Power LED s simplest light with constant current

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Power LED's - simplest light with constant-current circuit
by dan on January 7, 2007
Table of Contents
intro: Power LED's - simplest light with constant-current circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
step 1: What you need . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
step 2: Specs & Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
step 3: wire the LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
step 4: Start building the circuit! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
step 5: keep building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
step 6: add a resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
step 7: add the other resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
step 8: permanant-ize it . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
step 9: finish the circuit! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
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Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
simplest-light-with-constant-current/
intro: Power LED's - simplest light with constant-current circuit
Here's a really simple and inexpensive ($1) LED driver circuit. The circuit is a "constant current source", which means that it keeps the LED brightness constant no matter
what power supply you use or surrounding environmental conditions you subject the LED's to.
Or to put in another way: "this is better than using a resistor". It's more consistent, more efficient, and more flexible. It's ideal for High-power LED's especially, and can be
used for any number and configuration of normal or high-power LED's with any type of power supply.
As a simple project, i've built the driver circuit and connected it to a high-power LED and a power-brick, making a plug-in light. Power LED's are now around $3, so this is
a very inexpensive project with many uses, and you can easily change it to use more LED's, batteries, etc.
i've got several other power-LED instructables too, check those out for other notes & ideas
step 1: What you need
Circuit parts (refer to the schematic diagram)
R1: approximately 100k-ohm resistor (such as: Yageo CFR-25JB series)

R3: current set resistor - see below
Q1: small NPN transistor (such as: Fairchild 2N5088BU)
Q2: large N-channel FET (such as: Fairchild FQP50N06L)
LED: power LED (such as: Luxeon 1-watt white star LXHL-MWEC)
Other parts:
power source: I used an old "wall wart" transformer, or you could use batteries. to power a single LED anything between 4 and 6 volts with enough current will be fine.
that's why this circuit is convenient! you can use a wide variety of power sources and it will always light up exactly the same.
heat sinks: here i'm building a simple light with no heatsink at all. that limits us to about 200mA LED current. for more current you need to put the LED and Q2 on a
heatsink (see my notes in other power-led instructables i've done).
prototyping-boards: i didn't use a proto-board initially, but i built a second one after on a proto-board, there's some photos of that at the end if you want to use a proto-
board.
selecting R3:
The circuit is a constant-current source, the value of R3 sets the current.
Calculations:
- LED current is set by R3, it is approximately equal to: 0.5 / R3
- R3 power: the power dissipated by the resistor is approximately: 0.25 / R3
I set the LED current to 225mA by using R3 of 2.2 ohms. R3 power is 0.1 watt, so a standard 1/4 watt resistor is fine.
where to get the parts:
all the parts except the LED's are available from , you can search for the part numbers given. the LED's are from Future electronics, their pricing
($3 per LED) is far better than anyone else currently.
simplest-light-with-constant-current/
Image Notes
1. not needed
2. R1
3. Q1
4. recycled 6-volt, 400mA transformer
5. Q2
6. Luxeon Star LED
7. R3
step 2: Specs & Function

Here i'll explain how the circuit works, and what the maximum limits are, you can skip this if you want.
Specifications:
input voltage: 2V to 18V
output voltage: up to 0.5V less than the input voltage (0.5V dropout)
current: 20 amps + with a large heatsink
Maximum limits:
the only real limit to the current source is Q2, and the power source used. Q2 acts as a variable resistor, stepping down the voltage from the power supply to match the
need of the LED's. so Q2 will need a heatsink if there is a high LED current or if the power source voltage is a lot higher than the LED string voltage. with a large heatsink,
this circuit can handle a LOT of power.
The Q2 transistor specified will work up to about 18V power supply. If you want more, look at my Instructable on LED circuits to see how the circuit needs to change.
With no heat sinks at all, Q2 can only dissipate about 1/2 watt before getting really hot - that's enough for a 200mA current with up to 3-volt difference between power
supply and LED.
Circuit function:
- Q2 is used as a variable resistor. Q2 starts out turned on by R1.
- Q1 is used as an over-current sensing switch, and R3 is the "sense resistor" or "set resistor" that triggers Q1 when too much current is flowing.
- The main current flow is through the LED's, through Q2, and through R3. When too much current flows through R3, Q1 will start to turn on, which starts turning off Q2.
Turning off Q2 reduces the current through the LED's and R3. So we've created a "feedback loop", which continuously tracks the current and keeps it exactly at the set
point at all times.
simplest-light-with-constant-current/
step 3: wire the LED
connect leads to the LED
step 4: Start building the circuit!
this circuit is so simple, i'm going to build it without a circuit board. i'll just connect the leads of the parts in mid-air! but you can use a small proto-board if you want (see
photos at the end for an example).
first, identify the pins on Q1 and Q2. laying the parts in front of you with the labels up and the pins down, pin 1 is on the left and pin 3 is on the right.
comparing to the schematic:
Q2:
G = pin 1
D = pin 2
S = pin 3

