Does active low drive a led?
Hey,for the IC's like 74154 which are active low, if output pin 1 is enabled i.e. if pin 1 is low, then, can a led can be made to glow at pin 1?
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Normally you don't want to do that. LEDs require current to glow, and you don't want to be sending a lot of current through the IC. Check the specs on your chip.
Here's the datasheet:
http://www.ti.com/lit/ds/symlink/sn74154.pdf
On page 4, you see these values:
High-level output current: max 800 microamps
Low-level output current: max 16miliamps
So, yes. If you have a single LED, you could turn it on through that pin when low. Especially if this is just for testing or so. The pin can sink up to 16mA. But I wouldn't ever design a circuit to run at limits. I'd only be happy if the planned sink were 10mA or so.
Instead, have the pin drive something else (which has higher current limits) and sink there. Look for a buffer or line-driver.
It depends on the IC active low voltage rate and the LED is chosen. The older type low current RED LED 1mm size lights up quite dim at 1.3V and full brightness at 1.6V
The active LO for TTL mostly is between 0V to 2V. Therefore, some do and some don't .
Yes, with a pull down resistor driver. The output will sink 18 to 50 mA so it will drive an LED nicely.
R=(4.3-Vled) / Iled.
If you want to drive a red LED @10mA R=4.3-1.5=2.8V/.01 = ~280 ohms. (use 270).