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LM555 Night Light

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Brandon adds a LM555 and some LEDs to his toddler’s glow-in-the-dark headboard. Now his son has a beautiful night light that is on when he needs it and automaticaly turns off when he sleeps.

This article was submitted by Brandon Uhlig as part of the “Hobby parts for articles ” program. Let Brandon know that you appreciate articles such as this by posting a comment.

This project was built for my son. His toddler bed had some glow-in-the-dark stars and a moon built into the headboard. The problem was they didn’t glow very bright, or for very long. I decided to put some LEDs behind the glow-in-the-dark pieces to light them up. This brightened things up quite well, but I didn’t want them on all night long.

I needed a way to limit how long they would remain on. The timing circuit I came up with uses the ever popular 555 timer. It is wired as a monostable one-shot and using bits from my junk pile, came up with a light-on delay around 6 minutes. When wired this way, the output stays low until the button is pushed, and then goes high for X amount of seconds, based on the following equation.

X = 1.1*R*c/1000 (X is in secs, R is in K Ohms, and C is in uF)

Schem

In this case, I used a 1 M Ohm resistor and a 330 uF capacitor. This works out to X = 1.1*1000*330/1000 giving me an answer of X = 363 secs. Divide by 60 and I get roughly 6 mins.

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This charges the glow-in-the-dark pieces up fairly well, and my son can turn them back on if he wants by on using a momentary N.O. (Normally Open) switch. This way he gets the light when he wants it, but will turn itself off shortly after he (finally!) gets to sleep.

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The timing circuit was built on a 16 pin DIP socket, using point-to-point wiring (30 gauge Kynar, the wire used for wire-wrapping). The LEDs were wired two in a series, with six pairs wired in parallel. Power is from a 5 Volt wall wart.

Posted in Projects.


6 Responses

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  1. XTL says

    220 is typical for 5V (logic) circuits in my mind. You could get it higher using a switching component (bjt, fet). You could also get a more even current matching between the led pairs by using a current mirror (one R, 6 transistors).

  2. Eric says

    Actually, researching it a little more, the 555 timer says it’s internal voltage drop is 1.7 volts, which would leave you with 3.3 volts on the output. Hmmm….

  3. pK says

    Good work, but yes, can insert some resistors for LED’s. I think serial 220 Ohm should me ok.
    dFowler: Consider adding video to your greet articles 😉

  4. Eric says

    I think it is important to current limit the array, to avoid one pair hogging more current than another. When you put LED’s in parallel like that, one set can get favored over another, and you can easily get out of whack with your current distribution. Heck, a good resistor will set ya back 3 cents! 🙂

    So, output from the LM555 is 5 volts if I remember correctly? 2.4 * 2 is 4.8 so you proooobably could get by with less resistance than that since 4.8 is pretty darn close to 5.

    So we’ve got 5 – 4.8 leaves us .2 volts for the resistor (that much is probably sucked up by internal resistance, hehe). Anyway, .2 / .020 amps =~ 10 ohms. So yeah, 12 ohms is a fair value for that.

    Good job, by the way! I bought a bunch of LM555’s for a scouting project I was working on (the kids were building astable multivibrators for morse code oscillators), I might give this a go…I have all of those LED’s I bought from Alan and didn’t use them all for the last project.

  5. BrandonU says

    Eric-
    Good Point!
    I briefly considered it, but I’m cheap 🙂 I used a low current(150 mA) wall wart I had on hand. Bread-boarded the whole array to make sure nothing burned up. To be honest, not a lot of planning went into this, a free afternoon and a junk bin of parts equals the project above.
    The LED’s are spec’d at around 2.4V forward voltage and around 20mA current (bulk purchase, no idea on exact specs…). Probably not a bad idea to throw some in if you wanted to build one. According to an online LED array calculator ( http://led.linear1.org/led.wiz ), a resistor around 12 Ohms in series with each pair of LEDs would work.

  6. Eric says

    Just one question. Did you consider adding current limiting resistors to the LED legs?