Asuming that you find a LED to be visible at 1.5v ????. Asuming that the 1.5v is not a AAA or AA cells because they are not 1.5v it is possible to run an lm555 at minutes cycles and turn on the LED at not less then 95% of the minutes time. Asume 1 min then the on time will be 3 sec every minute.You might add battery life if you add a couple of farads across the battery. these caps are not expensive $2 each. The size is half the size of AA battery. So think before you proceed. Do you realy have a 1.5v source and a led of 1.5v

 

You're assuming too much, and most of it wrong. I designed the on/off timer for .1 sec on, .9 sec off. The LEDs in question are around 2.0V forward drop (for a total Vf of 6.0V), and will work fine with a 1.5V battery, though we are using 3V for brightness. This is because we are using a technique similar to a Joule Thief, which uses the inductor to create high voltage spikes that are harnessed by the LEDs. The only difference between "D" cells and "AAA" batteries is the lifespan of the charge.

 

Brand new name-brand AA alkaline cells have a rated capacity of 2500mAh but only if the battery discharges too low for the circuit. The capacity is about 1000mAh when the battery voltage drops from 3V down to 2V.
1000/24= 41.6mA per hour for a total of 24 hours.

The circuit steps up the voltage. Then the battery has much more current than the LEDs use.

The base design used 1.5V, which is below the 7555 rating, but worked fine. I suspect they are being a bit conservitive on chip specs (which is always good). Check the circuit description Wookie gave.

They claim 192 hour lifespan, 8 days continuous. That seems a little optomistic, but I figured I stretch it out by only having it on 10% of the time. I don't know if I have the wrong size capacitors, but my measured oscillation was around 18Khz, they said 222Khz.

 

Parts List
U1,U2 - Two 555s or one 556
Q1 - 2N2907A or equivanent
Q2 - 2N2222A or equivanent
D1,2,3 - Green LED
C1 - 1µF Capacitor
C2,4 - 0.01µF Capacitor
C3 - 470pF
R1 - 1.2MΩ
R2 - 150KΩ
R3,5,7 - 1KΩ
R4 - 1MΩ
R6 - 10KΩ
L1 - > 200µH Coil, High Q


Turns out I made a dumb parts error, C4 and C3 were reversed. Here are the values as they should be.

 

Bill....I'll rewire circuit in the morning and post results. Thanks for the heads up and fast reply. Just one more thing what's an OP? I suppose I've been called worse, but I've been trying to figure it out for a week:}

 

OP = Original Post

Used to get people back on topic, this thread is your's, you started it.

 

I made the circuit changes ( switched C3 and C4) on 1/6/09 at 9:00 am, it flashed until about 8:00 pm on 1/08/09. The flash stays steady, but the LED's showed signs of dimming yesterday, about 30 hours after start up. The original design boost 196 hours with the LED's on continulously. Go fish... maybe the magazine had to get published and they just threw out there to see if it would stick Any ideas on where to go from here, be kind remember I'm a girl.

 

R2 seems too large. I'd change it to 1k.

If you can, replace R1 with a total of 12 megohms of resistance, and reduce C1 to 0.1uF.

The 2nd 555 should be a triggered monostable by the 1st 555 rather than reset.

R5 is a very low value for being on the discharge pin. When the reset line is held low, the discharge pin will be connected to GND.

 

The second 555 generates a 222Khz signal, used to pump the inductor, a monostable will not work there. I see what you're trying to do, decrease current by any means possible, but I don't think 1.2MΩ to 12MΩ will make much difference to this circuit.

The second 555 could use some rework as you suggested. I lifted the design off the sheet you pointed out verbatum, but there are ways to reduce the current requirements.

R7 is 1KΩ, it could be boosted to a higher value.

I have a question about the coil, does it need a 50% duty cycle, 90%, or 10%? I think you need current flowing through it as long as you can to build up a charge. I wonder if it would be possible to use resonance to reduce the conductance from the battery to a minimum.

I still haven't finished mine, I'll try to do that this weekend. It would be good for a reference. I also have it breadboarded (separately) so we could try tweaking the values.

I was trying to make other variations of a flasher that may work. I'll have to try them and get back with you dogs2U.

 

Ok, try it this way instead
Cut down R2 to around 20k.

Then disconnect the power (pin 8) of the 2nd 555 timer, and connect it to pin 4! Also, disconnect R5 from the power rail, and connect it to pin 4. Basically, all of the current from the 2nd 555 will come from Q1.

In this particular case, removing C4 would be necessary.

 

Want me draw the changes for reference?

There is a RC charge curve, I'm wondering if we can't do something like a RL charge curve. The coil is high reactance at first, but goes down with time. Maybe we can find a good cutoff before it starts soaking the juice. What do you think?

20KΩ heh? that would reduce the flash duration from 100ms to 20ms, below the persistance of vision. Might work since they start so bright, but when they get a little dim it might be hard to see. Still, worth trying.

Personally I think C2 and C4 are pro forma anyhow.

 

Want me draw the changes for reference?

Sure, why not?

