I recently bought a bunch of 74xx chips to experiment (ok, play) with all those logic concepts I've only ever really read about. Turns out the ones I got are all 74LS and this page tells me those are not good at sourcing current.
For my purposes it's much more convenient to wire LEDs outbound from chip output to ground, for a lit LED when output is a 1. That's the 74xx sourcing current then, and even an LED is over the curent in that link.
Seems I should either switch to sinking with the LED inbound so to speak from the +ve and have an LED on all the time except when my output's high, or source the current but switch a mere LED through a transistor. Or I could use a huge resistor on the led to keep the current down and have a dim LED.
So to my question: is it worth all that kerfuffle, given that my purpose is demonstrating the logic to myself and the LED will be on for mere seconds now and then when I set the inputs? Can I safely use a 74LS to source short, infrequent bursts of current through an LED?
Over the years I have standardized my designs to sinking current to turn on LEDs.
You could use buffer drivers like ULN2803 or even O.C. SN7406 to drive LEDs.
Seems I should either switch to sinking with the LED inbound so to speak from the +ve and have an LED on all the time except when my output's high, or source the current but switch a mere LED through a transistor. Or I could use a huge resistor on the led to keep the current down and have a dim LED.
Seems I should either switch to sinking with the LED inbound so to speak from the +ve and have an LED on all the time except when my output's high, or source the current but switch a mere LED through a transistor. Or I could use a huge resistor on the led to keep the current down and have a dim LED.
How about something like this:
5V
|
R
|
74LSXX output ------•
|
+
LED
-
|
GND
Thanks- but I'm not sure what that does for me, sorry.
The LED will be ON when the 74LsXX ouput is high, OFF when low.
The 5V will supply the led current when on, so there's no need to worry about the low source current capacity of the 74LSXX.
The 74LSXX can sink about 16mA so it will have no problem turning the LED off when its output goes low.
Its just a way to have a normal LED (with up to 16mA source/sink) follow the logic of this IC family.
Meantime I just changed the 220R or whatever it was for a 3k3. Sourced current is measured below 1mA and the LED I'm using is more than bright enough to see for my purposes.
I have a 10xLED dip thingy permanently on my breadboard with resistors for 5V supply at 20mA or whatever: just going to swap a few for 3k3's for this 74LS experiment.
Stick a 300ohm resistor and source directly to the LED, it will glow to give indication but clearly not as bright it could be.
Just really 1 output from a "gate chain" if you know what I mean. An LED on 5V with a 300R is normal stuff so it's pretty right and probaly in the 10s of mA.
3k3 works a treat: just bright enough to see and <1mA sourced.
Now I'm confused. Does that mean it can source only 400uA which is 0.4mA? The link I gave says 2mA: I'm below a sourced 1mA with the 3k3 on the resistor, but over 0.4mA at about 0.8 I think.
But once again, I just upped the series resistor: 10k gives me an LED I can just see, needs hand shading actually, but current of 300uA.
Interesting thing here is, just how little current an LED really needs to be seen!