8 Channel Logic Probe





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.



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    6 Discussions

    Thanks. I tried to contemplate a pcb, but just couldn't figure out a way to do it any smaller, faster, or easier than just wiring it. There are 88 resistors in there, so any way you slice it, there's a lot of soldering to do. At least with protoboard there are no holes to drill. :)

    Must hav been hard to get your head around? Did you have to do any debugging? IE did it work first time? It sucks on large circuits when the DONT work first time, as it is damned hard to find the fault.

    Well, the only hard part was wrapping my head around the original logic probe circuit. Once I figured out how to arrange the resistors on the actual IC, the rest was pretty easy. It was just a matter of doing everything 8 times, like on an assembly line. This is actually the second one. The first one has been so useful, that I decided to make a new one with better materials. I also intended to document the build for an Instructable, but in the end, I struggled with describing what it actually does and what it was for. And the pictures just don't show the smd resistors very well! So I made it a slideshow, instead. It was a bit of a surprise, but both worked perfectly on the first try. :)

    What does a logic probe do...? Does it just tell you wether the pin in input or output?

    I imagine that could be pretty useful =)

    Yeh, pretty much. It distinguishes input (high impedance) from output hi, output low, and high frequency oscillation. And it does it without loading the lines. The impedance of the circuit is about 1Mohm. The real utility for me comes when you add reconfigurable pullup/downs and momentary switches. Then it becomes a generic button/LED array for use as a dev board. You can change the configuration of each pin of your dev board, individually. Just one wire can now act as a manual input, a pullup/down resistor, an output indicator, and a logic analyzer test point.