Comments on: LM555 Night Light http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/ Microcontrollers Electronics Hobby Tue, 24 Jan 2012 09:58:33 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: XTL http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/comment-page-1/#comment-457 XTL Sat, 24 Nov 2007 10:44:27 +0000 http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/#comment-457 220 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). 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).

]]> By: Eric http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/comment-page-1/#comment-456 Eric Tue, 31 Jul 2007 19:43:40 +0000 http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/#comment-456 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.... 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….

]]> By: pK http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/comment-page-1/#comment-455 pK Tue, 31 Jul 2007 11:11:06 +0000 http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/#comment-455 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 ;) 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 ;)

]]> By: Eric http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/comment-page-1/#comment-454 Eric Tue, 31 Jul 2007 05:43:09 +0000 http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/#comment-454 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. 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.

]]> By: BrandonU http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/comment-page-1/#comment-453 BrandonU Mon, 30 Jul 2007 23:46:41 +0000 http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/#comment-453 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. 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.

]]> By: Eric http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/comment-page-1/#comment-452 Eric Mon, 30 Jul 2007 22:51:15 +0000 http://www.uchobby.com/index.php/2007/07/30/lm555-night-light/#comment-452 Just one question. Did you consider adding current limiting resistors to the LED legs? Just one question. Did you consider adding current limiting resistors to the LED legs?

]]>