My Simple 8 Way Logic Probe - Page 1
Forum section: electronics / projects

author: David_AVD
author_rank: Super Contributor
timestamp: June 21, 2013, 11:41:23 pm
content: When debugging equipment, I often find it useful to be able to watch several logic lines at once. A lot of the time the signals are slow (switch inputs or relay drive outputs) so I don't need to analyse them as such - I just need to see the steady state or watch for a bit of activity. Moving a single logic probe from pin to pin is too fiddly and doesn't show the relative timing I want to see.I had already made an 8 way logic probe some years ago, but it's part of a eurocard rack system (up high on my bench) and means I have to keep looking away from the equipment under test to see it. So... I decided to make a small, portable version.Each input uses a dual comparator, clamp diodes and a few passives. The pair of LEDs on each channel are fed via a single constant current device as only one is ever on at a time. This means the LED brightness is constant over the 5V to 20V operating range.I have a resistive divider with taps for TTL and CMOS thresholds. A small slide switch selects those thresholds and feeds them to all of the channel comparators.The channel inputs connect via a 16 way IDC header. This means I can have multiple cables with probes or wires, etc. I can even leave the input harness connected to the equipment under test and move to another project. (I often have lots of projects going on at the same time) For some reason the part of silkscreen that shows the input and power connections got lost by the PCB makers - slightly annoying.The DC input is reverse polarity protected with a small polyswitch and shunt diode. In hindsight I could have used the FET + resistor trick, but this works fine. Anyway, just thought I'd share one of my projects with you all.

author: ftransform
author_rank: Frequent Contributor
timestamp: June 22, 2013, 02:23:32 am
content: can you post some schematics ? It seems like a useful tool, single sided board right?I might make one if you post the PCB.

author: David_AVD
author_rank: Super Contributor
timestamp: June 22, 2013, 03:14:41 am
content: It's a double sided board, mainly because it was easier and I chuck little projects like this onto mixed prototype panels we get done regularly.The footprint for the SMD electrolytic capacitor is wrong so I need to fix that in the PCB file. The value is not critical so I just put a smaller one in.Here's the channel and overall schematics in pdf format if anyone is interested.

author: Harvs
author_rank: Super Contributor
timestamp: June 22, 2013, 06:17:36 am
content: Thanks for sharing.

author: David_AVD
author_rank: Super Contributor
timestamp: July 03, 2013, 11:34:52 pm
content: Just this week I used my new probe for watching two IIC busses, an SPI buss and another select line. Seeing them all at once made debugging a lot easier and saved me time.

author: hlavac
author_rank: Frequent Contributor
timestamp: July 04, 2013, 11:50:45 am
content: I have designed something similar to this, but 16 channel and using dual color LEDs (antiparallel red/green between the level comparator outputs) so that I could cram more into the small space. As it was very slow (LM339 based) anyway it didn't matter. Used it to debug some LCD interfacing code.

author: David_AVD
author_rank: Super Contributor
timestamp: July 04, 2013, 12:19:02 pm
content: I did consider using bi-colour (2-pin) LEDs, but using separate ones has allowed me to see even relatively narrow pulses on those serial lines.

author: hlavac
author_rank: Frequent Contributor
timestamp: July 04, 2013, 02:45:59 pm
content: Too bad fast (10ns or less) comparators are so expensive.Makes this architecture too expensive for multiple channel logic probes...I was able to make a fast enough discrete component replacement input stage that is 10x cheaper and still picks up 10ns pulses but that takes a lot of board area...