Need a high-resolution discrete linear indicator? This design is a linear array of LEDs that has a resolution of 128 points/steps at 1 mm spacing. The display is controlled through a serial peripheral interface and and can be optionally dimmed with a PWM signal. The board dimensions are 25 x 150 mm.

This board combines 128 densely-placed SMD LEDs and shift registers to create a linear indicator that is easy to control from popular microcontrollers. The serial interface makes it possible to control 128 LEDs with only 6 pins.

The display can be controlled using the built-in SPI (serial peripheral interface) connection on most microcontrollers (Arduino, STM-32, ESP32, nRF5). A demo Arduino sketch is available on the Hackaday project page.

Pin descriptions and hook-up directions:

• VDD: supply voltage, must not exceed 6 V. 5 V and 3.3 V tolerant. Each LED draws 2 mA of current (without PWM dimming), so the total maximum current draw is 256 mA.
• GND: power ground
• CLK and SDI: clock and serial data lines. Connect these to the SPI pins (SCLK and MOSI, respectively) on your microcontroller. Send a string of 128 bits (16 bytes) to set the state of the LEDs. Bits are shifted from left to right (as viewed with the interface pins on the left), i.e., the last bit sent will control the first LED on the left.
• LE: latch enable. Pulse this pin (turn it on, then off) after sending the serial data to make the new values effective in driving the LEDs.
• OE: active-low output enable. Pull this pin low to turn on the display. Pulling this pin high will blank the display. Connect a PWM output pin to OE to dim the display, or tie to ground to leave the display permanently on at full brightness.

A 6-pin section of .100" breadboard header is included. For board integration, a footprint for a JST PA series connector is left unpopulated on the back on the board. The part number for this header is SM06B-PASS, available from many component distributors.