LED sign

Discussion in 'The Projects Forum' started by vjlmorgan, Aug 20, 2009.

  1. vjlmorgan

    Thread Starter New Member

    Aug 20, 2009
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    Needing to make a little sign out of tiny white LED's about 1.2mm across, which has to be on a dimmer switch that slowly fades up for 30 seconds then fades off for 5 seconds. I am a complete electronics novice, the sign is just 2 words using about 200 bulbs. Is it possible to find a dimmer that you can time and adjust yourself?

    Any help welcome

    James
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    It is funny (and not aimed at you) but it seems similar questions come in clusters. We've had a lot of fading LED queries lately.

    Are you wanting to make the text on this sign programable, or is it like a piece of plywood with a bunch of LEDs pushed through displaying the text?
     
  3. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    I picture a room full of students all asking at the same time.
    [​IMG]
     
  4. daviddeakin

    Active Member

    Aug 6, 2009
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    Perhaps you could power the LEDs from a 3-pin regulator IC, but instead of connecting the adj' pin to ground, connect it to the output of a very low frequency oscillator (555 timer maybe?). As the output of the oscillator ramps up, the output of the regulator increases and the LEDs glow brighter. Then the opposite happens.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    True, but the reason I asked my question for the OP is to establish what kind of circuit he was needing. If he wants something like a marque then a microcontroller is called for, and the special effects are part of the coding. If it is a fixed text sign I was thinking several variations of the 555. Basically we're waiting for the OP to get back with us.
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Do you have some specs. on LED's? A 48V power supply@ 300mA would be in the ball park, to keep strings as long as possibleand and about 15 strings.As to finding a dimmer , none known to me. Full bribgtness est. 20 mA for example, end of 30 sec, 6 mA, 35 sec 0 mA ??
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Just a quick outline. Have not established whether it is switch activated or free running, this works with either. 555 A starts with a push ow a switch, generating a + pulse long enough to charge Ct, the trailing edge of pulse is differenated as a start pulse for 555 B. Ct starts discharging via R1 with a time constant of about 50 sec. 555B outputs a + 30 sec. pulse which effectively opens R2. At the end of 30 sec. R2 is grounded, adding an additional discharge path, bringing Ct to ground in another 5 sec. Ct is then feed into a V follower OP amp & LED driver.On bread board Ct=10,000μF, R1 9.1kΩ, R2 680Ω., V follower LF353 has a nasty habit of output swinging to B+ after Ct discharges, maybe it does not like to operate with single-ended power supply. Also ground end of R1, was returned to a - supply[9V battery] with + side of C1 clamped to ground to prevent - excursions. It's a start.
     
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