Post by thread:Driving LEDs (was: Suggestions for new PIC person)

> > > The "hello world" of the PIC is to blink an LED tied to a pin. I have watched > > several people implement this and have one warning: > > If you can't see the LED blinking, check with a 'scope. PICs aren't very good {Quote hidden}> > at wasting time! The LED may be blinking faster than you can see it. > > I would suggest an easy remedy to this is to use a bipolar LED (one of > the two-lead red/green types, available at Radio Shack or any mail-order > electronics house) connected as shown: > > Gnd----[220ohm]---+---[220ohm]----+5 > | > +---[LED]------- Port pin > > Although this circuit will waste 10 mils when idle [there are ways around > that] it will provide a clear indication whether the port is solid high, > solid low, tri-stated, or switching between two of those states. Note too > that this can be a useful trick for driving more than one "independently- > controlled" LED from a port pin. Hmmm, I was having a discussion about bipolar LED's driven by PIC pins in one of my classes I'm teaching this summer just this morning. The objective was to drive 6 bi-polar LEDS (2 pin) using as few PIC port pins as possible. Each LED should be able to show red,green,yellow, and off. The design I hit upon after thinking a bit revolved around multiplexing the 6 LEDs utilizing a common pin on one side. Something like: Port pin---+-[220ohm]--+---[LED]------ Port pin | +-[220ohm]--+---[LED]------- Port pin | +-[220ohm]--+---[LED]------- Port pin | +-[220ohm]--+---[LED]------- Port pin And so on. One led can be selected by tristating all but one port pin on the right side. The selected LED's color can be picked by twiddling with the left port pin and the one non-tristated port pin. The question that popped into my mind during this design discussion was the relevance of the position of the resistor. Does it matter if the resistor is on the anode or the cathode side of an LED. Is putting the resistor on the anode side simply a convention or is there some other reason that justifies its placement. Should a bi-polar have resistance on both sides (I've done this in previous designs)? THe above configuration would be ideal because the left port pin could be tied to the common terminal of a resistor pack somewhat simplifing the wiring. Thanks for any advice, BAJ

{Quote hidden}> From: Byron A Jeff <spam_OUTbyronTakeThisOuTCC.GATECH.EDU> > > The design I hit upon after thinking a bit revolved around multiplexing > the 6 LEDs utilizing a common pin on one side. Something like: > > Port pin---+-[220ohm]--+---[LED]------ Port pin > | > +-[220ohm]--+---[LED]------- Port pin > | > +-[220ohm]--+---[LED]------- Port pin > | > +-[220ohm]--+---[LED]------- Port pin > > And so on. One led can be selected by tristating all but one port pin > on the right side. The selected LED's color can be picked by twiddling > with the left port pin and the one non-tristated port pin. > > The question that popped into my mind during this design discussion was > the relevance of the position of the resistor. Does it matter if the resistor > is on the anode or the cathode side of an LED. Is putting the resistor on > the anode side simply a convention or is there some other reason that justifies > its placement. Should a bi-polar have resistance on both sides (I've done this > in previous designs)? > It makes no electrical difference which side the R is on. Two 2-pin devices in series may be used in any order. BTW, the above circuit can also drive any 2 LEDs to arbitrary polarity so long as consistent brightness isn't a problem. How about 3 or more? Have to think about that... Regards, SJH Canberra, Australia

Byron!! Byron A Jeff wrote: It *DOES* make a difference where you put the resistor, since programmers invariably make *mistakes*. With only one resistor, it would be possible to burn out the left-hand port pin if you were to drive too many of the right-hand pins to the opposite polarity *simultaneously*. That's all. If you're careful, *really* careful, not to do that, then all is well with only one resistor. It makes absolutely no difference which side of the LED you put the resistor on, unless you want to use the roughly 1.3 V drop for some sort of measurement purposes. Use one "file" to hold bits to be driven during the "RED" cycle and another to hold bits to be driven during the "GREEN" cycle. Yellow gets a bit in both. Then cycle through both files, one bit at a time. That way you can be pretty certain you're not going to drive more than one LED at a time. How 'bout loading the flag file into the data output latches and then rotating the TRIS file with one bit cleared? :-) Now you know I'm going to have to build this thing, just to see it run. You might also wish to lower the value of the current- limiting resistor to allow the full 20mA that the pins can put through it. Since you're only going to be driving the LEDs for very small duty cycles (1/12 at most) you certainly won't have to worry about burning them out. -- Rick Miller <.....rdmillerKILLspam@spam@execpc.com>

electronics house) connected as shown: >> >> Gnd----[220ohm]---+---[220ohm]----+5 >> | >> +---[LED]------- Port pin >> This can also be used with separate red and green LEDs in inverse parallel, e.g. a panel display to indicate TOO HIGH (red LED); TOO LOW (green LED); or within range (tri-stated to open circuit - no LEDs). {Quote hidden}> Port pin---+-[220ohm]--+---[LED]------ Port pin > | > +-[220ohm]--+---[LED]------- Port pin > | > +-[220ohm]--+---[LED]------- Port pin > | > +-[220ohm]--+---[LED]------- Port pin > >The question that popped into my mind during this design discussion was >the relevance of the position of the resistor. Does it matter if the resistor >is on the anode or the cathode side of an LED. Is putting the resistor on >the anode side simply a convention or is there some other reason that justifies >its placement. Should a bi-polar have resistance on both sides (I've done this >in previous designs)? > In a series circuit, the same current flows through all elements, so it makes no difference which side the resistor is on. Also, if you only need to light one of the LEDs at a time, you can simplify this by using a single resistor at the port pin. .....................Reg Neale..................... "Ignorance is a renewable resource" P.J. O'Rourke