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Making a flashing mains indicator LED

Discussion in 'LEDs and Optoelectronics' started by seanspotatobusiness, Mar 29, 2019.

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  1. seanspotatobusiness


    Sep 11, 2012
    I previously added a mains indicator LED to a hot glue gun by connecting an LED and resistor directly to the mains connection with a diode across the LED to allow current to pass in the opposite direction when the direction changes. For some devices I'd prefer the LED to flash because I think it would get better attention. How can I make the LED flash with the fewest/smallest/cheapest components? I know you can buy LEDs with some built-in tiny components which make them flash but would they be damaged trying to hold back a mains voltage even with the resistor in place? Thanks!
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011

    Instant remedy with a catch: Instead of the anti-parallel diode already in place use a zener diode with a threshold a bit above the LED's pass voltage. A zener voltage of 3 V ... 5 V should be readily available. This zener diode (also anti-parallel to the LED) will limit the voltage across the LED in pass direction. However, as the input is a sinusoidal waveform the LED will be presented with a pulsating DC and it is therefore unlikely that the flashing works as expected. Instead you will need an additional comparatively large capacitor across the LED so it operates on (essentially) DC. The size of the capacitor depends on the current drawn by the LED:
    A few assumptions:
    • LED current of 3 mA.
    • LED pass voltage 1.6 V ( a red one)
    • Mains frequency of 5060 Hz or a period of 17 ms (rounded).
    In the on phase of the LED the capacitor will charge to 1.6 V (put in the real value of your LED used). During one period of mains it shall discharge by e.g. no more than 20 % (assuming this is sufficient to keep the LED "alive".
    The charge drawn during this time is Q = 3 mA * 17 ms ~ 50 μAs (approximating a linear discharge by the LED).
    The voltage drop during this time is 1.6 V * 20 % = 0.32 V.
    From these parameters with C = Q/V we get C = 50 μAs/0.32 V = 156 μF. A 220 μF capacitor should work just fine.

    Edit: you will also need an additional series diode to prevent discharge of the capacitor during the negative half wave:

    This is all pure theory. I leave it up to you to do the experimental part of this exercise. I'd love to see your report on the outcome.
    Last edited: Mar 29, 2019
  3. Bluejets


    Oct 5, 2014
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Very good. Hopefully there's space for the battery.
  5. Bluejets


    Oct 5, 2014
    I'd say plenty for one of those 12v "keyless remote" batteries with perhaps a small amount of circuit mod. if required.

    Actually the Op will be lucky if he ever gets it back together again.
    Ever seen inside one of those things...?
    Not even an "element" as one would expect.
  6. bushtech


    Sep 13, 2016
    Darn it Bluejets! Now you've got me looking at my glue gun and thinking: "Shall I........?:mad::D
  7. Bluejets


    Oct 5, 2014 it.
  8. 73's de Edd

    73's de Edd

    Aug 21, 2015
    HEY ! . . . . . Sir . . . . . " Solanum tuberosum " man . . . . .

    0% all pure theory . . . . . 100% all factual / functional reality . . . . and faultlessly working from 1946-2019 . . . to date, as the flashing POWER ON indicator on my self assembled isolation transformer + a following VARIAC in its metal enclosure.

    So that makes about 73 years of brilliant ORANGE blinky-blinky action . . . whenever so called upon, by a POWER ON condition .
    It consists merely of a rectifier diode feeding into a small carbon . . . voltage dropping / current limiting resistor that then feeds into a paper / poly capacitor shunting across a NE-2 neon lamp.
    ( A NE-51 version was used back in my construction days, since the mini all glass blob, leaded NE-2 had not yet come upon the scene.)

    The + nodes of the AC gets rectified to DC and then is voltage dropped to charge the cap that is shunting the neon lamp.
    Upon power up, the reduced DC voltage flows directly to the capacitor and startes charging it up.
    When it builds up an ~ 72-75 VDC level, where the threshold of ionization of the neon lamp is met, the lamp electrodes flashover from that stored level of charge within the capacitor.
    There is one brilliant orange flash, then the charge depleted capacitor starts a recharge to repeat that same timing cycle.

    You sort of have to evaluate the flash intensity and up the capacitance if you want a brighter flash and make a like decrease in the resistance of the resistor to get a quicker charge time , if the chosen capacitance value was raised.
    I remember experimenting, using .47----.68 ---- and 1 ufd caps with accordingly selected dropping resistors.
    I finally got my result of a bright orange flash every 2 seconds.

    Joe Blow and Colloqial Local Yokels can find neon lamps within the storage drawers of the electrical section of a FULL SERVICE . . . . . ACE Hardware store.

    Its circuit look-a-like dis . . . .


    On a droppable item like a glue gun, one might additionally place a clear plastic cover over the exposed lamp portion.

    73's de Edd . . . . .

    What has four legs and an arm? . . . . . . . . . . . . . . one HAPPY pit bull.

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