EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: sci4me on July 08, 2015, 03:36:04 pm
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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?
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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.
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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.
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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...
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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...
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You could use 74HCT where you want to drive LEDs... it can drive about 5mA in the high state.
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*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.