1.5 V blinking / flashing circuits

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BillRoberts

Newly Enlightened
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Dec 2, 2003
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I would like to make a VERY inexpensive blinking LED light that I can run off of 1 AAA battery. I was hoping I could simply use a capacitor. My thinking was the capacitor could be used to build up enough voltage to light the LED. But I have no idea how to size the capacitort to even try.

anyone have any ideas?
 
Re: blinking / flashing LED with Capacitor

Don't know any way with just a cap. National used to make a cheap IC, LM3909, just for this purpose. If I recall it requires only 1 external part, but you will have to use a red or other low voltage LED. I don't think it will flash a white LED, as they didn't exist when the 3909 was designed. Search for LM3909. Good luck!
Rob
 
Re: blinking / flashing LED with Capacitor

The lm3909 works great for flashing leds. I still have some unused chips. I bought some of them back around 1980 or so.

All you need with this chip is a capacitor, a led, and a 1.5 volt battery and you have a flashing led.

This chip has been discontinued by National, so the only sources are probably pack rats like myself. I do have a few hanging around and if you are interested I could part with some for a couple of dollars and postage. I could even include some circuit drawings, I might even still have a data sheet, but I would have to look.
 
Re: blinking / flashing LED with Capacitor

The LM3909 was a great way to very easily create a flashing LED. I wonder what they are using now since stopping production on that. There are still so many bicycle rear lights that use it.

Alternatives would be to use either the ubiquitous 555 timer chip or build a multivibrator from discrete parts (2 transistors, 2 small capacitors, 2 resistors). Actually, I may be wrong here. The 555 timer chip may not work as it might require more than 1.5V to operate. But the discrete-parts circuit should work.

Can you specify the colour of LED you will be flashing? And how many you intend to use? So that we have an idea of the power draw required.
 
Re: blinking / flashing LED with Capacitor

The LM3909 was an interesting chip. National still has the datasheet. Since the input voltage can be up to 5 V and the chip is a voltage doubler you should be able to flash LEDs other than red by using 2 or 3 cells. 3 cells at end of life would be 2.4 V times 2 is 4.8 V which is still more than the Vf of almost all LEDs. The spec. sheet says a red on a D cell for 2.6 years but I think mine used to run over 3 1/2.

Some places sell the chip for $6 (US) or more but others show it less than $2. There seems to be no shortage even though National last sold this chip in 1998.
 
Re: blinking / flashing LED with Capacitor

I have a design that will flash a white led with as low as 1.0volt input and is based on discrete components.
 
Re: blinking / flashing LED with Capacitor

I'm tring to build some cheap flashers for trail marking (so I can find my way back if it gets too dark)or toys for the kids, I'd like to run it off of 1 AAA NiMH so I can recharge them easily, I'd like to make about 10 or so.
 
Re: blinking / flashing LED with Capacitor

Electronic breadcrumbs! I like it! I think 2 LM3909s. One using a circuit similar to the LED booster (with an added capacitor across the output) to drive a second flasher circuit. I would use blue-green (cyan) LEDs since these appear at least 2 1/2 times brighter to the eye than any other color. Hmmm...may have to order some LM3909s and try this myself. I think the low current draw for only a few hours a night would make throw away batteries a better choice. I'm thinking a single AA would last over a year?

BTW - the LM3909 is the great granddaddy of switched capacitor power supply chips in use today.
 
Re: blinking / flashing LED with Capacitor

I guess I didn't write down what I did so now I have to reinvent it, but basically I use a coil that has pulses applied to it to increase the voltage to the led. Its basically what a step up circuit does only slow the frequency down to like around a hertz or a half a hertz.
Or for you oldtimers like me, RINGING CHOKE !
 
Re: blinking / flashing LED with Capacitor

If all you are looking for is some trail markers, I saw some neat little ones at Shopko, in their camping stuff. They had these neat little led beacons just for campsite markers. They had the Coleman name on them (I think), used a couple? LR45 sized button cells. I think the run time was about 20 hours. I refrained from buying them only becouse I am watching for the Dorcy led lights.

Wal Mart carries (carried?)a assorted package of the flashing beacons that look kind of like small bike reflectors. They had the Bell name on them and are in the automotive department. They came with 4 or 5 to a pack and (I think) are less than $10.
 
Re: blinking / flashing LED with Capacitor

I'm looking at the LM555 timer to blink an LED. I want it to blink once every 2 to 5 minutes for 1 second. The circuit works and I'm happy with it.

Now the next part. I want to build it into the head of a flashlight where space is kind of tight. Does the 555 come in various sizes/packages? How does one find the variations? I kmow the 555 is rather small, but I may want something even smaller to squeeze into lights the size of a Pelcan M6-LED.

Thanks for any help.

Daniel
 
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