Logic probe question?

[etstudent]

I'm just not understanding this question, and can't find the answer.

If a simple logic probe is fed a stream of digital logic data consisting of equal proportion high and low logic pulses,

1 the HI LED glows brightly
2 the LO LED grows brightly
3 both LED's glow dimly
4 both LED's glow brightly

My guess is 4, but I guess it could be 3? Any help would be great appreciated.

[amvakar]

Essentially, you'd be turning each LED on and off 50% of the time at high frequency, meaning that they would be dimmed compared to a steady high or low signal. Ample information can be found on the subject by looking up pulse width modulation.

[etstudent]

Thanks, i'm gonna go with both dimly lit. Its a weird question i think though, don't you? Thanks again.

[vk6zgo]

See,you got the same answer as on AAC!

[free_electron]

An led at 50% current will still glow bright... Led is rated for 20mA. So at 10mA it will still be pretty bright.

Besides, the logic probe will have pulse stretchers on board so the 50% incoming signal may turn the led on 100% if the base frequency of the incoming signal is high enough that the monoshot never times out....

There is no valid answer to this question as we don't know the internal build of the probe. It is also not stated what the voltage levels are so we don't know if the probe will even detect the signal. The schmittriggers at the input may reject it completely.




Random ranting and raving peppered with a few too many 4 letter expletives removed by Moderator

[AndyC_772]

I think it's just looking for an understanding that one LED lights when the signal is high, the other when it's low, and that the proportion of time a signal remains in a particular state will govern the brightness of the LED. It says a "simple" logic probe, and "simple" is usually academic jargon for "as simple as it can possibly be, regardless of how useless that makes it".

I don't think I've ever actually used a logic probe in my entire professional career - but if I were making one, I'd make the LEDs blink for a certain minimum period of time so you don't miss brief pulses on an otherwise steady output. I might even go so far as to include a third LED to mean "toggling".

[etstudent]

Woe, woe take it easy. Its not something to lose your mind with and run off to an island. They just went over it as something "you might use" in your work. They explained its rarely used. They went over all the ways of troubleshooting digital circuits, analog circuits too of course. This course is very thorough as far as I can tell, yes sometimes they give too much info, or stuff you may never use but they still go over it. On something like this, and others they say "you may never encounter this, but you should be aware of it anyway". See I like that, other programs may skip over it, but they still take the time to at least get you familiar with it "just in case" you see it in your work. I'm sorry the question made you so mad.

[ejeffrey]

More important than any question about the operation of digital logic probes is the observation that your eyes are terrible intensity detectors.  You will very likely not be able to tell the difference between a half-on LED and a full-on LED just by looking at it -- maybe you can, maybe you can't, but 50% is on the edge of what I would expect to be visible.  Your eyes are pretty good contrast detectors, so if you have a 'bright' LED next to a 'dim' LED you should be able to see it clearly, but without a reference it will be difficult.  Even after you get a feel for what that particular device looks like fully/half on you will be screwed if the ambient lighting changes.

Anyway, that is something to keep in mind when you are designing anything electronic with a human interface.  Small changes in the amplitude of a signal (visible or audible) are almost imperceptible.  Changing color/pitch is better, changing a low frequency blink/beep rate is better.  If you need to make something visible change brightness (or loudness) you need a logarithmic control.  This is something that most engineers who make LCD displays don't seem to understand.  Back in the good old days of CRTs with analog controls, the brightness knob gave a pretty big dynamic range.  Today most TFTs (at least the cheap TN ones) are too bright and have terrible dynamic range on the brightness control.  More expensive IPS monitors are less bright to start with, but they don't always do any better on the dynamic range.  This makes a bit of sense with CCFL backlights (they can't be dimmed very well), but even a lot of devices with LED backlights have terrible brightness controls.

[etstudent]

Interesting info, thank you. LCD's fascinate me, they have over taken plasma in sales but they have always had an inferior picture quality. They have now caught up for the most part, LED back lighting really helped that happen. Plus LED back lighting REALLY makes these new LCD's thin, unbelievable how skinny they have gotten. I have a very large Sony LCD, a few years old now, picture is fantastic, but its not real thin like LED LCDs.