Q1:
E = pin 1
B = pin 2
C = pin 3
so: start by connecting the wire from the LED-negative to pin 2 of Q2
simplest-light-with-constant-current/
Image Notes
1. Q2: power NFET.
2. pin 3
3. pin 1
4. pin 2 of Q2, connected to LED-minus wire
5. the LED-plus wire
step 5: keep building
now we'll start connecting Q1.
first, glue Q1 upside-down to the front of Q2 so that it is easier to work with. this has the added benefit that if Q2 gets very hot, it will cause Q1 to reduce the current limit -
a safety feature!
- connect pin 3 of Q1 to pin 1 of Q2.
- connect pin 2 of Q1 to pin 3 of Q2.
Image Notes
1. pin 3 of Q1 to pin 1 of Q2
2. pin 1 of Q1
Image Notes
1. connect pin 2 of Q1 to pin 3 of Q2
simplest-light-with-constant-current/
step 6: add a resistor
- solder resistor one leg of resistor R1 to that dangling LED-plus wire
- solder the other leg of R1 to pin 1 of Q2.
- attach the positive wire from the battery or power source to the LED-plus wire. it probably would have been easier to do that first actually.
Image Notes
1. LED-plus wire

2. R1: 100k-ohm
Image Notes
1. connect the positive wire from power source
step 7: add the other resistor
- glue R3 to the side of Q2 so it stays in place.
- connect one lead of R3 to pin 3 of Q2
- connect the other lead of R3 to pin 1 of Q1
simplest-light-with-constant-current/
Image Notes
1. connect R3 lead to pin 3 of Q2. wrap lead around for easy soldering
2. R3, 2.2-ohm glued to side of Q2
Image Notes
1. other lead of R3 to pin 1 of Q1
step 8: permanant-ize it
now test the circuit by applying power. assuming it works, we just need to make it durable. an easy way is to put a large blob of silicone glue all over the circuit. this will
make it mechanically strong and waterproof. just glob on the silicone, and make an effort to get rid of any air bubbles. i call this method: "BLOB-TRONICS". it doen't look
like much, but it works really well and is cheap and easy.
also, tying the two wires together helps reduce strain on the wires also.
i've also added a photo of the same circuit, but on a proto-board (this one is "Capital US-1008", available at digikey), and with a 0.47-ohm R3.
Image Notes
1. BLOB-TRONICS!
2. strain-reduction on the wires
simplest-light-with-constant-current/
step 9: finish the circuit!
now connect the negative wire from the power source to pin 1 of Q1.
you're done! we'll make it less flimsy in the next step.
Image Notes
1. negative from power source to pin 1 of Q1
simplest-light-with-constant-current/
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simplest-light-with-constant-current/
Comments
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Artificial Intelligence says:
Mar 19, 2008. 9:59 AM REPLY
Very cool, but not the simplest. Watch this /> dan says:
Mar 19, 2008. 11:03 AM REPLY
thanks, i covered your circuit here:
/>the problem with it is that using an LM317 it has a high dropout (3.5v) so your efficiency is poor unless you are using many leds in series. you can do a
little better with a different regulator model.
ewitte says:
Aug 30, 2008. 5:23 PM REPLY
Would this be good to run with 6 at 12v with a .33ohm resistor? That gives 1.515A with it dropping to .7575 running series two sets of 3. Some are
3W and some are 5W but I decided to run them all between .7 and .8ma because I have 50+ to power in all ;)
ewitte says:
Oct 2, 2008. 3:45 AM REPLY
I've got 16 on this now working my way up to the full 56. A tip I have for keeping the efficiency up. Usually what I do is use 3.6v for blue and 2.4-
2.5 for red and calculate without the 0.5v of the circuit. When I'm finished its usually barely warm if even that. I'll throw an extra 1v on to see if it
brings the light up, if it does I stay if not go back. Usually I'm working at 15-18v.
Artificial Intelligence says:
Mar 19, 2008. 12:06 PM REPLY
But I'm from Denmark and the LD1585CV or LM1084IT-ADJ is not available here.
Gopher says:
Jul 26, 2008. 3:11 PM REPLY
OK, I need a circuit to power a 3W luxeon LED that is powered by 4AA
batteries, which is easy enough in itself, however I would like to be able
to have the LED at quite a high frequency (5 - 10pulses per second) and

