digital_citizen
Newbie level 1
Hi, I just bought this clock:
And I want to replace the original display with a much larger one that requires 12V. In the link you will also find the schematic, but BE AWARE there is an ERROR: the displays are COMMON ANODE and not common cathode as written. The clock requires 5V. So I firstly need to use transistors like a switch (or better, ICs like ULN2803) connected to the MCU outputs (which is an STC15F204EA). Will this work, if I connect the ULN2803 inputs directly to the MCU outputs without resistors? The LED's cathode of the larger display will be then connected to the ULN2803's outputs with the limiting resistors, of course. Will this work?
Now for the anode side, I need to use 4 PNP (one for each digit) instead of the original 8558's, but I have trouble to calculate the new Rbase (that will replace the old 4.7K resistor) because I don't know if I have to use the max current output of the STC15F204EA and if yes, what's it's value?
So, before I order all the parts I need to know if such thing could work, and how to calculate the Rbase of the PNP transistor. And BTW the new display does not require much current, maybe 2mA in TOTAL (it shoud be very dim to be used in a bedroom).
Please if you find errors, explain it as clearer as possible and with understandables formulas to calculate the values, because I'm not an electronic engineer.
Thanks.
And I want to replace the original display with a much larger one that requires 12V. In the link you will also find the schematic, but BE AWARE there is an ERROR: the displays are COMMON ANODE and not common cathode as written. The clock requires 5V. So I firstly need to use transistors like a switch (or better, ICs like ULN2803) connected to the MCU outputs (which is an STC15F204EA). Will this work, if I connect the ULN2803 inputs directly to the MCU outputs without resistors? The LED's cathode of the larger display will be then connected to the ULN2803's outputs with the limiting resistors, of course. Will this work?
Now for the anode side, I need to use 4 PNP (one for each digit) instead of the original 8558's, but I have trouble to calculate the new Rbase (that will replace the old 4.7K resistor) because I don't know if I have to use the max current output of the STC15F204EA and if yes, what's it's value?
So, before I order all the parts I need to know if such thing could work, and how to calculate the Rbase of the PNP transistor. And BTW the new display does not require much current, maybe 2mA in TOTAL (it shoud be very dim to be used in a bedroom).
Please if you find errors, explain it as clearer as possible and with understandables formulas to calculate the values, because I'm not an electronic engineer.
Thanks.
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