Saturday, January 28, 2017
Today I learned how to correctly drive a common cathode display device with transistors. Moreover, I learned to control higher-voltage devices with standard logic levels. My main mistake with previous efforts was using NPN transistor arrays to control common cathode devices. The circuits were overly complicated and posed a real risk of housefire. I should've been using PNP transistors. Once I realized that, the principle of operation became easier to understand.
I also learned that an NPN driver is needed before the PNP driver to boost the 5V logic signal to the higher voltage for the display. This arrangement allows me to use positive logic to control the display segments. With the NPN-based circuits, I had to send a 0 to turn a segment on, and a 1 to turn it off. That should've told me something right there.
I tested these concepts with an IV-6 vacuum fluorescent display tube and got satisfactory results. It takes a BC549, a BC559, and three 1kΩ resistors to drive each segment.