preferably with a constant current (thought the last bit isnt essential)
I cant give specifics of the Luxeons as I intend to use it on a range of models/makes etc so something to cover most bases will be great
does anyone have a diagram of such a circuit or an idea where to finf one etc
samphantom says:
Jun 23, 2008. 5:43 AM REPLY
Hello every one.
Hello Dan, I like your circuit.
I have a Led @1W from digikey part # 475-2587-1-ND.
I implemented a driver with an LM317 an couple resistors, the LM317 has heat sink, the problem is the LED needs 1.4A to give the tolerance brightness with
no problem @3.5V.
This LED needs also a heat sink to avoid overburn, I put a 0.27R resistor @1/2W with no problem but the leds still need a little fan to avoid over heating.
Could you help me to make a circuit for this specific LED avoiding heatsinks? I armed the circuit because I saw it on internet but yours seems so simple.
Thank you beforehand.
1up says:
Apr 13, 2008. 1:21 PM REPLY
This looks nice! I might use this sometime.
Could this circuit also be used as a driver for a laser diode?
raygromer says:
Apr 1, 2008. 4:29 PM REPLY
Great one Dan. I've used it successfully on a couple projects already.
But it won't work on the project I'm working on now because my LED cathode legs must connect directly to ground (battery negative).
Is it possible to change the polarity of your circuit so the LED anode side is "regulated" and the LED cathodes connect directly to ground?
Thanks!
krunal_299 says:
Apr 1, 2008. 3:03 AM REPLY
Hi!
My requirement is for 350ma LED and Vf=3.5v.
So does it require Heat sink in this ckt.?
Thanks & regards!
simplest-light-with-constant-current/

johnpombrio says:
Mar 30, 2008. 7:26 PM REPLY
Yep, NO REASON AT ALL to make this anymore. a Constant current source for use in flashlight making are dirt cheap and work great. I have 20 of them
that have 5 functions, work off of 2-3 batteries and cost all of Three dollars a piece:
/> bilti says:
Mar 22, 2008. 10:39 PM REPLY
Congratulation Dan for you contribution!
I am very interested in your work and I`m wondering how can this idea can be adapted to 220V (Europe). I want to connect few power LEDs. directly to the
220v power . Any suggestion?
Coffeebot says:
Jul 6, 2007. 8:09 PM REPLY
I'm looking at building a lamp for my living room using this technique (possibly the RGB adjustable one, if my wife lets me
_
).
The lumileds are rated at about 80 lumens (for the whites), and the RGBs are 30-40 (give or take). An average incandescent bulb is about 1000 lumens.
My question is whether or not the intensity is additive. That is, would I honestly need more than 10 luxeon 3s to come close to a standard 75 watt bulb?
Surely I'm missing or misunderstanding something here.
computerwiz_222 says:
Mar 19, 2008. 7:51 AM REPLY
lumens are lumens. the lumen is a measure of light output. There is no different scale for LEDs and light bulbs. So yes, you would need over 10 to reach
the output of a light bulb.
Just think though, the, the luxeons should never burn out.
Coffeebot says:
Mar 19, 2008. 8:32 AM REPLY
Well, I guess it's not so much the fact that I need 10 LEDs, as it is the amount of heatsink they would need in order to not burn out.
I suppose it could be done with a torch lamp, given the size of the average "bowl" on top. But, sadly, we threw all of ours out after the last move.
Damian7 says:
Sep 29, 2007. 5:13 PM REPLY
I'm working on a project that will used LEDs and will be powered by a 120v AC input 12v DC output adapter. You can't powered directly LEDs to 120v
AC, since they work with DC. As soon as I finished my project I will post it. My idea is to replace regular bulbs where they can fit with the adapter.

