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The cree LEDs are brightest, i'm told, when drawing 600-700mA. The voltage drop, i believe, is 3.3v per LED, and they are best wired in series to avoid thermal runaway (not sure why this happens, but it's been talked about on several forums i've seen, so i think it best to avoid it altogether) I would probably start with 2, but may use as many as 4 li-ion cells for this purpose.
The cree LEDs are brightest, i'm told, when drawing 600-700mA. The voltage drop, i believe, is 3.3v per LED, and they are best wired in series to avoid thermal runaway (not sure why this happens, but it's been talked about on several forums i've seen, so i think it best to avoid it altogether) I would probably start with 2, but may use as many as 4 li-ion cells for this purpose.


===Additional tasks===
==Additional tasks==
*reflector design - will likely try to use the reprap for this. are there software packages for this sort of thing?
*reflector design - will likely try to use the reprap for this. are there software packages for this sort of thing?


===questions===
==questions==


*What voltage regulator should i use?  
*What voltage regulator should i use?  
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* where is this circuit physically (might be no where at the moment, might be bits and pieces on a bike or on a shelf...). --[[User:Jstockford|Jstockford]] 11:21, 26 November 2008 (PST)
* where is this circuit physically (might be no where at the moment, might be bits and pieces on a bike or on a shelf...). --[[User:Jstockford|Jstockford]] 11:21, 26 November 2008 (PST)
== Rachel's automated lighting ==
The point of my system is that I don't have to remember to turn it off (or on).  The first rev will be battery powered; a later revision may take advantage of Rigel's work and connect to a dynamo.
Rev one will contain a couple bright white LEDs in front and some red LEDs behind.  The front will be steady, the rear blink, as is often seen on bicycles.  The base is a seat cover, in which is embedded a pressure switch such that the lights are only on when the seat is occupied (a later rev will change this to leave the lights on for 2 minutes after the seat becomes unoccupied).  An additional switch, to prevent unnecessary daylight use, is a photoresistor which will shut off power to the lights if it sees more than X amount of light.
=== parts ===
* 1x or 2x 555 timer chips
* 1x LM393 (op amp) analog comparator for voltages
* 1x photoresistor
* 1x pot
* various resistors & capacitors to produce the right 555 outputs
* 1x fabric pressure sensor
* fabric for seat cover
* batteries as needed
=== current state ===
I have the 555 circuit modelled for a nice rate of flash for the rear LEDs, using 3V.
The photoresistor registers about 140k ohms in inside natural light, held under the bike seat.  In the bathroom with the door mostly shut, simulating nighttime, it registers 22M.  Tentative initial ON value: 20Mohm.
Learned a bit of comparator chip circuit theory from David... Have the op amp circuit breadboarded out such that it turns on one LED when the photoresistor is occluded.  Circuit includes a pot for calibration, forming the second voltage divider.
Notes on my pots:
blue pot ranges from 0 ohms to about 2.35k ohms.  the pair of leads swap resistance levels compared to the single lead, such that the total resistance is always about 2.35k ohms.  When the pot is turned all the way to one side, resistance between the single lead and one of the doubles is 0, and from the single lead to the other of the doubles is 2.35.  When it's turned all the way to the other side, the resistances on the double leads, against the single lead, is swapped.
Have the 2 circuit bits combined with a transistor, fully functional.  Drew circuit diagram & breadboarded additional version from diagram to test diagram; made a couple small corrections.  Diagram is correct (possible resistor values s/b changed for optimality as NB supply hadn't anything smaller than 100 ohm).
=== next step ===
Big(?) capacitor behind seat switch s/b sufficient to keep lights on for some time after seat is vacated.  How big must it be?  Test.
Alternately, I have a scribbled diagram that uses a Schmitt trigger which is claimed to be usable for this purpose... research & poke at this more.
I can create the seat cover with embedded pressure switch whenever, it's just a switch, so it can wait until last.
[[Category:Projects]]
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