Cheap/Good Logic Probe - Page 1
Forum section: electronics / beginners

author: SuperMiguel
author_rank: Regular Contributor
timestamp: November 20, 2010, 08:24:25 pm
content: Im looking to get a logic probe, any recommendation?

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 20, 2010, 08:41:08 pm
content: they are alot in ebay?

author: oPossum
author_rank: Super Contributor
timestamp: November 20, 2010, 08:49:21 pm
content: HP 545A

author: tyblu
author_rank: Frequent Contributor
timestamp: November 24, 2010, 05:50:54 pm
content: You need to define your bandwidth, input capacitance, 'scope attachment, and maximum voltage. Attachments are usually BNC, some with an extra contact for probe features (attenuation info). It needs to be able to compensate for your oscilloscope input capacitance (5-20pF).You are likely looking for a 10X probe that exceeds your oscilloscope bandwidth. Measurement bandwidth will be reduced by the probe by this equation:f_system = ( 1/f_probe^2 + 1/f_scope^2 )^(-1/2)For example, if you have a 20MHz 'scope, in order to get 19MHz measurement bandwidth you'll need >60MHz probes. If you have a 60MHz 'scope, in order to get 50MHz meas. bw. you'll need >90MHz probes.<edit> TI has a decent parametric search for probes to give you a starting point in your search.

author: tyblu
author_rank: Frequent Contributor
timestamp: November 25, 2010, 12:33:16 am
content: Wait a tic... you said "logic". I thought you meant 'scope probe. Mybad. You can make your own logic probe for a few dollars.http://www.swansontec.com/sprobe.html

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 25, 2010, 03:26:21 am
content: One modification to that design i would suggest is to connect a piezo speaker between either side of the LED and ground. This way, low frequency signals (<20kHz) can be distinguished by tone. Or add a piezo speaker from input to ground with a series 10k or so resistor to reduce capacitive loading.

author: GeoffS
author_rank: Unknown
timestamp: November 25, 2010, 03:32:33 am
content: There's always theSuperprobe.I found an old Micronta/RadioShack digital probe in a drawer today so was planning to use the case for a Superprobe.I just have to design a PCB for it.

author: Zero999
author_rank: Super Contributor
timestamp: November 25, 2010, 06:19:59 pm
content: Who uses logic probes nowadays?

author: Mr J
author_rank: Regular Contributor
timestamp: November 25, 2010, 06:26:23 pm
content: you can make your own, here is the schematic. On my logic probe I made I changed the pinout and used a LM324 instead of the LM339 depicted. Why the change? I have many LM324 on hand so I used them. Works really well and I fit the whole thing in a 1/2 PVC pipe and two end caps, used a brass nail as the tip. Basically it's a window comparator, a high will light up green and a low red, both on it's a clock signal.

author: Unknown
author_rank: Unknown
timestamp: November 25, 2010, 06:45:50 pm
content: I believe most commercial logic probes are a little bit more complex, with features like pulse stretching, ability to show pulse trains, detecting tri-stated outputs and a memory.I don't think they're terribly useful in modern electronics, especially if you have a scope, but I use one (HP 545A) occasionally for signal tracing or spotting bad connections. The ability to distinguish between high, low and tri-state is something a scope can't easily do. They're only useful for simple signals, however, not much point using it to troubleshoot an I2C bus.

author: oPossum
author_rank: Super Contributor
timestamp: November 25, 2010, 06:50:36 pm
content: Quote from: Hero999 on November 25, 2010, 06:19:59 pmI use one almost as much as a 'scope.Using a scope requires probing and then looking up at the scope display and possibly adjusting the controls. A logic probe is much faster for a quick check. No need to look away and adjustments right on the probe.There is a big difference between a simple hi/low logic level indicator and a quality logic probe like the HP 545A. The ability to catch glitches, indicate rep rate, adjustable threshold, well diffused indicator, etc...

author: tyblu
author_rank: Frequent Contributor
timestamp: November 25, 2010, 07:42:41 pm
content: Quote from: Hero999 on November 25, 2010, 06:19:59 pmI use a simple logic probe more than any other measurement tool, including DMM, in embedded development. It's faster to use and read than any of them.

author: djsb
author_rank: Super Contributor
timestamp: November 25, 2010, 08:37:43 pm
content: Try to get a HP 547a (current tracer) HP 545a (logic probe) and 546a (logic probe) probe set if you can. They are quality tools that do come in handy.David.

author: Mr J
author_rank: Regular Contributor
timestamp: November 26, 2010, 04:37:07 am
content: Quote from: tyblu on November 25, 2010, 07:42:41 pmYes I agree, I've tried to use multimeter in the past and there just to slow to read pulses. I still use a lot of 40XX and 74XX series logic in production.