question about logic probe

Discussion in 'Homework Help' started by mr.harm, May 6, 2009.

  1. mr.harm

    Thread Starter Member

    Nov 24, 2008
    29
    1
    Hello everybody,
    Today while I was testing some ICs. I used the logic probe to test them. One of the ICs was connected to LED and the LED was lighting (the output was high). I tested the output pin which is connected to the LED with the probe but it didn't read high. I tried it again when the output was low and it gave the right reading (Low). Can you help me in explaining that

    and I need some information about this probe because I should submit a report tomorrow about it in addition to the explanation of the question above.

    Thank u all
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    A logic probe has some voltage breakpoint between HIGH and LOW indications. Assuming a HIGH can be read by the probe for the IC when it is not driving an LED, the voltage on the output pin of the IC must have dropped below the breakpoint in order to supply enough current to drive the LED.

    Was there a resistor in series with the LED that you were lighting? This is a standard setup because each LED has a specific voltage (NOT 5v) that must be over it in order to light it up. A resistor is used to limit the current from the IC to an acceptable level for the LED.

    You also must make sure the IC has enough current capability in the output to light the LED.
     
  3. mr.harm

    Thread Starter Member

    Nov 24, 2008
    29
    1
    Actually there was no resistor in series with the LED. About the breakpoint, do you mean there is a voltage range for the probe to light red or green (i.e. let's assume 2-3 volts for low and 4-6 volts for high) that the probe won't light up in it. If there a range could you give me the range for both.

    How do u explain the working of the probe when the output is low (it was indicating low).

    I want to ask about the button on the probe. It was strange. The one I know it should change between TTL and CMOS. But the one I found in the lab had two options indicated with numbers (400 and I do not remeber the other).

    Thak you a lot. If u could just I want anylink that give me more information about how this probe is working cause I have to submit a report.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You could read up on LED's from our Ebook - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

    No logic, TTL or CMOS, is intended to drive a LED with no series resistor. Most logic is unable to source or sink enough current to light a LED without overloading the output stage.

    Get the data sheet on the logic IC that you have driving the LED, and examine the limits of each output. Combine this with the Ebook information on LED's, and you will have a good idea as to why the logic was not reaching a full HIGH level.
     
  5. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    The probe worked at the logic low level because it is not trying to source current. The IC can, therefore, get to the logic LOW state without any trouble.

    To get more information about your probe, you will have to look up exactly what probe it is, part numbers and such. I am not sure what the numbers are on the probe. Maybe someone else can jump in here for that.
     
  6. mr.harm

    Thread Starter Member

    Nov 24, 2008
    29
    1
    At last, could u give me some information about that button.

    Thank you all for helping me. I really appreciate that.

    If there somebody knows where the numbers of the probe are written or how to recognize it, please give me a hand.
     
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