Logic Probe

Discussion in 'General Electronics Chat' started by mentaaal, Aug 4, 2008.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    Hey guys, was looking for a logic probe shematic and came across this:


    Been trying to figure out how it works...
    I dont understand how the high LED lights when the test signal is high as the 74ls00 can only sink 8 ma?
    Even if the second NAND gate assisted by sinking curent the Iih is only 20uA?

    Also whats the point of the pull down restor to earth on the bottom input of the left most NAND gate?

    Thanks guys!
  2. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007
    The LED must be low current type, <8mA.

    The inputs are clamped to 5V and ground through the resistor diode combination. So, if the input is 0.7 more negative than ground, then it will start conducting and dissipate through the resistor. If 0.7 more than 5V, then it will do the same. It's an old trick to protect inputs / level change an input.

  3. SgtWookie


    Jul 17, 2007
    It's not a very good schematic. You'll find lots of that on the Internet. The LEDs might light up, but they won't have anywhere close to their full brightness. The way he has the test input wired, it would place a fair amount of load on the circuit under test.

    Ideally, the test probe would have a very high input impedance to prevent loading the circuit under test.
  4. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    Man i have a lot to learn!
    I am not worthy! ;-)

    Thanks guys.