I built a basic CMOS logic probe circuit, but the 'high state' LED stays on even when the probe is floating. However, the 'high' LED does turn off and switch to the 'low state' LED when it makes contact with a low state. Everything appears to be connected properly, and I took a couple voltage measurements (schematic and measurements coming in a second here). Is there something fundamental I could be missing?

Question

mr._scampy · Answer

attachment

mr._scampy · Answer

Both LEDs should be off when the probe isn't making contact with anything, yeah?

kenljw · Answer

probe > 4 V high
probe < 2 V Low
The floating state is an undetermined state, just like when coupling logic
One thing about using IC's with multiple logics, and, nand, or, ect, or 
Op-Amps it's best to make connections for all unused, V+, grounds, and inputs because leaving these floating may cause internal feedback or other abnormalities.

kenljw · Answer

If this fails, add test point of 3 V, which should turn both off

mr._scampy · Answer

Thank you very much, I'll give it a try!