CMOS Logic Probe Project

THE_RB

Joined Feb 11, 2008
5,438
The most important feature of a logic probe is the ability to display extremely short pulses (using a monostable effect).

Just showing HI and LO on a LED is not worth any more than, well... A LED. ;)
 

Thread Starter

AlphaDesign888

Joined Jul 27, 2014
193
The most important feature of a logic probe is the ability to display extremely short pulses (using a monostable effect).

Just showing HI and LO on a LED is not worth any more than, well... A LED. ;)
You're right. So how do I do it?
Kindly show me the way ...
 

THE_RB

Joined Feb 11, 2008
5,438
Have a look at "555 monostable" on google.

Basically if the logic circuit under test makes a very short pulse (like a few 1uS) it triggers the 555 monostable which makes a fixed LED on pulse of say 100mS, which is long enough for your eye to see.

Sometimes instead of a 555 timer they use some logic gates and a cap.

There are a lot of logic probe kits you could also google, to get some schematics. :)
 

Thread Starter

AlphaDesign888

Joined Jul 27, 2014
193
Hello,

The probe uses the 74ls14 chip, so it will be conform the LS-TTL norm.

Bertus
I guess providing that the chip is within specs. But won't one of the LEDs always be on since the output of the chip will be either high or low? There is no tri-state.
I mean also to have the input of an logic chip floating falls into the area if unpredictable output. Bad logic as my teacher back in 1997 said.
 
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