computerwiz_222 says:
Mar 19, 2008. 12:35 PM REPLY
they can be run off of AC. The LED will only run at a 50 percent duty cycle. The LED will have a 60 hz flicker. Luckily, the power companies designed
electrical systems to alternate faster than our eyes can see, so you should have no issue.
Do some research, it is never safe to connect directly into 120VAC.
Coffeebot says:
Oct 1, 2007. 7:11 AM REPLY
I'm curious to see what you come up with. I've been thinking of doing something like that myself.
They already have some LED bulbs like this (thinkgeek.com had them they don't anymore), but I'm skeptical as to how bright they are (note Dan's
comment below, and my earlier thoughts on the subject above).
I would ultimately like to have them lighting up my office in "futuristic" light bars across the top of the walls but that would be pricey, and probably
kick out a fair amount of heat.
dan says:
Aug 16, 2007. 4:24 PM REPLY
no, you are not misunderstanding. that's why you are not seeing LED lightbulbs at home depot yet.
johnpombrio says:
Mar 30, 2008. 7:13 PM REPLY
Closer than you think, now (1 1/2 years later)! Home Depot has LED stickies (3 LEDS using 3 AAA batteries), 7 LED light Pucks on a transformer
(similar to the Halogen pucks). IKEA DOES have LED small reading lamps which I use at night when others are asleep.
Richard21 says:
Feb 13, 2008. 11:17 AM REPLY
I would really like to make this project, as the LM317 has too much overheads. Problem is searching for Fairchild 2N5088BU returns no results in
Google.co.uk Buying from Digikey makes the whole project too expensive with European postages :-( Does andy one know where to get these parts or
equivalent in the UK
Thanks and hoping. Richard
simplest-light-with-constant-current/
callmeshane says:
Jan 31, 2008. 12:23 AM REPLY
Uhhhh N-Fet's, F-Net's, Fet-Net's and Nut-Fat's - does anyone ever use F-it's?
I am trying to be funny, and overcome my default setting at retarded, mentally defective and lazy

Actually this device is pretty much what I want to make up, to run a similar system.
Only I like to tell me self, (whining tone of voice, tears in my eyes, professional victim to the max) "Oh it's too hard, my brain hurts, I am too busy etc "
When all I really need to do is to ge a pencil and possibly a really big sheet of paper and do a few diagrams, a few sums and sort of hope to figure it out, and
when I get REALLY stuck to then look for more clarity and or ask some people some questions
However being a sniveling victim is easier, because I can blame them for my not making it work.
Anway I shall start machining the aluminium billet for my head lamp case 2 knight
Cheers
Shane.
surftom says:
Jan 17, 2008. 12:29 PM REPLY
Just a comment/Question your Text says an N-FET but your schematic is a P-FET. I assume you really want an N-FET as the DigiKey part number is in fact
an N-FET
Tom
dan says:
Jan 17, 2008. 1:25 PM REPLY
yes, NFET
rmccurra says:
Dec 6, 2007. 5:25 AM REPLY
Dan,
Is there a way to use the simple circuit with a number of LEDs in parallel? I want to keep my voltage down so I can power with a homemade alternator where
the input RPM could fluctuate (and therefore voltage). Voltage would most likely run up from zero and hover around target of 6V but could spike if RPM
momentarily increased. I would like to use 6 bright white leds (3.4V and 6*.02A=.12A). Is my only option to put a resistor next to each one and make the
circuit less efficient? Is there a better way than the simple circuit for this application?
Thanks
dan says:
Dec 6, 2007. 12:16 PM REPLY
you can do this if:
(1) all your parallel leds are the same
(2) they are all mounted near each other, so they will have the same temperature when operating.
rmccurra says:

