Introduction: 8 Channel Logic Probe
This is a logic probe based on a circuit I found on this link:
It is made using 4x LM324 quad opamps, giving it a total of 8 channels. Using 4 sets of DIP switches, each channel has an individually configurable pullup or pulldown, as well as a tactile switch that is individually configurable to either the positive or ground rail. The 10 wire harness plugs into the top port: 1 lead for each channel and the other two wires for power and ground. The bottom port gives access to the same connections, less the positive rail - for easy connection to my 8 channel logic analyzer.
The logic probe circuit works very well, but it does have one limitation. The LM324 does not run very close to the rails. It need about 4V before the LEDs become clearly visible. But it works well at 5V.