Thursday February 23, 2017

Using a Logic Probe

Okay How Do I Use This Thing?
Well, lets first figure out if the logic probe is the most appropriate tool for the job.  The logic probe is very useful but like any tool you wouldn't want to use it when it's NOT the right tool for the job.  After all you wouldn't hammer a nail in with a screwdriver or dry your hair with a flamethrower would you?  If you answered yes, you may have more problems than can be fixed with a logic probe.

So how do we determine if a logic probe will solve our problem -- say, fix that non-working pinball circuit board or computer motherboard that keeps rebooting?  Well, that's where experience and some knowledge about the circuit come in.

You WOULD NOT use a logic probe if..

  • Voltage of component being tested is unknown or exceeds 15v
  • There is physically something damaged (visual inspection of components)
  • Smoke, fire, burning smell, sparks or anything else dangerous occurs when circuit is powered on
  • You don't have a clue how any of the components on the circuit board are working
  • The device's power LED is not lit (suspect the power supply first!)
  • You're not examining an IC, transistor, etc that's operating on digital logic (+5v, 0v)
  • The components being tested are too small (ie. surface mount) or 3.3v logic
  • You're trying to impress a woman

You WOULD use a logic probe if..

  • You know the component being tested is operating on digital logic (+5v, 0v)
  • You have an idea how the circuit should work (ie. press a button and +5v should appear input pin 2 of IC14) which then enables its output pin 15.
  • You have schematics and have determined at which points a HIGH, LOW or pulsing (if it's a data bus) signals should be seen.
  • You've visually examined the components and nothing looks damaged.  Additionally you've checked the power supply & the proper voltages are present.

Those are just some examples to give you an idea, but the point is that the logic probe ISN'T always the right tool for the job.  Sometimes the right tool is your multimeter, sometimes the right tool are your eyes and nose.  Sometimes none of these are the right tool and you need yet another tool to diagnose the circuit.  Knowing how to identify when the logic probe IS and ISN'T the right tool is important and sometimes that just comes from experience.  Probing a data bus line for example would just show blinking of HIGH & LOW signals since the data is moving so fast, but you would know that there was data being transferred and that the data line wasn't *stuck* HIGH or LOW.

No Really, Just Tell Me How To Use It
So you're all ready to jump in and test your circuit.  Okay, with the power off, hook the black alligator clip to ground, the red alligator clip to +5v (usually via a test point on the circuit board).  Make sure the alligator clips aren't shorting across any other components.  If they are touching anything but GND (black clip) and +5v (red clip) you'll need to figure out a better way to hook them up so they aren't shorting anything.  Also be sure if you're stringing some wires around the board that you don't have GND & +5v shorting together.

Next, turn the power on to the circuit under test & then use the tip of the logic probe to test various points in the circuit.  Usually you would be testing a pin on an IC, or transistor that uses digital logic.  The logic probe will indicate whether the signal is HIGH or LOW.  It does this by using the reference voltages at the alligator clips.

Interpretting the HIGH and LOW signals is really up to you and the circuit under test.  If for example, you probe an IC pin and it's stuck "HIGH".. well, maybe it's not stuck.  Maybe that signal is being held HIGH for a reason.  You'll need to examine the circuit and schematics (if available) and may also have to read the data sheets on the devices you are testing to learn how the circuit *should work*.  The logic probe can't tell you if a HIGH signal or LOW signal is expected.  Only the schematics, datasheets of the components and expectations of how the circuit should work can give clues on whether a component is faulty or working properly.

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Re: Great article.
Posted 09/30/12 10:55AM by AceBHound
Thanks for pointing out the spelling mistake! Always happy to fix any of those cause they drive me nuts too =)
Great article.
Posted 09/27/12 5:01PM by Anonymous Techdoser
Great article. Just one small nit-picking point. On page two, you use the word "here" when you mean "hear". Sorry about that. I saw this mistake made twice today.