Hello and good day everyone,
Firstly, I'm really new and have very basic electronic knowledge. Please excuse my newbie question.

I need to design and build a circuit for LED that can operates for at least 10 hours and adjustable brightness. This LED will be attached on helmet for caving application or other outdoor activities.

I've chosen Cree LED with forward voltage of 3.1 V @ 350 mA. My power supply is 4 AA which is 4.8 V. I've read some article that involves the usage of 555 timer but I don't quite understand it. What components do I need and if it's not too much, how the circuit will be connected to achieve at least 10 hours operating time and adjustable brightness.

Thanks in advance

 

There are DC-DC converters, also called LED drivers, that would be the best option here. High efficiency, and constant current output.

Take a look on e-bay for instance using those keywords and you'll find tons of cheap options. Look for one that meets your voltage in and out specification, can be adjusted to 350mA constant current, and is rated for 500mA or more. (You want some overhead to allow for any exaggeration of the specs, and in general it's not wise to run anything at full capacity for long periods.) You'll need to devise a way to limit current (and brightness) so that no matter how you turn your adjustment dial, you cannot blow the LED. Murphy's law guarantees that the knob WILL get turned to both extremes.

Hang on, I forgot to put two and two together. You said you need 10 hours at 350mA. That's 3500mAh and far exceeds the capacity of a AA, which might offer 2500mAh at 100mA current draw. Do you have any bigger battery options? Maybe rechargeable Li-Ion cells?

 

Unfortunately no, my option is just 4 AA batteries. I'm not sure if I give the correct spec of the LED. Here's the full spec sheet
http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Discrete-Directional/XLamp-XPE

If I use the DC-DC converters, do I need to use step up or step down and do I still need to place resistor? Because seems the current is already controlled by the converters.

 

The specs show things are even worse. That LED maxes out at 1A. So if you want to get close to maximum brightness, say 900mA, that's only a couple hours at best from your AAs.

There's just no way around the energy balance. That LED can use up to about 3 watts, so each hour at full brightness requires 3 watt-hours of energy. A 2500mAh AA battery puts out maybe 1.25V at 1A for two hours. Four in series deliver 5V at 1A, or a total of 5 watt-hours. Of course if you turn down the brightness you extend the battery life.

You won't need a resistor with the DC-DC converter, which will be strictly a buck or step-down converter. That's one way the converter improves efficiency, by eliminating the wasteful resistor. Many of the converters will provide both buck and boost capability, but you may save money choosing one that is buck only.

 

What about the 555 timer? I've read on some other thread that it can help in reducing the power usage by make it blinking over 30-40 blips per second that makes it look continuous.

What max current of the LED would u suggest to make it possible to operate for 10 hours? I still have option to choose other LED that suitable to use as a portable lamp as long it has reasonable brightness.

By the way, I've search that the average lumen for flashlight is around 20 lumens. From the spec sheet, the Cree LED has 307 lm maximum light output. So i guess around 10% from max current is enough to provide reasonable brightness? Do I understand it correctly?

 

A 555-based circuit can provide dimming and save power by virtue of the light being "off" more of the time. Brightness is roughly proportional to current, and thus inversely proportional to operating time. Half brightness last twice as long. Quarter brightness, four times longer. It's even a little better, because as you reduce the draw rate, the battery can come closer to delivering its rated capacity.

One problem with the 555 approach comes if you need to limit the peak current when the 555 turns the LED to on. You can control the average current by limiting the duty cycle of the PWM signal from the 555 to a maximum of, say, 80%. But if you need a resistor to limit the peak current when the LED is on during each cycle, you'll suffer the power waste from that.

 

I see. In other word, instead of reducing the brightness by limiting the voltage supply, 555-based circuit still reduce the brightness by reducing the "ON" time. And overall the operation time for the LED is still the same. Do I understand it correctly?

I've just edited my post and perhaps u didn't notice my other question.
I've search that the average lumen for flashlight is around 20 lumens. From the spec sheet, the Cree LED has 307 lm maximum light output. So i guess around 10% from max current is enough to provide reasonable brightness. Do I understand it correctly?

 

I've re-read all your comments, and I guess I'll go for the same LED but just use 20% of the max current of the LED which is around 200 mA.

My question now is on the DC-DC converter. Does it provide the feature to adjust the brightness? I've search on it and many recommend to use potentiometer to vary the resistance and thus changing the brightness. But as u said before, the resistor will cause me to suffer power waste. What is the best component option for the adjustable brightness feature?

 

Yes, I think most DC-DC converters with a constant-current mode will allow adjustment of the output current, and thus brightness. It will have a potentiometer to allow you to make this adjustment. Note that in this kind of circuit, the pot is adjusting a very small "information" current to control the output, not the main "power" current flowing through the LED. A pot in series with an LED – really in any "power" application – is a poor dimming solution and would be not be practical in your application.

If you want us to take a look at whatever you choose before you buy it, just post the link here.

 

After some search, I found this one here for the converter.

http://uk.farnell.com/texas-instrum...14983481469&CATCI=pla-77217964501&CAGPSPN=pla

Is it suitable for my application? I'm sorry again for my silly question, but I don't quite understand on how do I adjust the potentiometer in the converter. Will it come with a knob or something?

 

Unfortunately no, my option is just 4 AA batteries. I'm not sure if I give the correct spec of the LED. Here's the full spec sheet
http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Discrete-Directional/XLamp-XPE

If I use the DC-DC converters, do I need to use step up or step down and do I still need to place resistor? Because seems the current is already controlled by the converters.

The ideal is a buck boost converter so you can run the cells all the way down - but I think you'll need at least C cells to do what you're talking about.

 

The ideal is a buck boost converter so you can run the cells all the way down - but I think you'll need at least C cells to do what you're talking about.

I guess I'll try make it dim and see how it goes then. But for the converter, can u suggest the converter that u mentioned? I've tried to find but seems there's like so many of them. And will it have the option to adjust the brightness as well?

 

Here's the sample circuit that I found using Texas Instrument LED driver. Can I just use the circuit? How do I choose the capacitor, inductor and the diode?

 

Not even close, if you mean the traditional 9V "transistor" batteries. Lots more volts, but nowhere near as much power.

 

Here's the sample circuit that I found using Texas Instrument LED driver. Can I just use the circuit? How do I choose the capacitor, inductor and the diode?

That diagram shows many LEDs in series, whereas you want to light just one. But I have not investigated that chip. I highly suggest you exhaust any commercially available options before considering building something. A build will cost you more and take time, and is unlikely to have the features already available in cheap commercial products. It may be fun to build and I'm not trying to talk you out of that, if it is indeed fun. But if you just want to get done, buy something.

The buck-boost solution could allow you to drain the batteries completely but I assumed you wanted to use rechargeable batteries, right? Some cannot tolerate being completely drained. Nicads can. So it comes back to your battery choice.

There are a lot of things like this on e-bay. Another. Many require a 5V input, that first one says 4V but uses the same IC. It would be nice to find one with a lower input voltage requirement.

 

Not even close, if you mean the traditional 9V "transistor" batteries. Lots more volts, but nowhere near as much power.

You are absolutely correct. Based on the datasheets, they should go with atleast a 'D' cell battery.

 

If you use LM3410 then you still need the pwm signal, so I just drew the TLC555 and IRF540 to drive the LED.

 

Not even close, if you mean the traditional 9V "transistor" batteries. Lots more volts, but nowhere near as much power.

Did this thread get edited or moved or something? Or did I respond to the wrong thread?

 

Thank you very much guys.
Actually this is a part of my final year project, which somehow is quite out of my scope of study. I've only learnt the basic of electronics (which is very basic!) during school. So this kind of circuit and calculation really like an alien language to me.

If you use LM3410 then you still need the pwm signal, so I just drew the TLC555 and IRF540 to drive the LED.

If you need to calculate the frequency, here is - 555 astable multivibrator calculator.

View attachment 108274

Thanks a lot for taking your time designing the circuit. Really appreciate it. Still, I have one more silly question, what frequency do I need to make the LED illuminate continuously without blinking? From what I read, 555 timer makes the LED blinking, but by make it blinking so fast, human's eye won't be able to detect the blinking. What is the minimum frequency for that?

 

Thanks a lot for taking your time designing the circuit. Really appreciate it. Still, I have one more silly question, what frequency do I need to make the LED illuminate continuously without blinking? From what I read, 555 timer makes the LED blinking, but by make it blinking so fast, human's eye won't be able to detect the blinking. What is the minimum frequency for that?

Since this is the homework and I moved it to homework help forum, the homework can't give you the direct answer, but you didn't say that this is the homework, and the circuit is easy to get from internet, so I kept the circuit.

human's eye won't be able to detect the blinking.

You can google Persistence of vision and you can easily to get the answer, the others as old CRT used ?Hz, the movies used ? films/sec, city power AC110V/? Hz, AC220V/? Hz, the most important thing is you have to do the experiments and to try the frequency, it is not so hard, you just need to try it.

Whatever you try, using at least five times of the frequency that beyond the frequency with no blinking .