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Using an indicator lamp with 120v

Discussion in 'Electronic Basics' started by kevreh, Dec 13, 2004.

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  1. kevreh

    kevreh Guest

    I want a simple on/off indicator for a switch that's controlling an
    outlet. It sounds like a neon indicator lamp is the way to go. What I'm
    wondering is if the amps pulled by whatever I have plugged in could fry
    the bulb. Or do they have a resistor built in to the lamps to protect
    against this? If not, should I include something?

    Also, should the lamp be wired in series or parallel FROM the switch?
    Btw, would using an LED be more hassle (wiring)?

  2. Mike

    Mike Guest

    Would an illuminated wall switch suffice? Leviton makes these...

  3. John Fields

    John Fields Guest

    They can't.
    on some lamps there's a resistor built into the lamp, but not to
    pritect it from the amps the load is pulling, it's to keep the current
    which is making the lamp light up from getting large enough to hurt
    the lamp.
  4. The indicator needs to be parallel to the load.

    You can use an LED by using a diode pointing in the other direction in
    parallel with it, and a resistor to drop the current to something

    A 1/2W 100k resistor will drop the current to 170/100k = 1.7mA peak. The
    dissipation in the resistor will be 120^2/100k = 144mW.

    You also should use a 1/8A fuse. You can get ones that look like little
    resistors that will work perfectly for this.

    Line --- fuse ---- resistor --- antiparallel LEDs --- Neutral

    Line is the little rectangular hole, Neutral is the big rectangular hole.

    Make sure Neutral is within 10VAC of the big round hole, ie, ground. If
    not, don't do this, and call an electrician.


    .-------. 100k 1/4W
    | | _ ___
    -----| | | |-o \_/o--|___|----o------.
    | | | | | |
    | | | 1/8 A | |
    | | o | V - LEDs or Diodes
    | | | - ^ One Up
    | '-------' | | One Down
    | | | |
    | | | |
    | |
    | _ |
    | / \ |

    AC Meter Should
    Read < 10VAC

    Tape everything upstream of the 100k resistor
    (created by AACircuit v1.28 beta 10/06/04

    If the LEDs aren't bright enough, you can use a lower value resistor. Be
    careful, however, about dissipation. The formula for heat generated is


    So, as R decreases, the heat increased. 1/2 R will double the heat,
    meaning a bigger (1/2W maybe?) resistor.

    A 47k resistor in place of the 100k resistor would dissipate about
    300mW, so a 1/4W resistor wouldn't work.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  5. Bill Bowden

    Bill Bowden Guest

    If you use a neon indicator in parallel with the switch
    (from one side to the other), it will light when the
    switch is off and there is something plugged into the
    outlet which is turned on. The neon indicator bulb should
    have a 100K resistor in series for 120VAC operation.
    Some are built in, others are not.

  6. kevreh

    kevreh Guest

    Dumb question, but would the resistor be before or after the lamp (I'm
    guessing before since its their to prevent excess amperage from being
  7. kevreh

    kevreh Guest

    Are you sure? I was planning on taking the hot FROM the switch to one
    lead of the lamp and a neutral lead that supplies the outlet to the
    other lead of the lamp. My terminoligy isn't all that great so some of
    what I'm saying may be misleading :eek:)
  8. John G

    John G Guest

    And if your terminology is bad your understanding of electricity is even

    Much of the advice given above, though technically correct, could lead
    a novice such as yourself to your DEATH.

    I never cease to be worried about the technical and complicated answers
    given on some of these Electrical news groups to absolute novices who
    are way out of their depth in what they are attempting.
  9. John Fields

    John Fields Guest

  10. John Fields

    John Fields Guest

  11. kevreh

    kevreh Guest

    Not so sure about that. I understand and respect electricity, just a bit
    fuzzy on the right words to describe the concept I'm trying to explain.
    This is intimidating but not helpfull, especially for a forum about basic
    electronics. Am I supposed to give up on this project? Why don't you share
    your insight on the SAFE way to proceed raise the bar on the quality of this
    Again, the group is titled "BASIC"
  12. John G

    John G Guest

    I am sorry, I do not want to be insulting, but you do not have the
    remotest understanding of even the most basic electric principles when
    you ask questions, as you did above, like, Is the resistor to stop the
    load drawing too much power? or Should the lamp be wired in series or
    parallel? or Should the resistor be before or after the lamp?

