaper for assembly. Double-sided boards require two trips through a pick-and-place machine and the reflow oven. Depending on the process, adhesives have to be used on the first side. This procedure adds complexity and cost to the final product. Secondly, reverse mount LEDs allow for a smooth front surface to mount both the diffusers and wave guide material. I want that part of the design to be very flexible and easy so users can make their own guides.

The second way I narrowed the search was selecting only blue LEDs. The brought the results down to 34. I wanted a blue clock, but I chose blue for another reason. Blue is a difficult LED color (difficult enough to win researchers a Noble Prize). Blue LEDs require the highest voltages, which means that, if blue LEDs work, lower-voltage LEDs, such as green and red, should also work. I'm also interested in a manufacturer that has a family of similar LEDs that differ only in color. Because of that feature, if I want to make variants of the segments with different colors, I can use the same boards and just swap out the LED part.

After further narrowing the search to cut tape packaging and in-stock items, there were only six results. Cut tape is important because I want small quantities for prototyping but reels for manufacturing. I have assumed that, if cut tape was available, I can also get the product in reels, but I will double check my final selection later.

At this point, any of the LEDs would work for prototypes. The next two columns I looked at were unit price and quantity available. If I make 500 boards with 22 LEDs per board, I'll need 11,000 LEDs. Digikey now lets you sort by price at a quantity. At production quantities, the Lite-On LTST-C230TBKT blue LED stood out as less than half the cost of the second least expensive option. It also had the highest quantity available, at over 51,000 in Digikey stock.

In my next post, I'll go over my process for double checking my LED selection.

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.

Click to enlarge.

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.


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.