Mathematics

In the Schematics I have added a little bit of math to explain my choice of using 2k7 resistors for interfacing the matrix. The problem with this schematic is, that is was designed for a more powerful matrix that was not build. I have reused this PCB for interfacing an 8x8 dot matrix from TOPLITE.

The basic concept for this is the relationship between the base current and the collector current of a transistor. That relationship is denoted by the B-factor. Look into the datasheet of 2N17711.

For the case of the big matrix I have 8 LEDs per row with a Forward Current (I_F) of 20mA, I expect a maximum flow current of 160mA per row. So I want the transistor to not exceed 160mA, thus I set the base current with the 2k7 resistor to 1,6mA that results in a maximum collector current of 160mA with a B-factor of 100.

A base resistor for a transistor is calculated with the desired base current and the Forward Voltage of the base diode (U_B).

As we have no transistor stage with the 8x8 dotmatrix, the matrix will look a little bit dimmer than normal. But that is okay for my re-usage of the PCB.

The math for a series resistor of a LED is fairly simple. You just calculate what amount of voltage you want to catch with the resistor given a current forced by LED. Same concept on calculating the series resistors for the dot matrix display.