As all of the others had also inquired, I also was desirous of further precise information as to the end application of the LED. Initially, I might have surmised it to be some light reference source. Then as the tale unwound it seems to be merely used in an indicator function, with max consideration towards optimum conservation of its sourcing battery.
I think what I might throw at you for consideration is my K.I.S.S. circuit...and heavy on those S's....please. Let it be a criterion/benchmark for its performance vs utter simplicity comparison, should you proceed further towards the definitively more involved complexities of DC/DC converter utilization, u/p incorporation, adjunct light output detection and its introduction into a feedback controller loop for further PWM modulation of the sourced power fed on to the LED.
This is something that I built up waaaaaay back, circa '68, to fulfill a definite shortcoming being eternally confronted in common flashlights. That, of course, being its quite rapid, ever dimming performance; from a cluster of cells initial 1.5 VDC fresh threshold, on down to its 1.0 VDC level...and the resultant weak, yellow beam output down towards and below that level.
I retrofitted the flashlights stock lamp with a #49 lamp with its common voltage level of 2.0 VDC and its miniscule current requirement of only 60 ma. Yet, this still provided the close use, typical illumination level that I required, but with yet ever more stringent current demands than the typical ~ 500 ma conventional flashlight bulb behemoth. Then I could use a set of four cells placed within the confines of flashlight housings that were initially intended for the 6 VDC common ignition cell….. or later on… the actual availability of designed units that were actually made for accommodating 4 individual "D" cells. The 6 V battery with either its spring terminals or terminal stud configurations has now skyrocketed in its pricing (low demand).
For a final comparison of performance , had the conventional 5.95 VDC lamp at its .5A drain been used, the initial battery cluster's 30watt power capability would have been drained down to the 1 VDC per cell (yellow light onset threshold) with only a provision of ~3.5 hours of use and a final reserve/yet untappable, 20 watt power level being retained in the battery stack, at that point where the dimmed light output condition had onset.
What a waste!
With the utilization of my depicted circuit inline, from the battery to the lamp, a constant power level was sent to the lamp
all the way down to the point where the battery cluster had tapered to its 3 VDC level of output. Now, that battery pack gets bled fully all the way down, with only its final 1.5 watts of capacity left to be wasted !
Now I can pick up that flashlight and expect a constant white light performance for upwards of ~125 hours of use ! Problem solved.
Eventually, I even used a conventional stick style 5 "D" cell flashlight and incorporated the circuit housed within a top located dummy cell, its same size housing being made up from poly plumbing pipe.
I re-plotted its circuit for your referencing, wherein I merely scaled down the initial circuits designed power level from producing ~2.0 VDC @ 60 ma down to the more conservative 10-20-30 ma levels to be used for a LED....all in accordance to the heftiness of the particular LED(s) used, with the typical 20 ma medium power level being more of the norm.
WOW
…. that LED will just hang in there at a uniform output. Should you not have access to an adjustable power supply , it would be permissible to use a string of typical 9 VDC NEDA batteries; start with an aged unit that might be down to that depleted 6 VDC level and then start series polarized / top connector clipping units in order to progress on up to the higher voltage levels.
Now, in a repeat evaluation, should a LED have been otherwise solely utilizing a series c/l resistor, in your testing, somewhere approaching the increased voltage threshold of ~12VDC, along with the increased ramping up of its brilliance, that LED's emitter die should have reached a hyper-bright level along with a then abrupt emissivity cessation and its transformation into an intense black "fly speck"at its positioning within the locus of its reflective parabolic housing .
However, using the circuit in line, it is immediately receptive of any voltage/change within the previously specified ranges, with the LED merely holding at its initially established bright level.
Accordingly, with no discrete capacitive elements incorporated, it is receptive to quick turn on-off or supply voltage level transitions.
The selection of the ultra-reliability qualified 2N2905's was merely due to stock and familiarity with it in NASA/Aerospace/Military design and its companion lower level 2N2907 and their NPN counterparts. Here, it is loafing at 50% or less of its BVcbo and current/power capabilities specs. The selection of another equivalent electrical spec'd industrial/commercial/consumer grade unit is certainly permissible.
In your further experimentations, you might still want to incorporate the inline circuit, if solely for protection of the LED from damage.
73's de Edd
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Hey !...I just got to thinking……….Do radioactive cats have 18 half-lives?
<small>[ December 18, 2005, 08:13 PM: Message edited by: Edd Whatley ]</small>