Dec 7, 2007. 12:28 PM REPLY
Just to make sure I understand, you are saying no resistor is ok with the conditions you mention? Would it add any safety factor to put like a 1ohm
resistor in series with each led?
Thanks!!!
dan says:
Dec 7, 2007. 1:45 PM REPLY
correct. if you follow the above notes, i would not bother with resistors unless this is a commercial product you are making. yes - adding a resistor
will improve the safety factor, target maybe a 0.2v drop on the resistor, so that's more like 10ohm in this case.
maroneyi says:
Oct 29, 2007. 5:17 PM REPLY
I built this but realized it is not supplying enough current (even removed both resistors) I have it hooked up to two of those high power LEDs. Putting 4.5 V at
700 mA through it.
What can I do to increase brightness?
mori says:
Oct 21, 2007. 5:29 PM REPLY
Hi.
I'm planning to wire up this circuit as an addon for this product; to produce an uber alarm clock.
/>With a 460nm star LED; which according to this:
/>is the wavelength of light responsible for stopping production of melatonin.
However as this "simulates" dawn by increasing and decreasing the voltage to the plugged in devices, thereby increasing and decreasing the luminosity of
the light. I'm not quite sure if it does this by varing the current or the voltage of the devices, but from what you're saying this device is either ON or OFF? re:
"it keeps the LED brightness constant no matter what power supply you use or surrounding environmental conditions you subject the LED's to".
So is there anyway of modifying the luminosity of the LED via the supply voltage? or is it one brightness solves all?
simplest-light-with-constant-current/
mori says:
Oct 21, 2007. 6:02 PM REPLY
ooo Already answered i think by a different thread.
/> dan says:
Oct 24, 2007. 11:05 PM REPLY
see my 'power led circuits' instructable, it has a couple methods of converting this circuit for dimming.

midlander says:
Oct 15, 2007. 12:29 PM REPLY
Hi, I wanted to use this circuit to drive 1W LED (3,35 V/350 mA) from 4 AA NiMH cells. You declared 0,5 V dropout voltage, however, from 4,9 V on the
battery (circuit switched on) I can get only 3,3 V (330 mA) on the diode. With 6 cells it's fine, so there must be higher dropout for some reason. As I am from
Czech Rep. and could not get the transistors used by you I bought BC548C (common NPN) and IRFZ46N (as MOSFET) which should have similar
parameters. But I'm not an expert, so maybe I missed something important in the specification. I use 100k and ~1,47ohm resistors. With 6 cells I got 354 mA
and 3,34 V on the diode, with 4 cells I got only 3,29 and 325 mA even if I used 1,22 ohm instead of 1,47. Do you have some idea what's the reason for it?
Thanks a lot.
midlander says:
Oct 21, 2007. 12:12 AM REPLY
Shame on me! It was the bad probe wire! Its resistance was causing additional 1.2 V dropout while measuring the current with the multimeter. :o)))
But I have another question: the current is still dependent on the source voltage. Well, not much, but it is. E.g. if I increase the source voltage from 5.2 to
7.8 the flowing current rises from 351 to 370 mA. Is there some kind of formula that can express this dependence?
Jamshed says:
Jul 11, 2007. 12:24 PM REPLY
Hi Dan,
In step 2, you mentioned turning off Q2 reduces the current through the LED's and R3. Assuming Q2, is turned off, where you will trigger the base of Q1
from.
And you have mentioned Q2 is acting as a variable resistor, so if 1 of our led gets short, then the LED Voltage drop will be across Q2?
dan says:
Aug 16, 2007. 4:19 PM REPLY
1) as Q2 turns off, voltage at base of Q1 goes up, turning it on. this is a feedback system.
2) yes, if a led shorts out then the voltage drop will be across Q2.
Jamshed says:
Oct 17, 2007. 10:03 AM REPLY
Hello Dan,
I mean to ask, if an led gets open then there will be no voltage drop across the NFET Q2. So, this means Q2 is off. Now with Q2 off(because of open
led), how the base of Q1 is going to be triggered?
trons says:
Oct 16, 2007. 7:39 AM REPLY

