How essential is a logic probe? - Page 1
Forum section: electronics / beginners

author: geratheg
author_rank: Regular Contributor
timestamp: July 17, 2014, 01:40:40 pm
content: Hey Guys!I've been planning on learning more about electronics through the Make: Electronics book, for which I'm still ordering parts and was wondering about whether a logic probe would be needed?Are these useful?Any recommendations on a logic probe? Do the cheap autoranging digital ones on ebay work well enough?

author: Simon
author_rank: Unknown
timestamp: July 17, 2014, 01:58:39 pm
content: depends on the speed of the signal, if you know what you are doing you can make your own with a comparator, some resistors and an LED. If you want to look at anything fast you need a scope.

author: bwat
author_rank: Frequent Contributor
timestamp: July 17, 2014, 02:03:12 pm
content: I wouldn't buy one until you need one and when you need one you will know. I remember using a cheapish USB probe connected to PC to debug a serial bus connecting two devices (a smartphone application CPU and the modem CPU). Troubleshooting is much easier when you've got everything on the screen, compared to going through timestamped log files.

author: pyrohaz
author_rank: Regular Contributor
timestamp: July 17, 2014, 02:07:22 pm
content: I've got a really cheap Saleae rip off logic analyzer that I bought off ebay for a couple of quid, its REALLY useful when trying to debug LCD's or other lower speed devices connected to an microcontroller. Considering its only <£20, I'd recommend getting one for the future, even if you don't use it immediately, its really good for accurately checking frequencies <100kHz, such as pulse length and width from a 555 timer.After having a quick look, if you buy it from Hobby components themselves, its <£10!http://hobbycomponents.com/index.php/hobby-components-usb-8ch-24mhz-8-channel-logic-analyser-compatible-with-saleae-software.html

author: DJohn
author_rank: Regular Contributor
timestamp: July 17, 2014, 03:43:12 pm
content: I used one frequently when my work was mostly repairing the company's main product. You already know that the design works, and after a while you get to learn what each signal should sound like (if you get a probe, get one with a beeper). It saves a lot of time to be able to probe around without having to look up at a screen.For new designs, they aren't very useful. Knowing that there's some sort of signal there doesn't mean it'll be the right signal. I haven't used mine since I stopped doing repairs. Even the cheapest oscilloscope will be much more useful.

author: mrflibble
author_rank: Super Contributor
timestamp: July 17, 2014, 03:59:37 pm
content: Quote from: bwat on July 17, 2014, 02:03:12 pmTimestamped log files +gtkwave= everything on the screen. Used that many times. It's not perfect, but fairly useful. Definitely beats staring at log files.

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2014, 04:01:07 pm
content: Quote from: Simon on July 17, 2014, 01:58:39 pmGood logic probes include pulse stretching so they can visibly display narrow pulses.

author: Simon
author_rank: Unknown
timestamp: July 17, 2014, 04:28:48 pm
content: Quote from: David Hess on July 17, 2014, 04:01:07 pmYea, one pulse at a time, that works for single pulses and was a compromise of the day, nothing beats a scope even if it's that crappy vellman thing that I treat as a single channel logic analyzer due to the bandwidth limit which is still far better than human reading speed of an LED

author: bwat
author_rank: Frequent Contributor
timestamp: July 17, 2014, 07:37:27 pm
content: Quote from: mrflibble on July 17, 2014, 03:59:37 pmI'm actually quite a heavy gtkwave user. I use it when I'm looking at output from my verilog test benches. It needs a few bug fixes (sometimes it doesn't start up properly with a lot of input, and I keep on getting messages saying it tried to free unallocated memory or something like that) but it's quite stable and does the job I need done.

author: rob77
author_rank: Super Contributor
timestamp: July 17, 2014, 07:52:19 pm
content: Quote from: David Hess on July 17, 2014, 04:01:07 pmactually a simple pulse counter is better.. - i built myself a logic probe with 3 x 4 bit counter stages with 74HC193 - with LEDs on the outpus. - you can catch several short pulses and actually count them (if you "read" binary)another advantage is that you can make a rough guess what the frequency is - by looking at rate the leds are blinking