This is a tristate logic probe that I use to use about 30 years ago before I got a oscilloscope. I built this from a circuit I found in a book I got from Tandy called Practical Digtal Electronics by Mike Tooley. It will show ones by green led zeros with red led plus the third state pulse by the blue led. The logic train and switch are attached to the point where probe input should be. The use of a LM393 for the comparator in the circuit plus 555 are the IC's. The bottom half of the LM393 test for 0 the top test for 1 the 555 shows the third state when both the output of the zeros and ones are high. When using this probe hook up to a TTL supply for TTL and a CMOS supply for CMOS. Again note to all new comers to Everycircuit. If you click “in Chrome” or tap “in app” on a user names you will see their profile and a list of their public circuits for viewing.
