look up the data sheet, depending on the logic family, most of them can drive 5-20 ma, some that are also buffers or are open collector can drive more. in any case, most chips won't be damaged, though you may get less current through the led than you expect. as long as the chip doesn't dissipate too much power there is rarely a problem, even with shorted outputs. a 7404 can very safely handle 10 ma. most data sheets will state an output voltage and current, or output voltage vs. current for "normal" logic levels. 74 series chips should be able to sink 10ma, that is pull that much current to ground, if the led were connected between ground and the logic output then you probably would have trouble getting enough current through it but you still probably wouldn't damage the gate. if it's a 74ls i believe it's 5 ma, for other logic families i'd have to look it up. national semi parts are usually "standard", some ti parts have improved specs over industry standard, so be careful if you look at the ti data sheet but use another brand of chip. for instance, i just checked the 74hct04 data sheet from ti (because i have it handy, i'm using one in a current project) and it can sink 4 ma for something like driving an led. you might check my numbers, it may only be 8 ma for straight 74 series and 4 for 74ls, though like i said you wouldn't do damage, the output voltage on the chip would just be higher which is usually only a problem if you are trying to drive an led and another gate from that output. Matthew Brush wrote: > > Or switch that NPN to a PNP and re-arrange the circuit > accordingly. Or you could use two NPN transistors. > Not sure if either of these are the "right" way to do > it, but they'll both work fine for an LED. > > If all you're driving is an LED, I'm sure the inverter > IC can handle that. > > Cheers. > > --- rixy04 wrote: > Just tie the > collector to + and put your 300R and > > LED in the emitter to ground. > > Rick > > > > Robert Rothe wrote: > > > > > I'm still learning, so please forgive my > > ignorance. I have an IR detection module (an old > > radio shack one) that detects IR signaling at about > > 40 kHz. It drives its data line LOW when a signal > > is received. I have it hooked up to shut OFF an LED > > when a signal is present using this circuit: > > > > > > VCC 5Volt > > > + > > > | > > > o------------------------- > > > | | > > > | | > > > | .-. > > > | | |300R > > > | | | > > > | '-' > > > | | > > > | | > > > | | > > > | V LED > > > | - > > > | | > > > | | > > > 2 o | > > > ___ |/ > > > IR 1 o--------------o|___|--| NPN n2222 > > > Recvr 1K |> > > > | > > > 3 o | > > > | | > > > | | > > > | === > > > === GND > > > GND > > > > > > I wanted to invert the process so that the LED > > lights when a signal is present so I tried this > > circuit: > > > > > > VCC 5Volt > > > + > > > | > > > o------------------------- > > > | | > > > | | > > > | .-. > > > | | |300R > > > | | | > > > | '-' > > > | | > > > | | > > > | | LED > > > | > > +------->|------+ > > > | | > > | > > > | | > > === > > > | | > > GND > > > 2 o | > > > ___ |/ > > > IR 1 o--------------o|___|--| NPN n2222 > > > Recvr 1K |> > > > | > > > 3 o | > > > | | > > > | | > > > | === > > > === GND > > > GND > > > > > > This seemed to work OK except that I get some odd > > indications on the LED that IR is being detected. > > I suspect they were there in circuit #1, but > > couldn't be seen. (I grounded the metal can of the > > detector module. Putting a sunglass lens over the > > module helped a bit, too) > > > > > > Are these (especially the second) the right way to > > do this? > > > > > > My second question is, if I decided to just use an > > inverter (ie, 7404), I'm unclear how to determine in > > the data sheet how much current it (the 7404) can > > sink. > > > > > > Sorry for the simple questions. Any help would be > > greatly appreciated. > > > > > > Rob > > > > > > -- > > > > -- > > http://www.piclist.com hint: PICList Posts must > > start with ONE topic: > > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other > > [BUY]:,[AD]: ->Ads > > > > ===== > MJ Brush > LeftClick.ca Internet Media Services > mbrush@[NOSPAM]leftclick.ca > > ______________________________________________________________________ > Post your free ad now! http://personals.yahoo.ca > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics