Anatomy of a headlamp

Well, yesterday I finally bought this China made Head Lamp after been in the look out for one for over an year now. Cheap enough at Rs 180/- it has 5 ultra-bright LEDs. When the light went out at night I had a chance to try it. The light it threw gave me a thrill, it is quite a flood light! It gave my pet torch quite a beating.

As usual, being prudent (paranoid?) about safety i decided to give it a burn test. And so it light my table in the darkness for 20 minutes. Then, did i notice a dimming? The casing was getting quite warm. Is it the LEDs? but any component getting so warm is not liable to last much.

So I removed the cover to find the 3 x "AA" zinc-carbon batteries plenty warm. Which means the torch is consuming too much current & eating away the batteries. ummmm... so comes out my dusty multimeter of two decades. I had once burned its current circuit and patched it up, dunno how accurate it is now, but than we don't need to be exact in this case. It shows 300mA current drain! the batteries will be out dead in about 3-4 hrs. You can imagine the light fizzling out there in the middle of nowhere in the Ladakh heights & cold... One resistance on the board is also getting pretty hot, it surely is sinking more current than its size would make it capable of. i prefer to walk on safer grounds...

Anatomy :
Plus points :
1. the casing is quite compact & light.
2. the LEDs are what you want, the latest, ultra-bright LED. And five of them, when even one is quite good.
3. three push button setting, normal light, bright light & blink.

Minus points:

1. Consumes too much battery. For trekking point of view, I think i would want something that lasts me for 10 nights while i may be away from civilization. So about 10x7hrs= 70 hrs of battery life. (Is it asking too much? with the new LEDs around i wonder...) This one will last 4hrs maximum on normal "AA" battery. Magnesium batteries will last longer of course. Perhaps we will need to trade off luminance to save on battery.

It has three switch setting:
1. normal light : burns 300mA at full battery, down to 200mA after 30 minutes
2. bright light : not much change perceptibly, burns much over 300mA, the meter needle went of the charts!
3. blinker : it is good, consuming about 150mA.

2. The switch circuitry uses a Chinese COB (chip on board) design. Well we all know how reliable Chinese gadgets are! My MP3 player went dead in Sandakphu, and became alive again in Darjeeling. Problem is temperature range in which these circuits will operate. Cheap stuffs have their trade-offs. For a hardy thing I would opt for mechanical switches. There is hardly any need of an electronic circuit in a flash light except for the blinker. And then this COB itself will be consuming some of that precious battery power. We need to save on every bit.

To cure: uummmm... let's see... i need to check out the LED data sheets & get the optimum current/power settings... search for some switches which can fit into the compact casing.. will get back once i have it working again. I have started with putting the COB in deep freeze.. let's see how hardy it is...

And the correct price? Considering the Chinese costs, retail price anything over Rs50/- is stretching it... The only problem is the volume perhaps which is keeping the price high in local market.

Two Days latter :

Here is a true copy of the last dialog i had with the COB :

COB (ruefully): " you are not a nice person Sanjay".
Me (puzzled) : "huh?"
COB (sadness its voice): " My brothers & sisters have brought much joy in the life of other masters, but u were not happy"
COB : "first you dishonored me in public.."

Well, what can i say, Chinese stuffs have made a name for themselves. It is not for me to cover up.

COB : "then u froze me..."

well... all in the good cause of saving my dear life...

COB : " then u fried me"

that was a genuine mistake... I connected the batteries in reverse by mistake and it fried...

COB : " i can't think straight anymore... i behave erratically... i can't blink..."
COB : " earlier i was the master of 5 LEDs, who did my bidding. U chopped me off them."
COB : " i am no good any more. You have destroyed me. And i thought u were a good person when i first saw u..."

well what can i say... we got to do what we got to do... ummm... but kinda sad... may be i will give it a decent burial in the Ganges tomorrow...

Here is the results of reworking of internals. The measurements are for fresh batteries at 4.4v, at 4v the current drain cuts down to about half.


My temporary work bench : The flash light glowing away at full power before i decided to cut off 2 LEDs making it a 3LED headlamp instead of the original 5 LED.


The results below are with 3 LEDs instead of 5. The idea was to drive the LEDs closer to their optimum operating range which would be 20mA. This would give the best luminance efficiency & rated white light. The constraint of reducing LEDs comes due to our targeted battery efficiency. But i need to test & confirm. let's see...

Photos with the Flashlight kept at a distance of 2' from the wall :