Universal logic probe circuit - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: September 09, 2023, 07:09:29 am
content: I don't like how the usual logic probes usually have the Hi/Low LED's still light up with pulsed signals,Also how they usually have the Hi/Low LED's wired up in that crappy way, and because of that it has a constant 2k load across the supply.Even tho those "problems" could be fixed just by using the quad NOR gates more efficientlyAlso since they use a CMOS logic IC, it only works for those logic levels.Also they don't really show a floating input, it always shows either high or low signal.My design aims to fix all those minor frustrations.It doesn't quite work down to LVCMOS logic levels, although it tries to (in spice sim) just that there's not enough gain to give ample current to the LED's to actually make them glow, probably with some cleverer choice of transistors (I just used 3904 and 3906s), it could work for that too. Also with more clever HFE choice you probably could delete the LED current limit resistors altogether.The idle current is all from the voltage dividers, a few microamps, so you could just leave it powered in the case for battery power.The reason I'm mentioning battery power for a logic probe is because I originally designed it for an op amp tester, and liked the idea of minuscule idle current without any op amp installed, so I don't need an on/off switch. Later I realized I had made a glorified logic probeI don't like the input circuit, the forward drop of the transistors is a pain since with 5V levels it would fully trigger only at ~4V.Not wanting to complicate the circuit even more I just added a couple biasing diodes (not actually turning the transistors on, just shifting levels), But this made it so it fully triggers around 3.5V so that was much more ideal.Q1 and Q13 are the because the biasing current of the Hi/Lo circuit is enough to turn on one of the pulse LED'sI'm wondering tho how fast will this circuit operate with plain bipolars?[ Attachment Invalid Or Does Not Exist ]

author: janoc
author_rank: Super Contributor
timestamp: September 09, 2023, 02:20:19 pm
content: There are plenty of these probe designs around that work on multiple voltage levels (5V, 3.3V, CMOS), so no need to reinvent the wheel. A fixed divider is not good for TTL/CMOS distinction - the voltage levels don't completely overlap and your probe will happily indicate high but the circuit expecting a CMOS level will read low or oscillate because that signal is too low for it to register.One tip concerning the pulse indication circuit - these pulse stretching/latching setups are universally awful.Long time ago when I have been still using my probe and not a scope, I have got a tip from a more experienced guy - put a counter (something like 74xx90) on the output of the probe and attach LEDs to the outputs. That allows you to not only see that there are pulses on the input but also how many - e.g. useful to see whether some contact is bouncing or some signal is being asserted multiple times - or whether there is a continuous pulse train going. With the usual "Pulse" LED you are completely in the dark with regards to that. I haven't seen this on any commercially made probes, even though it is so useful feature.Quote from: ELS122 on September 09, 2023, 07:09:29 amProbably way faster than anything you will care to use a probe like this with anyway. Logic probes are pretty useless these days unless you are working on some retrocomputers or really slow designs - and even then a cheap oscilloscope or a 10€ logic analyzer will do you much better service.