[T4P]

Interesting info, thank you. LCD's fascinate me, they have over taken plasma in sales but they have always had an inferior picture quality. They have now caught up for the most part, LED back lighting really helped that happen. Plus LED back lighting REALLY makes these new LCD's thin, unbelievable how skinny they have gotten. I have a very large Sony LCD, a few years old now, picture is fantastic, but its not real thin like LED LCDs.

And they don't chew power like a pig, not even with CCFL, well for one you can feel the heat of plasmas from a few cm away ...

[M0BSW]

I've a logic probe it has 2 switches on it mem/pulse & cmos/ttl 3 led red,yellow, green, thats all I know about it , how you use them is still in progress, I think it maybe a you tube job at some stage," some of the gear , with no idea.

[AndyC_772]

I still think plasma has the edge for motion, and certainly for consistency of colour when viewed from different angles.

Also don't forget that when it's on, it's nicely warming the lounge so the boiler doesn't have to!

[Simon]

true but gas costs 1/3 the electric and it won't be an asset in the summer 

[etstudent]

Yes, I forgot to mention motion, which has always been LCD's short coming. Better than it was, but still not as good as plasma. Everything has caught up, except that. 

[T4P]

Unless you have been buying the cheapies ... My new TV display is fantastic even when compared to a plasma
http://www.sharp.com.sg/web/products/prodDetail.asp?prodId=403

[6502nop]

M0BSW
I've a logic probe it has 2 switches on it mem/pulse & cmos/ttl 3 led red,yellow, green, thats all I know about it , how you use them is still in progress, I think it maybe a you tube job at some stage," some of the gear , with no idea.

Oddly enough, I just used mine the other day. I have the Micronta (Radio Shack) 22-302 model I got back in the early 80s. I still find it useful. Anyway...

The Micronta's switches are "Normal/Pulse" and "CMOS/TTL", which should behave as yours does:

Normal = Just your average high/low probe.
Pulse = Adds the pulse LED to show you that the changes are being clocked while using the high/low LEDs to show relative logic levels of those pulses. That is, if the pulse LED is blinking, with the low LED bright and high LED dim, then you've got pulses with a duty cycle/rate below 50%. If they're the same brightness, it's about 50%, and if the high is bright, over 50%.

Also, on my probe, the high is paired with a piezo beep of a high frequency, and the low with a lower frequency. Simple, but annoying after a while.

CMOS/TTL = The logic levels defined by the various families set where on the pulse the logic low becomes a high. Since TTL is a 5V standard, it has a very strict definition. Since CMOS operates from about 3-18V, those levels are adjusted. If you're not sure which to use, pick CMOS over TTL.

For a quick go/no go check, you can't beat a logic probe. No batteries to leak, portable, and very versatile. In fact, I used the probe to check MOSFETs on the PCB of an R/C Helo. Since it ran off a 3V Lipo, I just set it to CMOS/Pulse, and probed the gate pins for PWM pulses from the uController.

nop

[M0BSW]

After reading your post  I tryed it out , and understand it a little better , so thank you.I'll be using it alot more now so I understand it better.
Thank You
Paul

[IanB]

I think the piezo beep is also useful, since it can be a pain trying to position probes on tiny pins around a chip and look away for a visual signal. An audible confirmation of high and low (similar to a continuity buzzer) can help you keep your eyes focused on what you are doing and be invaluable in simple troubleshooting.

You should always try the simplest tools first before getting out the more complex tools like scopes and logic analyzers. Even a simple LED, resistor and ground clip has its place.

[etstudent]

Great info guy, great discussion. I'm learning a lot just listening.

[M0BSW]

I take all this very slowly, the help on here is fantastic, the octopus component tester was the last thing I built, experts  would put it together in minutes, took me about 2 hours, but the pleasure of it working was priceless. Id just to say a Huge Thank You to all the very helpful people on hear.
Paul