Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Small LED on screwedriver

  1. Mar 1, 2008 #1
    I have a small screwdriver which has a small LED in it. When you stick it into something like an outlet, and hold your finger on the other end, the LED lights up. Today though I held both ends of the screwdriver and the LED lighted up. This confuses me tremendously... Doesn't that indicate there is a potential difference between the two fingers? Otherwise what makes electricity flow and light up the LED? Does anyone know these screwdrivers? My best hypothesis is that it must have a battery in it or something, and some type of a small ammeter or something?
  2. jcsd
  3. Mar 1, 2008 #2
    It actually does have a battery inside. Lol, you thought you could obtain energy from your fingers? Thats great. Such a device is chiefly called a Voltage Detector or Voltage detection screwdriver.
    C.f.: Patent 6100679 - Abstract & Schematics

    (google is sortof starting to scare me with its versatility)
    Last edited: Mar 1, 2008
  4. Mar 2, 2008 #3
    no i thought that the LED just requires VERY little voltage to start shining... And it's pretty dim too so i thought it was a possibility that I was charged or something... but not uniformly? sounded weird. Anyway the thing that confused me the most was that the screwdriver is REALLY old, and the battery would have to last a really long time as well.
    ok cool good to know... and i was right :p
    thanks for the link

    there still is the issue however of why it lights up on contact with my skin...
    "A tool for indicating the presence of potentially dangerous voltages to the user includes circuitry for detecting the presence of an AC voltage..."
    i'm still not exactly sure how it works. it's probably easy to detect high voltages, but the voltage drop between my two fingers should be pretty much 0.
    Last edited: Mar 2, 2008
  5. Mar 2, 2008 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    When I hold the tip of an oscilloscope probe between thumb and forefinger, I see a periodic trace on the scope.

    Similarly, I have an iPod docking station with speakers. It includes a cable (mini plug to mini plug) for connecting auxiliary devices other than the iPod to its speakers. When I connect myself to it (i.e. I turn on the unit, plug one end of the cable into the mini jack, and hold the contact of other end between thumb and forefinger), I hear a buzzing noise over the speakers.

    The human body emits some type of electrical signal. I think it's related to what keeps one's heart beating, but I don't know too much about this topic.

    I also am not sure if that is what is lighting your LED.

    Can anyone explain the cause of these phenomena in more detail?
  6. Mar 2, 2008 #5
    The human heart gives off a small electrical field. Thats how most heart rate moniters work, by picking up this field. Also has to do with why they shock you to get your heart started agian.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook