# Led digital logic detector

#### mentaaal

Joined Oct 17, 2005
451
Hey guys, i want to replicate the logic detection circuit used in the logic labs that i use in college. I want to be able to feed a digital level into the output and the level will be represented by either the red led turning on (for logic high) or the green led turning on (logic low.) its easy to do this by putting two leds in series with each other and feeding the output between these so that the output is either sinking or sourcing current to one or the other of the leds. Want i cant figure out how to make it such that the leds dont light when there is now output feeding them? Any ideas?

thanks,
Greg

#### beenthere

Joined Apr 20, 2004
15,819
If it's powered up, the input has to be either high or low. It is most conventional to pull up an input, so the probe will be indicating a high state when not actually connected. Letting an input float usually results in an oscillation at the operating point, so you would see both LED's on. Be easier to use a press to operate switch than to figure a way to sense a circuit connection.

#### mentaaal

Joined Oct 17, 2005
451
thanks guys, i had a hunch there wasnt an easy way to do this. Will do. I am wanting to do this for 8 logic outputs so i think i will definitely just run with the easier way!

#### bloguetronica

Joined Apr 27, 2007
1,540
Hey guys, i want to replicate the logic detection circuit used in the logic labs that i use in college. I want to be able to feed a digital level into the output and the level will be represented by either the red led turning on (for logic high) or the green led turning on (logic low.) its easy to do this by putting two leds in series with each other and feeding the output between these so that the output is either sinking or sourcing current to one or the other of the leds. Want i cant figure out how to make it such that the leds dont light when there is now output feeding them? Any ideas?

thanks,
Greg
You can do this that way, but use a 74HCT buffer (or inverter), to serve as interface between the test circuit ant the LEDs. Some circuits cannot source or sink enough current to drive the LEDs. Using a buffer/inverter from the 74HCT family offers both advantages of TTL and CMOS, namely the fact that such logic can be driven by TTL without the need of additional pull-up resistors and is capable of supplying enough current to light the LEDs (when sourcing and when sinking).