Step 4: The anode multiplexer
Required parts:
Placing the parts:
This resistor (4.7 kOhm)
must be soldered on
before the first test of
the matrix. It sets the
output of the shift
registers before they
are initialized. Otherwi-
se, the LEDs will light
uncontrolled and can
burn out.
2*UDN2981A
Darlington arrays
2*74HC595
shift registers
4*100 nF
1*pin header
and Jumper
Just like all the other chips, make sure to line up the notch on the chip with the mark on the board. The 74HC595
shift registers and the UDN2981A Darlington arrays face opposite directions.
You can use the jumper to change the voltage input of the Darlington arrays. If it is set to „5V“, then they will get the
same 5V as the rest of the board. If it is set to „Vext“ then it will receive its input directly from the external power
supply.
You can use this to change the brightness of the LEDs. However, if you use this then you must figure out, on your
own, what value to use for the resisters on the cathode multiplexer (R12-R27). You would need to use an oscillo-
scope to measure the voltage on the output of the Darlington arrays and calculate the appropriate resistor value
using one of the many online resistor calculators. As you can see, this is an advanced option intended for experts.