Fullscreen HTML5Med Res (??MB)HTML4

Displaying digits using single RGB LEDs With embedded systems these days, engineers must often deal with very small PCB footprints, so suitable displays (especially for debugging) can be difficult. My idea is to display multidigit numbers using just a few RGB LEDs, where the digital value of 0-9 is shown using standard resistor colour codes (black, brown, red, …, white). The circuit shown in Fig.1 is minimalistic, using just one high-brightness RGB LED controlled by a tiny PIC10F200 (the world’s smallest 6-pin microcontroller). It uses three simultaneous PWM pulse trains, each with an independently controllable duty cycle. By varying the duty cycles, the intensity of the red, green and blue elements within the RGB LED create different colour mixes. While seven of the required colours are easy to obtain: red, green, blue, yellow (red+green), purple (red+blue), white (red+green+blue) and black (led off), achieving the three remaining colours required some experimentation and fine-tuning of a look-up table (see the table at right). The current-limiting resistors have also been individually adjusted to obtain the same brightness precisely, using a lux meter. The RGB led should be used with a white diffuser to mask the individual red, green, blue elements (even with SMD RGB LEDs, these elements are distinctly visible). Pin 6 in the circuit shown monitors the status of pushbutton S1, which allows you to cycle through the available digits values/colours (Fig.1). This pin has an internal pull-up enabled for that micro pin so that it can detect button presses. As shown in Fig.3, this circuit can be used in cascade, with discrete logic OR gates, creating a multi-digit display. Alternatively, a single 8-pin microcontroller can drive a four-digit LED display, as shown in Fig.2. The source code (1dgtRGB.ASM and PWMLEDS.INC) is available for download from siliconchip.com. au/Shop/6/5781 This suits the 6-pin PIC10F200 and would need to be modified for the PIC12F617 or other micros. Its size is optimised, and it uses macros to ease understanding. It occupies just 116 program words and nine data bytes. The code is fully commented, to make it easy to adapt to other microcontrollers. Benabadji Mohammed Salim, Oran, Algeria. ($100) Fig.1 Fig.2 Fig.3 siliconchip.com.au Australia’s electronics magazine March 2021  41