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Old 08-23-2012, 06:58 PM   #1
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Default Simple slow flash circuit??

I have need to create a slow flashing light for a project and I'm not all that good with circuit design.

I'm not picky about what voltage type and amount. I think I can work with 110VAC, 12VDC, 5VDC, 3VDC, etc. incandescent or LED.

Practically speaking, a 110V light bulb *might* be the easiest rig up, but I could create a separate DC LED with no hassle to do the job too.

I'm guessing it might use a 555 timer, but I'm not sure how to get it to slowly light up the lights, and then dim them slowly, then back on, and so forth. And then how to vary the speed of the flash to get it "just right"...

Anyone have any suggestions?

I've seen some circuits online, but they either have a great looking circuit and no visual to see how it actually works, or a nice video and no circuit to help me build it.

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Old 08-23-2012, 07:07 PM   #2
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I found this site. How does this look? I could rig this up with a 9V battery, or a 9V DC adapter pretty easily.



Adjust the flash rate by changing C1? How about a variable capacitor instead?

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Old 08-23-2012, 07:14 PM   #3
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What's that got to do with beer?? Lol

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Old 08-23-2012, 07:25 PM   #4
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What's that got to do with beer?? Lol
It's a secret...
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Old 08-26-2012, 05:45 PM   #5
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Yes, varying the value of C1 will change the flash/fade rate. Depending on parts availability and price, you may want to go with a trimmer cap that is undersized for your needs and put it in parallel with a fixed capacitor which effectively adds their values.

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Old 08-26-2012, 06:44 PM   #6
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Instead of a trimmer cap, I'd just go with a trim pot. You can get everything in your circuit at radio shack or fry's electronics for pretty cheap. Just put a 10k resistor in series with your trim pot in place of R1. That will give you control of your frequency. You'll need to adjust R3 for the LED that you choose (based on the max current). Don't sweat it if you can't find the BC547B, any general purpose npn should work. If you want, you can simulate it pretty quick in LTSpice... I just did.

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Old 08-26-2012, 10:39 PM   #7
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Quote:
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[...]If you want, you can simulate it pretty quick in LTSpice... I just did.
I like it!

Now, how about simulating how big a cap he'd need to stick on Q1's emitter to make the on/off transition slew rate slow enough to providing a noticeable fade-in/fade-off effect?

Cheers!
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Old 08-27-2012, 04:50 AM   #8
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I like it!

Now, how about simulating how big a cap he'd need to stick on Q1's emitter to make the on/off transition slew rate slow enough to providing a noticeable fade-in/fade-off effect?

Cheers!
43 Farads... or 7 femto Farads, either will work

Another option is to get a cheap microcontroller. You could get an MSP430 dev board for 20 bucks (usb programmer and one micro). Plus 3 extra micros for 10 bucks. You could fade a dozen LEDs with PWM at any frequency. Just a thought.
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Old 08-27-2012, 05:05 AM   #9
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This wouldn't have anything to do with that recent TARDIS project now would it?

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Old 08-27-2012, 04:28 PM   #10
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First, you guys rock! I think I have enough to get this going.


Quote:
Originally Posted by SeattleMatt View Post
43 Farads... or 7 femto Farads, either will work

Another option is to get a cheap microcontroller. You could get an MSP430 dev board for 20 bucks (usb programmer and one micro). Plus 3 extra micros for 10 bucks. You could fade a dozen LEDs with PWM at any frequency. Just a thought.
I already own an Arduino board, but I don't really want to dedicate a logic board for a project that really ought to run on a few dollars worth of components. But truth be told, I could probably program this pretty easily on that board.

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This wouldn't have anything to do with that recent TARDIS project now would it?
I have no idea what you're talking about, sir.
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