There is a RC charge curve, I'm wondering if we can't do something like a RL charge curve. The coil is high reactance at first, but goes down with time. Maybe we can find a good cutoff before it starts soaking the juice. What do you think?

Well, I'm assuming the fellow who wrote the article already did that. However, if the inductor is going into saturation earlier than expected, that would be a big drain of juice.

BTW, you've seen Ronald Dekker's flyback converter page, right?
http://www.dos4ever.com/flyback/flyback.html
The whole page is well worth reading, but 1 screen below here:
http://www.dos4ever.com/flyback/flyback.html#ind2
on the right; inductor saturation circled.

20KΩ heh? that would reduce the flash duration from 100ms to 20ms, below the persistance of vision. Might work since they start so bright, but when they get a little dim it might be hard to see. Still, worth trying.

Well, the idea was to cut down on the current drain. Reducing C1 and increasing R1 would help some; that power is just getting used up on resistors. The real power killer was holding the reset line low though.

Personally I think C2 and C4 are pro forma anyhow.

Well, they do help to keep the upper threshold level consistent for timing, after the initial charging period - but in a case like this, we'll be toggling power on the whole timer, and C4 wouldn't have time to charge. That would make the timer PRF very much higher than you'd calculated, as the threshold levels would start off nearly the same on each cycle. I'm too tired at the moment to figure out how long it would take to charge a 10nF cap through a 5k resistor.

Actually, I don't know what the value of the resistor divider network is in the CMOS version, and I haven't seen it specified in any of the CMOS 555 timer datasheets that I've downloaded.

OK... just found a note in the Maxim ICM7555 datasheet that basically says "...due to the comparators having such high impedance, capacitors are not required decoupling capacitors on the control voltage pins."
The BJT versions of the 555 have much lower input impedance on the comparators, which is why they need the decoupling caps.

 

C2 and C4 are just for filtering, as far as I know. I suspect that pin 5 (7555) is for the VCO functions more than the filtering. It'll take me a couple of hours for the redraw, so I'll get back with you.

 

OK, here are the revised drawings. It actually simplified them a little.

Parts List
U1,U2 - Two 555s or one 556
Q1 - 2N2907A or equivanent
Q2 - 2N2222A or equivanent
D1,2,3 - Green LED
C1 - 0.1µF Capacitor
C3 - 470pF
R1 - 12MΩ
R2 - 1.5MΩ
R3,5,6,7 - 10KΩ
R4 - 10MΩ
L1 - > 200µH Coil, High Q

I'm going to work with the original a bit first before giving up on it though. Whatta ya think, eliminate R4?

 

Built Bill's latest version of this circuit today and it's up and flashing. I will keep you posted on battery life. Also during this build I realized that I had used an LM556 (not a cmos part) so I corrected that problem. Also I used a different inductor with a high Q of 90. The first one I used had a Q factor of 20. Anyway time will tell. I've one more question.....I am currently using two AA for 3 volts, if I use 2 C size batteries (ugh) how much longer should the battery life be? If the AA batteries last two week how long would the C batteries last? Thanks in advance to all that have taken their time to respond

 

Can't answer outright, but the difference between AAA and C is huge. I'm guessing (a pure guess mind) that it will be 10X at least. Battery chemistry will also make a difference, even among alkalines there are significant differences. Something like lithium (too expensive for my tastes) would last even longer.

I'm surprised the 556 worked at all, given the low voltage you're feeding it.
Please let us know how it works out, I am curious.

 

The circuit is missing a supply bypass capacitor that MUST be used with a battery powered circuit. It holds up the supply voltage during the 400mA supply current spike from the output of an ordinary 555 (shoot-through current). Since your supply voltage is so low then the shoot-through current will also be low.

The continuous supply current of a 556 is 12mA max. The supply current of a Cmos 556 is almost nothing (400uA max) and it will keep working when the supply voltage drops to 1.5V.

I am surprised the 556 works with a supply as low as 3V. I bet it stops long before the battery has dropped to 2V.

AA alkaline batteries can supply 7mA for 14 days when the voltage of two in series drops to 2V.
C alkaline cells can supply 7mA for 33 days when the voltage of two in series drops to 2V.

 

It was CMOS that was specified, so the cap isn't needed. With a battery than is less than 3" away I don't usually bother with bypass capacitors either way, not with a single chip on the power supply.

I'm suprised it worked at that low a voltage too.

************************************

Opps, never defined a CMOS IC. DOH!

 

Day three and I'm still up and flashing. Also I'm back looking for absolute perfection. Here what I think I know.... If I attempt to use 6 green LED's I would have to forfeit some led brightness and perhaps lessen my battery life. Neither of which is a good option. The problem with sticking with 3 LED's is that they don't show up that good in the daytime. Currently I'm using a 5mm with a 2.2vf. I spent time looking at other LED's with more MCD, but the forward voltage on these are much greater, 3.3vf to 3.6vf. So would more battery power be in order?

 

Is that what you wound up using? I'm going to eventually update this one even more, looking at it there is considerable room for improvement. R1 is way too low for this application, for example.