The shift registers receive bit patterns for a given row of LEDs via a serial I/O pin. You can add or remove shift registers to suit your requirements. Nine eight-bit shift registers give me 72 current sinks. My sign needed 70 current sinks, but few MCUs we can reasonably work with have 70 or more I/O pins, so my controller circuit uses eight-bit shift registers connected in series. Because the controller turns on LEDs row by row, the current-sink circuits will carry only the current for a single LED. Next, we need circuits to sink current from the common-cathode pins for LED columns. Circuit diagram of the eight FETs and a ULN2803A that supply current to the seven or eight rows of LEDs in a single-line display. I/O pins on 3.3V or 5V MCUs can’t handle a 12 volt MOSFET gate-control voltage, so I chose a ULN2803A Darlington-transistor array as an interface between my MCU’s I/O pins and the MOSFET gates ( Figure 3).įIGURE 3. I had many IRF9520 power MOSFETs on hand and used eight of them one per row. We must figure out how to electronically drive current through a sign’s LEDs. However, each LED column - all 70 in my sign - needs its own on-off circuit to let current flow to ground so we can select individual LEDs. This arrangement means we supply current to only seven or eight LED rows, one at a time. In a horizontal line of LED modules, the common-anode pin for a given row of LEDs connects to the same common-anode pin on all other LED modules. When a controller rapidly turns on LEDs in one row at a time, this sequence appears to our eyes as the letter B. Figure 2 shows the sequence of illuminated rows for the letter B.įIGURE 2. A circuit supplies current for a row at a time and ground connections, or current sinks, determine which LEDs turn on for each row.Ī commercial controller illuminates LEDs in one row at a time, but quickly enough so our eyes don’t see any flicker as one row turns off and the next row turns on. A 5x7 LED matrix with common-anode rows and common-cathode columns. That gives us an easy way to control LEDs a row at a time, and that’s how the controller will work.įIGURE 1. To turn on LEDs in a row, we apply current to that row’s anode pin and sink current through the cathode pins for the corresponding LEDs we want to turn on. The LEDs for our project require common-anode connection for the rows and common-cathode connections for the columns. To start, look at a 5x7 LED module that provides a matrix of LEDs ( Figure 1).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |