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.