Could I hook up 2 Luxeon I's in parallel to this circuit? Or can you only safely go parallel with strings of minimum 2 series LEDs? I was thinking of using an
mATX PSU's 5V line to drive the Luxeons in pairs, each sharing a driver with 700 mA output. (I have 12 LEDs, 8 are from the same batch 0473468, the other
4 are batch 0473467) Thanks in advance.
Tomteslakt says:
Sep 7, 2007. 3:30 AM REPLY
Cool! If this circuit is used in a flashlight with a Luxeon K2 LXK2-PW14-V00 LED running at 1000-1500 mA powered by batteries (3 C-cells?), are there any
modifications needed? This is a bit more amps than the 200mA mentioned in the instructable And what is most important to keep the voltage drop of the
circuit as low as possible?
Tjp says:
Sep 3, 2007. 6:38 PM REPLY
Cool stuff. I am looking for a way to have a 12 Volt + signal turn off the LED supplied by the 12 V+ supply (nominal automotive supply actually around 13.8
but can be 18VDC in normal operation and as high as 80V in extremes.) Basic concept is to turn one constant current LED source on with +12 VDC and
then turn on an alternate constant current source for a different color LED array while turning off the first one It gets a bit more complex in that the first one
needs to have a 30% brightness and 100% brightness but that's another issue maybe. Maybe something that shunts the FET gate to ground when positive
bias is applied?
BlessedWrath says:
Sep 3, 2007. 9:14 AM REPLY
Can this circuit be used as a driver circuit for laser diodes? I assume so, but wanted to be sure.
Is there any reference material I can download to begin to understand why these components are necessary, and what is happening with the physics of the
electricity?
simplest-light-with-constant-current/
technomorph says:
Aug 23, 2007. 3:59 PM REPLY
Hey Dan,
Love it,
How would you add more LEDs to the circuit.
In series? In Parrallel?
Is there anything else that needs to be modified?
thanks
evox says:

Jul 10, 2007. 2:41 AM REPLY
Hi Dan i realize this is a stupid question and i am very new to this stuff. but when i build your circuit i get the same voltage on the led end of it (basically what
ever the input voltage is the output voltage is the same) i was just wondering if this is what is supposed to happen. i am trying to power up 1w leds. if anyone
knows what i am doing wrong they can email me at thanks for the help guys.
dan says:
Aug 16, 2007. 4:23 PM REPLY
how are you measuring? with no leds connected, you will see approximately the full input voltage at the output. you must connect leds to see the reduced
voltage. also, measure the output voltage across the leds only.
johnfischer06 says:
Jul 8, 2007. 7:26 AM REPLY
Two questions:
1) Why is R1 100k ohms? What would happen id you were to change it?
2) When deciding the value of R3, why did you divide .5/R3? What is the .5 in this equation?
dan says:
Aug 16, 2007. 4:17 PM REPLY
1) as noted it is 'approximately 100k ohms' - anything from about 20k to 500k will work fine. what it does is turn on Q2 in a 'soft' way so that Q1 can over-
ride it to turn off Q2.
2) 0.5 is the turn-on voltage for Q1. in reality it is somewhere between 0.4 and 0.6 depending on the Q1 type used, and the air temperature. the specified
Q1 model was chosen because it is closer to 0.5 over a wide range of temperatures than most other models.
KT Gadget says:
Jul 6, 2007. 12:33 AM REPLY
hey dan. im wondering if a 9V battery will work. will the resistor be able to take 9V and lower the voltage to the appropriate voltage?
dan says:
Aug 16, 2007. 4:08 PM REPLY
yes, that's the point of this circuit
drips says:
Jun 30, 2007. 2:46 AM REPLY
So easy a primate can do it! Thank you dan for all of your LED Instructables. I built my first one already and it was a-ok. I was checking out some other LED
Instructables and found this which uses a large capacitor before the LEDs to get a fade-out effect. Would this be possible with your circuit and if so any
recommendations for the caps? I would love to have that nice blue light do a soothing fade as it drifts off into sleep

altaria1993 says:
Jul 3, 2007. 2:21 AM REPLY
all the parts just should be a bit bigger to hold :P
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