Do I need a logic probe if....

[easilyconfused]

....I have an oscilloscope? Thanks

[tycz]

Yes. You will need two, one for yourself and one for your oscilloscope.

[NiHaoMike]

You only need a special probe if your oscilloscope has a logic analyzer built in or if the logic circuit is so sensitive that a normal oscilloscope probe causes it to stop working.

[JohnS_AZ]

Well, I've been in this game for a few decades. I've had a logic probe the whole time, and have MAYBE used it three or four times.
Usually when I need something like that, I'll just toss an LED and a gate onto my protoboard to kind of make one.
At any rate, if you want one don't buy one!! You can easily make one out of less than a dollars worth of parts.

Go to Google ...
type in "logic probe" ...
click the "images" button ...
choose from several THOUSAND logic probe schematics.

( ... You will need two...   lol)

[alm]

Appears to be down to personal preference. You certainly don't need one, so if you've a tight budget, get more important things first. The advantage of a logic probe over a scope is that it gives you a visual/audible indication of logic level, without looking up at the scope screen. Another advantage is that it can detect tri-stated outputs (high output impedance). It's also a lot smalle than a scope. Some people prefer them to scopes for logic work for these reasons. Personally I don't, in my opinion they might have been useful in the time of discrete TTL logic with hundreds of gates, but there aren't too many simple parallel signals in modern equipment. A scope shows the same and more (eg. noise), and is much more versatile.

[Simon]

someone at work gave me a logic probe thinking he had done me a great favour but really i have no idea what I'd use it for, I see no point with it if I have a scope but then I've not done any probing as such but would first try the scope, so much more information. However I'll keep the probe as a hisotical artifact, might come handy one day when I can't carry the scope with me.

[McPete]

When I play around with relatively simple digital circuits (4000/7400 etc) I find them handy, just as a visual representation of what was happening in the midst of the circuit in operation.

I build a really nice SMD one from a kit and put it inside a pen case, which can be seen in my forum Avatar. The kit I used was this one; http://www.altronics.com.au/index.asp?area=item&id=K2587

[easilyconfused]

Well that was quite a mix of responses. They range from needing two (not one) to you don't need one at all. You guys have no idea how barely into this I am. So I think I'll just wait. I've built some very simple circuits and probed the 555 with my scope. With that I can see how the voltage varies from one leg to another and I can even see how it rises slowly as it fills a cap. My gut tells me that a probe is merely more handy due to its size and portability. Right now I'm just sitting at a card table trying to figure all this stuff out. Not going to be making any service calls. Thanks for all the responses.

[mikeselectricstuff]

If you have a scope _with you_ then a logic probe isn't much use. If you need something small & portable then it may be of some limited use, depending on what you need to do.

[djsb]

I have a full set of HP probes (545A , 546A , AND 547A) in a soft lined bag that I bought off Ebay

Same as these

http://cgi.ebay.co.uk/3-HP-PROBES-545A-546A-AND-547A-TESTED-GOOD-CASE-/290467007571?pt=LH_DefaultDomain_0

The injector is useful for basic audio injection as well as digital debugging. The current tracer is interesting and the logic probe could come in handy. I really just bought them because I liked the look of them.

David.

[Bored@Work]

Using a logic probe is kind of a habit. It can be replaced with other things.

It is kind of the digital version of the continuity mode the multimeter. Like you don't do everything measure with the continuity probe you don't do that with a logic probe. But they are good for quickly checking if there is a signal or not, instead of having to dig out the oscilloscope probes under a mess of wire, adjust the oscilloscope (trigger, horizontal, vertical), or, the non sportsmanship version, press the auto set button (and then spend the next minutes to undo the settings auto set did ...).

While someone still fiddles with the oscilloscope I already know there is a kind of clock signal on the SPI SCK line, there is some PWM output on another pin, the UART is receiving someting, and when the button is pressed, there is indeed a low (yes, I prefer active-low buttons) on my MCU input pin.

[alm]

I think logic probe vs. scope and continuity mode vs. resistance mode is a pretty good analogy, both are basically a subset of the latter, but are more suited for a quick check (although it's hard to find a new DMM without continuity mode). Their limitation is that they're only useful for simple signals like parallel buses or SPI, not really for more complex serial buses like I2C or 1-wire (you'll know that there is a signal, but not if the slave responds or if the address is correct).

[Zero999]

I don't see the point in having a logic probe, even a DVM is much more useful.

The only thing that might be handy is if it has many channels, I have a breadboard which has a logic probe and some LEDs to indicate when the output is high which is useful because there are many of them.

[djsb]

The logic injector is much more useful. With the HP 546a you can code in one of its six ROM-programmable output pulses (single pulses; pulse streams of either 1,10, 100 Hz; or bursts of 10 or 100 Hz; or burst of 10 or 100 pulses).  this feature allows you to continually pulse a circuit when necessary, or it also provides an easy means to put an exact number of pulses into counters and shift registers. The 545a logic probe can stretch fast pulses and that about all. The 547a current tracer traces current so can be used to find individual wires in a bundle for example. It's main uses is to trace low impedance faults. Not sure about other makes of probe can do.

David.

[wd5gnr]

In nearly 40 years of doing this stuff I've never needed a logic probe. However, I did build a "Super Probe" which does have a logic probe in it: http://www.hotsolder.com/2010/05/superprobe.html

Pretty cool device. The logic probe part uses a weak pull up to detect the difference between high/low and open. It also stretches pulses. But it does a lot of other things too.

I find now that I have a handheld scope and a tiny Rigol -- ahem -- 100MHz scope that I just use a scope for everything but continuity testing and reading ohms. My old Tek was too bulky to just move around.