    Yes this is a Basic group but it is not possible to teach first
    principles of a subject as complicated as Electricity by answering
    random questions. Some understanding is first required.

    And I repeat, from your very limited knowledge displayed above you hould
    stay away from even 120volts, it does kill.
  13. Randy Day

    Randy Day Guest

    kevreh wrote:

    No, you're supposed to get yourself to a community college,
    and take a short course in electric/electronic circuits so
    you can understand what we're talking about when we use the
    *basic* terminology in the subject. It doesn't get much more
    basic than schematic/series/parallel/resistor/ground/hot/
    I just did. Find a night course at a local high school
    offering a beginner's look at electr(on)ic circuits.

    Preferably with hands-on experiments, wiring practice
    and reading of schematics.
    Just like with BASIC physics, BASIC calculus and
    BASIC carpentry, you're expected to have some
    familiarity with BASIC math, BASIC equations and BASIC
    power tools before you attempt any of the above.

    Don't cop an attitude, pal. Get to a beginner's class,
    and build the birdhouse before you attempt the
    two-storey, split-level bungalow.

    120 volts can *KILL*. 120 volts can *burn your house to
    the ground.* Or both.

    You've been warned.
  14. The traditional 120V neon pilot assembly is apparently obsolete - the
    stores group at work used to stock them, but last time I bought one, I
    was surprised to find that it contained an LED! for pilot light use,
    the LED version works as a direct replacement for the original neon
    part, but someone depending on the neon characteristic could be in for
    a rude surprise.

    (This change was particularly surprising, as my boss and I are
    supposed to be consulted for any changes to part numbers or suppliers,
    and neither of us new of the change.)

    Peter Bennett, VE7CEI
    peterbb4 (at)
    new newsgroup users info :
    GPS and NMEA info:
    Vancouver Power Squadron:
  15. It is also useful to know the prefixes in the number system and the most
    common units. 975mhz means nothing. 975mHz means 975 milliherz, 975MHz
    means 975 million Herz, 1 kHz means 1000 Herz, etc..

    Learn the prefixes kilo, Mega, milli, micro, nano, deci, etc.. and how to
    use them.

    Learn some basics about Ohms law, series and parallell, etc..

    Don't touch any voltage above 40 Volt until you know a lot more about
    electronics and safety precautions.

    It is all available at any public library in the world, or through
  16. Kevin,

    Your questions show an extreme lack of knowledge and experience. So to stay
    on the safe side do not work directly on the mains outlet of your house. Use
    a plug and a cable to connect your signaling outlet to the wall outlet.
    Build your lanp, switch and outlet close together and make sure nobody can
    touch the live wire when you put your plug into the wall outlet. As for the
    switch, lamp and outlet: lamp and outlet parallel, switch in series with
    them. If you need the lamp for signaling only, buy a neon type with a build
    in resistor meant for your local mains voltage. Does not matter how you
    connect it to the outlet. Once more, be carefull. Live wire is killing.

    petrus bitbyter
  17. Actually, the number of electrocutions in the US each year is incredibly
    small. Only 140 contractors who work with electricity on a daily basis
    (many of whom don't really understand it, above "don't touch that") were
    killed by it last year.

    It's patronizing for me to say it, and I'm sure you are quite careful,
    but remember to keep one hand in your pocket when you are working on
    powerline AC.

    If you do that, the worst that will probably happen if you get careless
    is you'll get your hand burned.

    It is probably more likely that an old hand will get zapped, due to
    becoming complacent.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  18. kevreh

    kevreh Guest

    Sorry if it sounds like I'm "copping an attitude" (I feel like I'm in
    high school or something). Since this forum is titled BASIC I thought
    people here would be fine answering what I thought would be basic

    There's nothing wrong with taking classes or reading books as you've
    mentioned. But there's also nothing wrong with reading and learning on
    discussion groups like this.
  19. kevreh

    kevreh Guest

    I regret ever posting my question here. The feedback I've gotten has reduced
    me to someone who doesn't know anything about electronics- which is far from
    the truth.

    I'll find my answers elsewhere, as there are many other forums out there
    that aren't as antagonistic towards someone. Most forums run on the mantra
    of "there's no dumb questions", however this one seems to thrive on telling
    people how ignorant they are.

    Take care-
  20. John Fields

    John Fields Guest

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