Driving LEDs with 74LS logic

[sci4me]

Hey guys. So I want to add indicator LEDs to different parts of my logic circuits so I can see what it's doing as it runs. Looking at the spec sheets, the high level output currents are like <1 mA... so... what's the right way to drive an LED (and other gates) with that? Can I just add a transistor to drive the LED?

[SeanB]

TTL is better at sinking current, so run the LEd's from the 5V rail with the logic low turning them on. Use 5mA as a current and most modern LED devices will be very bright at this current. Typically this means a 470R resistor per LED.

[retrolefty]

A typical 74LS00 output pin can sink significant current to drive a standard LED. Just wire the cathode to the output pin through a suitably sized series resistor (calc for maybe 10 ma, plenty bright) to then to +5vcc. You just have design for circuit so that a LOW  output is the active (lite) state.

[sci4me]

A typical 74LS00 output pin can sink significant current to drive a standard LED. Just wire the cathode to the output pin through a suitably sized series resistor (calc for maybe 10 ma, plenty bright) to then to +5vcc. You just have design for circuit so that a LOW  output is the active (lite) state.

*sigh* I suppose I have to switch to using 0v as 1 instead of 5v instead of 1... I really wish I had done this from the start...

Hmm...

[Mr.B]

If you don't want the LED to indicate the reverse logic you could use logic level N-Ch FETs, but its going to get busy/messy...

[edavid]

You could use 74HCT where you want to drive LEDs... it can drive about 5mA in the high state.

[MarkF]

*sigh* I suppose I have to switch to using 0v as 1 instead of 5v instead of 1... I really wish I had done this from the start...

Hmm...

Use a SN74HC240 or SN74HC244 to drive your LEDs depending on which logic level you want to light the LED.  The '240 inverts and the '244 does not.