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Step 6Making the PCB and fitting the parts.
I made my PCB using the "toner transfer method", using an old iron to transfer the toner onto a piece of PCB board. I won't describe how it is done here as other people have probably made good instructables or web tutorials about it.
The pcb layout is in the PDF document ready to be printed at the right size (no scaling as before).
You could easily build the circuit on "stripboard" instead, it is not too complex.
Fit the parts as shown, making sure that you use a socket for the microcontroller.
The ICs all face in different directions because it made the pcb layout easier.
(I don't usually do that - but I wanted to avoid the need for wire links and keep the board small)
Make sure you fit them with the pin1 marks matching the layout.
Ensure the electrolytic capacitors are fitted with the negative polarity mark matching the layout.
The diode is also fitted with the white band matching the layout marking.
Connect the Rows and Columns wires in the right order.
You can either solder them directly into the pcb holes or use 2.54mm pitch header pins.
The pcb layout is in the PDF document ready to be printed at the right size (no scaling as before).
You could easily build the circuit on "stripboard" instead, it is not too complex.
Fit the parts as shown, making sure that you use a socket for the microcontroller.
The ICs all face in different directions because it made the pcb layout easier.
(I don't usually do that - but I wanted to avoid the need for wire links and keep the board small)
Make sure you fit them with the pin1 marks matching the layout.
Ensure the electrolytic capacitors are fitted with the negative polarity mark matching the layout.
The diode is also fitted with the white band matching the layout marking.
Connect the Rows and Columns wires in the right order.
You can either solder them directly into the pcb holes or use 2.54mm pitch header pins.
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Dec 7, 2008. 12:58 PMdanking
says:
What values did you use for the resistors? Did you spec if for full current draw with one LED or eight?
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1
Dec 8, 2008. 1:35 AMunusualelectronics (author)
says:
I used 56 ohm resistors to ensure no damage to LEDs or the PIC. Each port only has to drive one led at a time and the max allowed current per port is 25mA according to the Microchip datasheet. The voltage drops of the leds (about 3.2 - 3.6V) and the ULN2803 drivers (about 1V) actually limit the current to around 10mA so you could use lower value resistors if you want. Probably the minimum value you could get away with would be 33 - 22 ohm depending on your LEDs spec. I have seen designs that don't use any resistors at all but that may risk damaging the PIC or LEDs if it stopped multiplexing for any reason.
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Dec 8, 2008. 12:48 PMdanking
says:
Hmmm. Clearly I failed to undetand the design. I thought each port would have to drive an entire row (or column) at a time. Or, I missed a bit of subtlety. Thanks!
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