Connect with us

Logic probe

Discussion in 'General Electronics Discussion' started by Dschneider327, Sep 9, 2018.

Scroll to continue with content
  1. Dschneider327


    Sep 9, 2018
    Hi I'm taking digital circuit class and I have to build a logic probe tester. Has two led green for 1. Red for 0 .when I power it up both led come on when testing only the red goes out. Its using lm339 chip. He is a pic of schematic and bread board.Any help would be nice . Thanks . Kinda new to this

    Attached Files:



    May 20, 2017
    Had a go at simulating your circuit. The window comparators exhibit a lot of hysteresis which is part of your problem.
    Start off by ditching R6 and C1 then reduce R5 in value to 1K then replace the LM339 with a TL2272 and it should work.

    Edit: Or you could just use a 4 x 4093 gates paralleled together the do the job with LED's connected to the output. All the threshold are sorted out for you then.
    Last edited: Sep 10, 2018
  3. AnalogKid


    Jun 10, 2015
    There is no hysteresis in the circuit. The circuit is a window comparator with a wide dead band. However, there is an overcurrent condition with the LEDs. You need to add one current limiting resistor (220 ohms-ish) between the Vcc input terminal and the D1/D2 anodes. Since at most only one LED will be on at a time, only one resistor is needed to protect both LEDs. Other than that, I would not make any changes to the schematic until the basic circuit is functional.

    Also, you need to add decoupling capacitors across the IC power pins. Something in the 10 uF - 100 uF range where the red and black wires from the chip enter the vertical distribution rows, and a 0.1 uF ceramic directly across the chip power pins. Trim the leads as short as possible.

    A logic chip does not work well as a logic probe, because it does not have precise level detection. Also, hysteretic CMOS transition levels are incorrect for monitoring signals that are supposed to be compliant with TTL levels. A logic probe has a wide dead band so that no lights indicates an output that is not making a *valid* signal.

    Last edited: Sep 10, 2018
  4. Icenesis wayons

    Icenesis wayons

    Dec 14, 2018
    Hope you have the answer by now but if not there is a discrepancy in the board and the schematic. If you look at the plans, the LED's are leaving U1 pin 14 in series w/ea. other. Starting with the cathode of LED 1 to pin 14, anode of LED 1 to anode of LED 2, cathode of LED 2 to pin 1 of U1, anodes of LED 2 and LED 1 are then connected to vcc 5v pwr. Then to R 1, R 1 To U1 pin 7, Then R 2 @ U1 pin 7, R 2 to R 6, R 6 to pins both 6 & 9, R 3 junctions @ R 2 and R 6, other side of R 3 to pin 8, R 4 to pin 8 other side of R 4 to ground. C 1 connected across R 5 and one leg of R 5 to pins 6 & 9, other side of R 5 connects to probe end. Hope this helped I tested this on my breadboard and it worked. What kind of cap are you using? If ceramic replace with tantalum, if electrolytic check for resistance and or go to .50ufd @ 15 volts. Sometimes resistance of jumpers and various tolerance of components may play a necessary role in changing of values of certain pieces. Hope this helped. Happy holidays!
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day