Circuit for flashing LED once per second

[Cemoi]

I would like to build a bike tail light, direct driven by two NiMH AAs, using the following LEDs:

1. LED1 is a red Luxeon I. When connected to my batteries (Vbat=2.6V) in series with a 1 ohm resistor, I=118mA and Vf=2.40V.
2. LED2 is a red Seoul. With the same Vbat and resistor, I measure I=270mA and Vf=2.10V.

What circuit and components should I use so that:

• LED1 is permanently on (medium brightness, since I=118mA, which will give about 20 hour runtime)
• Once per second (must be adjustable by changing e.g. a resistor value), LED2 flashes, much brighter than LED1 thanks to its lower Vf.

I need a timer of sorts in the circuit, which connects LED2 in parallel to LED1 once per second.
Thanks in advance for your help.

 

[alpg88]

you can build one using 555 chip, and using different capacitors, and resistors, change timing of your flashes, and their duration.
there are tonnes of diagrams on the net, just google 555 flashing circuit.

 

[lctorana]

LM3909, if you can find one.

 

[Cemoi]

you can build one using 555 chip

This was my initial idea, but according to the datasheet:

• It requires a 4.5V minimum supply voltage. I read somewhere that it can handle as low as 3.6V, but I doubt it will work with 2.4V (2 NiMH).
• The maximum output current is 200 mA (although this can apparently be bypassed by adding a transistor).
• The output voltage is 1.7V below the input voltage, hence too low to operate my LEDs.

@lctorana: I could find a source for a LM3909, but very expensive.

 

[VidPro]

the 555 will run on about 2.5-2.6v minimum, and when you change the voltage from there, the speed changes slightly. (oops Corrected)

the 555 output is not enough for an high powered led, but it would drive a mosfet, or a relay. i have used a 555 to run 90watts of LED strobe

there is "logic" versions of the 555 , they use less power, not sure if they will work with enough less voltage.
most all microcontroller or curcuit board parts will want the 5v to run them, even if they arent driveing 5volts outputs :-(

you could set it up with a simple buzzer relay and capacitor , charge the capacitor, relay switches, discharges the capacitor, relay switches off. its a real kludge , and takes some messing with it to get the timing , probably not good for whole seconds though.

a transister vibrator curcuit, driving a relay, or a low voltage mosfet. that is probably the most efficent thing.

Anyvolt micro, is the cutest smallest little voltage thingee you have ever seen, it can be used to convert small currents from one voltage to another , in quite a WIDE range, not very efficient.
lowest input on it is 2.6 , it can be used to drive ONLY the 5v stuff to get the electronics working. then you can get a simple 555 going, and drive the mosfet gate with these low voltages. http://hobbywireless.com/cart/index.php?main_page=product_info&products_id=130

step up your battery to the 5v and halve out the led, have the 555 switch a relay from the 2 leds in seires, to 1 in series with a resister, then the one light does the pulsing, the other light kinda goes crasy to but stays on. half the time its 2xDD , the other half the time one is on resistered.

find cute little kit things like these http://www.electronickits.com/kit/complete/ligh/ck1006.htm that run on low voltage, and use the led output to instead drive a logic mosfet. http://astore.amazon.com/electronic-project-kit-20/detail/B0006HJSLY
http://www.elecfree.com/electronic/dual-led-flasher-by-2n2907/

 

[alpg88]

or you could just get iq switch for 2aa maglite it has 2 flashing modes, one of them looks a lot like a flash every second, i'll time it when i get home, and tell you exactly the interval.

 

[VidPro]

oops, i just checked one of my 555 things, by lowering the voltage till it stoped, and it went down to amazingly low levels, for a 5v speced part, and it was not the "logic" version of it i dont think.
it was still working at <2.5v , from a power supply, not a battery dwindling in power or being pulled down by driving a led.
that is weird, so i double checked it, and it cant be getting power from anywhere else.

