i want to build a logic probe which will indicate a high logic level, a low logic level and a tristate. this probe should be designed using only logic gates (no 555 timers, PLDs or PICs etc). i have tried to make a few designs but im not sure about them. i also want to make an enclosure for it. so plz suggest a good enclosure for it.

 

Hi
Try this one: http://www.uoguelph.ca/~antoon/circ/probe2.htm

Ante :roll:

 

thanx ante for your reply. im sorry i didnt mention that i want to build the probe to test TTL circuits. the circuit u gave tests CMOS circuits. i need a probe that will only test a HIGH, a LOW and a TRISTATE for TTL circuits.

thanx for the help in advance

 

Hi again!

How about this one then?
http://www.solorb.com/elect/logprobe/

Ante :roll:

 

looks nice but it isnt what i want. i dont want to indicate pulses. just a HIGH, LOW and TRISTATE. this one doesnt have an indicator for TRISTATE.

 

Heya Cheetah,

I think I have what you're looking for, only I have the schematic at home right now. The probe shows High, Low, Tristate (oscillation) VIA a 7 segment display. The schematic uses 2 IC's and a couple resistors and other minor parts.

I'll post the schematic when I get home (a few hours or so).

 

Here you go. TTL only, unless you develop a switch to change the resistor for the display input, and use CMOS equivalent IC's.

The display will light an "H" for high, and a "L" for low. For the oscillation probing, both H and L will seem to be lit. This is due to Persistence of vision because the oscillations occur too rapidly for the human eye to distinguish between H and L displays. Thus, you'll see an "8" unless the oscillations are extremely low frequency.

I've seen some neat enclosures, which enclose the circuit in a tube which is about 1 inch I.D. or so. (PVC tubing is cheap!). The probe is fasted (Epoxy) through the center of a pvc tube cap, which fits onto one end of the PVC tube. The other end has a similar cap, with holes to allow the wires through. You can then paint the PVC tubing (Which looks nice if you do it right). After you finish, you'll have something that looks like an oversized marker, or an exploded view of a ball-point pen. The enclosure shouldn't cost more than a couple of dollars.

Attached Images

Tri_state_logic_probe.GIF (54.2 KB, 1101 views)

 

thanx Johnson. i must admit that the solution u gave is great. but there is one problem. i said that it should only have logic gates. but this one also has a timer (74121). i want a TTL compatible logic probe that will indicate a HIGH, a LOW and a TRISTATE (i.e niether high nor low) by using logic gates only.

thanx for all the help!!!!

 

AHHHHHH horse pucky!!! I forgot about the monostable multivibrator. Sorry about that. I don't have any ideas with using only logic IC's.

Good luck!

 

You can build a monostable with a couple of gates, a capacitor, and a resistor or two - but in any case, it's still not detecting tri-state, only pulses.

The spec required doesn't really make much sense, it asks for TTL compatibility, but then wants to detect tri-state - which isn't a TTL compatible state?.

Would this be another homework question?.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

I was just thinking the same thing. Wouldn't you be building a discrete monstable using logic gates etc?

Can we define Tri-state? I'm thinking we all have different definitions. I interpret tri-state as being oscillations (High, flat, low--low flat high--high, flat low etc etc.). Can someone set me straight?

 

As I understand it (from tri-state buffers), if you set the pin as 'tri-state' it is effectively disconnected - so the pin can be affected by other connections to it, but the tri-state one doesn't interfere with it in any way.

As far as PIC programming goes, setting an output pin to be an input effectively 'tri-states' it.

I wonder if whoever set the question thought of it as an oscillation?.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

I'm confused. When you say "TRISTATE (i.e. neither high nor low" do you mean probing a point that has no voltage? Wouldn't this be accomplished by probing the point and finding that no readout occurs? Thus, no voltage is evident?

What is your definition of TRISTATE? Maybe we can help with this.

I'm under the impression, that TRISTATE refers to a probe that can measure:
First state : Lows
Second state: Highs
Third: oscillations

Thus, a TRISTATE (or three state) probe is accomplished. Any ideas?

 

I mean he need to detecting the floating input...

 

Three-state logic, also calle tri-state logic ( a trademark of National semiconductor Corp, who invented it ) is a misleading name. its not digital logic with three voltage levels or HIGH-LOW and the LOW-HIGH type of thing. it's just ordinary logic, with a third output state: open circuit. the predecessor to TRISTATE was open collector logic.

TRISTATE isnt TTL compatible???????????? i dont think so

yes it is