Hi All this is not really a strictly arduino related problem but there is a great community here so i thought i would ask.
I am working on building a word clock i.e. one of these http://qlocktwo.com/ but cheaper. I have the logic worked out mostly i think but have run into a snag, to drive the words i am using 3 HC595 Shift Registers and 3 ULN2003A Transistor arrays to provide 21 lines to power the words.
Probably not my greatest soldering but it all works without shorts
Since i am running using a MSP430 micro controller and both the chips all run at 3v i bought a 3v all wart to power the whole thing thinking i would just be able to power the leds from that (the white LEDS i bought have a typical forward voltage of 3.2v but i don't need them to be full brightness) saving me from having to solder 98 resistors to leds and then solder the leds together.
However i wasn't aware of what i believe is called "saturation voltage" or something along those lines meaning that the ULN2003A chip is only able to bring the outputs to within 0.6v of ground, meaning that the voltage difference on the LEDS is only 2.4 volts and ends up being way to dim.
What i would like to know is what is the best way to make this work without having to do hours and hours more soldering. The only thing i could think of was running of a slightly higher voltage (say 5v) with a 3v voltage regulator to supply the logic and a 3.6v regulator to provide for the leds which should come out to be 3v after loosing the .6 though the transistors.
I also thought about using one resistor for each word and being able to ditch the second regulator but apparently you are not supposed to use one resistor for multiple LEDS in parallel is this correct?
Hugoagogo:
I also thought about using one resistor for each word and being able to ditch the second regulator but apparently you are not supposed to use one resistor for multiple LEDS in parallel is this correct?
Yes. The parallel LED's will not divide the current evenly. Some will be over current and some will be under current.
Actually if all the LEDs are the same batch and at the same temperature it should be OK (the eye isn't going to notice +-20% brightness variations, its response is logarithmic).
One solution is to find a plug-in-replacement for the ULN2003A chips that uses just single transistors rather than darlington outputs - darlington outputs are always going to lose 0.6 to 1.2V (depending on current). Might take some searching (I'd love for there to be a MOSFET version for instance, don't seem to have found one yet (not in thru-hole at least).
[ aside: To be honest there's no excuse for Darlington arrays to be made any more given the performance of single bipolar and MOSFET transistors! ]
If i was to use the 5v supply a voltage regulator for the logic and then a resistor for each word is this correct for calculating the resistance needed
R = (Vs-Vf)/I
Where Vs is the supply voltage, Vf is the forward voltage of the led and I is the combined current of the leds?
i.e. for 4 leds @15 milliamps
R = (4.5-3.1)/(0.015*4)
R = 23.3333333 Ohms