 

[alpg88]

just timed iq switch, fast strobe is like.5 sec apart, slow is like 2.5 sec apart

 

[Light Sabre]

Takel a look at some of these flasher circuits, see if any will work for you:

led flasher - Google Search

 

[Illum]

This was my initial idea, but according to the datasheet:

• It requires a 4.5V minimum supply voltage. I read somewhere that it can handle as low as 3.6V, but I doubt it will work with 2.4V (2 NiMH).
• The maximum output current is 200 mA (although this can apparently be bypassed by adding a transistor).
• The output voltage is 1.7V below the input voltage, hence too low to operate my LEDs.

well....its good that your referencing the datasheet but then again its sometimes better to consult the application first
http://www.circuitstoday.com/lamp-flasher-using-ne-555

Note that at times you don't need dedicated timing ICs to get flashers to work. here's an example of a 1 amp capable flasher using the familiar LM317, the current setup its about 5Hz, [the frequency of the flashing depends on the value of resistors R1 to R3 and capacitors C2 to C4.]
http://www.circuitstoday.com/lamp-flasher-using-lm317

LM3909, if you can find one.

LM3909 was discontinued I think a year or two ago :candle:

I think Holtek once sold a 1Hz flasher that comes in TO-92 packaging for 5mm LEDs....but they appear to be discontinued as well.

For luxeons or higher your going to need a "power flasher circuit" to take the current, I dunno how much current 555s can eat before they start complaining

Here's a circuit that pulses 250ma from 3V at 0.5Hz...I'm not sure what you could do to bring the frequency to 1Hz
http://www.discovercircuits.com/DJ-Circuits/images/PowerLEDFlasher1.pdf

this might also hint some ideas
http://home.cogeco.ca/~rpaisley4/Flashers.html

 

[Mugrunty]

You could try the cmos version of the 555 timer. Example: LMC555. It is guaranteed to run down to 1.5v.

 

[Cemoi]

Thanks for your replies.
I should have mentioned that I'm rather noob at electronics: I'm able to read basic diagrams, but sentences like "step up your battery to the 5v and halve out the led, have the 555 switch a relay from the 2 leds in seires, to 1 in series with a resister, then the one light does the pulsing, the other light kinda goes crasy to but stays on. half the time its 2xDD , the other half the time one is on resistered" are difficult for me to translate into a diagram.

its sometimes better to consult the application first
http://www.circuitstoday.com/lamp-flasher-using-ne-555

This one is driven by 230V.

here's an example of a 1 amp capable flasher using the familiar LM317, the current setup its about 5Hz, [the frequency of the flashing depends on the value of resistors R1 to R3 and capacitors C2 to C4.]
http://www.circuitstoday.com/lamp-flasher-using-lm317

This one requires 12V. Does it work with only 2.4V, with what modifications?

Here's a circuit that pulses 250ma from 3V at 0.5Hz...I'm not sure what you could do to bring the frequency to 1Hz
http://www.discovercircuits.com/DJ-Circuits/images/PowerLEDFlasher1.pdf

5 transistors, 8 resistors, sorry, too complicated for me.

this might also hint some ideas
http://home.cogeco.ca/~rpaisley4/Flashers.html

The "basic flasher" is the circuit I have found for the 555, with the limitations mentioned in post #4.

Take a look at some of these flasher circuits, see if any will work for you:

Thanks, I will have a look.

You could try the cmos version of the 555 timer. Example: LMC555. It is guaranteed to run down to 1.5v.

OK, but the output current seems lower (150 mA?) than the basic 555, so too low to drive my power LED.

 

[VidPro]

before transisters even existed, a person could use a relay resister (or 2), and capacitor to basically do the same thing the 555 did.

the 555 itself charges/discharges a Cap and flips a circuit in one way or the other. in reality it doesnt DO timing at all the external components added to the flip flop did all the timing (its not a timer , its all been a cheap trick to sell a flip flop)

same thing you could do before, the flip flop being the relay, because the relay can move enough energy all by itself, it is the "simplest" possible curcuit that you can still use to do it.

that is what i was trying to say.
with the 555 you still need the resisters and the capacitor, and you still need the realay or mosfet, to get the power.

with the relay, you can put in just the same (basic) stuff on a relay and the relay itself is capable of driving the thing without any addditions. but then we all forgot that when digital took over and took 2x as much stuff to do things 1/2 as well