Simple Logic Probe

[Clifton Harp]

Ok guys (and girls) I am fairly new to electronics but I have a question on building a simple logic probe with 1 red and 1 green LED to signify Low and High states. I have breadboarded several different schematics I have seen online and nothing works. I even double-checked the data sheets for the pin assignments. The first one I tried was using a standard hex inverter (SN74LS04 from www.ti.com) Then I figured I would try an opamp so I pulled out my LM324 and hooked it up in another configuration. I ran both of these @ 5V but I still can't seem to get anything to light the LED's. When I get a little more time I will try to post a couple of the schematics I used and maybe some pics of the breadboard layout. In the meantime, do you guys have any ideas on something that might work? I would really appreciate the help.

[TerminalJack505]

Your circuit may not be working because it doesn't share the same power supply as what you're probing.  Your probe needs to (at least) have the same ground as what you're probing.  Ideally, it should also share the same positive supply rail.  Basically, you'd want the circuit under test to power your probe.  This will be fixed at 5V since you are using a 74LS series chip.

Also, if you're using 74LS04 inverters then the the inverters have to sink the current that lights the LEDs.  That is, they can't source the current.  The 74LS04 can sink 8ma but only source 400uA.

Something like the attached should work.  (With props to Forrest M. Mims III, whose circuit I based it on.)

[T4P]

[Clifton Harp]

I am building the circuits on a "Trainer Pad" that has dedicated +/- 5V, +/- 12V, Variable voltage from 0 to +/- 18V, as well as clock, function gen, TTL switches, momentary switches, and various other goodies.  I used the same +5V supply to test the circuit as I do to power it. I will try the circuit you posted. Maybe I will have more luck.
I have attached a pic of what I am trying to describe.

[Clifton Harp]

Just an update. I built the circuit using MultiSim (from school) and took screen shots of it in all 3 cases (Low, Open, and High). Then I spent some time with the trusty breadboard and actually built it. It works. Thank you again for the help. I will post some pics later on today of the probe in action!!! BTW...I am NOT taking this off the breadboard until I absolutely have to...lol I have attached the 3 screen shots of the simulated circuit

[T4P]

I built it a couple of times to train my perfboard skills ... takes time but it's fun

[hlavac]

Try my logic probe, I posted the schematic in this thread. It uses cheap components and detects very short/fast pulses (not only static logic levels).

[Clifton Harp]

I may try this one...most likely I will...but I think the one DaveXRQ posted will be sufficient for the time being. I just started my first hands on class (I'm about halfway through now) and we just started learning about logic gates and (fun fun) Boolean Algebra  We haven't even talked about the use of capacitors or diodes in a circuit yet...I'm just a little ahead of the rest of the class....and I have all this stuff to play with Plus I'm laid off of work right now so I have loads of free time so why not? right? I will definitely try it though. Who knows...maybe I'll get some extra credit...can't hurt to ask!

[Clifton Harp]

Ok let's try this again. Last time, apparently, the thread didn't like me. Anyway, here is a pic of the probe on the breadboard. I know it doesn't look pretty but it works and that's all I need. Still no word from the professor about extra credit...lol Sorry about the picture quality, its a crappy camera but it's all I have at the moment. If you look closely you will see the TTL Level switches on the right side...handy feature to have...just a row of 8 toggle switches the go between High and Low level states.

[codeboy2k]

congrats, it's working for  you... but I was curious, who makes that breadboard trainer in a suitcase. ?

[Clifton Harp]

It's all the stuff sent to me for my electronics courses I am taking at Devry University Online. The manufacturer is Knight Electronics Inc.
10557 Metric Drive
Dallas, Texas 75243

The model of this particular unit is the ML2010 Mini Lab. I am pretty impressed with it so far. Check out the specs and everything it has on it in one package

AC and DC power
Dedicated +/- 5 and 12 V
Variable -18V to +18V
Function Generator
Sweep ramp Generator
Pulse Generator
BNC hookups
Serial ports
undedicated momentary and toggle switches
8 TTL level switches
2 TTL pulser switches
speaker hookups (8 ohm 1/4 W max)
Input protection and breadboard

And that's just what I can remember. Overall it's not a bad little setup. If I think about it tomorrow I'll grab some closeup pics of the different things on it. maybe even scan the schematics that came with the manual. I have to take some better pics of the probe anyway. My professor said to put together a procedures document and submit it for some extra credit WooHoo!