dim an LED with a photosensor?
- Thread starter docb
- Start date
mmmm.. I could suggest something off the top of my head, that would work and be relatively simple, def not ideal though...
Acquire a buckpuck with the control pins. This will run you around 15$, is a constant current device avaliable in 350, 700, and 1000mA versions. The control pins will allow you to dim the leds by applying resistance, or if you use one pin, voltage. Using voltage, 0v is around 95% dimmed, 5v is around 0-5% dimmed, or around 2kohms is 95% and 0ohms is 0-5% when using both pins...
This gives you practically unlimited control options.. There's probably a unlimited number of ways to do this...
A easy, cheap way would be to find a photoresistor (CDS cell) that changes from as low as resistance you can find, to atleast 2kohms. That would be pretty much plug and play... the tighter of a scale you can get of 0ohms-2kohms the better dimming and response you're going to get. I would be surprised if you couldn't find one that was pretty dang close to 0-2000 ohms... Talk to the buckpuck manufacturer if you go this route, I can't remember the best max value of resistance for dimming, I think it's closer to 2.5 or 3k...
You could also ditch the photoresistor or use an adequately specced photodiode or phototransistor to leech current into the control pin as the light dims...
Another way would be to use a voltage comparator or ADC to convert a voltage divider/photoresistor setup to 0-5v and drop it into the control pin. This way you could use any photoresistor you could find, and scale it correctly (0-xxxxxxx kohms = 0-5v)
You could also use a PIC or audrino REAL easy if you got the stuff to program em! If you decide on using a buckpuck, and you want more help, let me know.
Somebody with good circuit skills could easily design you a quick and simple FET based powersupply that could be dimmed with a photoresistor, you'd have to do alot of self soldering though.
Good luck! update us!
Acquire a buckpuck with the control pins. This will run you around 15$, is a constant current device avaliable in 350, 700, and 1000mA versions. The control pins will allow you to dim the leds by applying resistance, or if you use one pin, voltage. Using voltage, 0v is around 95% dimmed, 5v is around 0-5% dimmed, or around 2kohms is 95% and 0ohms is 0-5% when using both pins...
This gives you practically unlimited control options.. There's probably a unlimited number of ways to do this...
A easy, cheap way would be to find a photoresistor (CDS cell) that changes from as low as resistance you can find, to atleast 2kohms. That would be pretty much plug and play... the tighter of a scale you can get of 0ohms-2kohms the better dimming and response you're going to get. I would be surprised if you couldn't find one that was pretty dang close to 0-2000 ohms... Talk to the buckpuck manufacturer if you go this route, I can't remember the best max value of resistance for dimming, I think it's closer to 2.5 or 3k...
You could also ditch the photoresistor or use an adequately specced photodiode or phototransistor to leech current into the control pin as the light dims...
Another way would be to use a voltage comparator or ADC to convert a voltage divider/photoresistor setup to 0-5v and drop it into the control pin. This way you could use any photoresistor you could find, and scale it correctly (0-xxxxxxx kohms = 0-5v)
You could also use a PIC or audrino REAL easy if you got the stuff to program em! If you decide on using a buckpuck, and you want more help, let me know.
Somebody with good circuit skills could easily design you a quick and simple FET based powersupply that could be dimmed with a photoresistor, you'd have to do alot of self soldering though.
Good luck! update us!
Id like to do it with a photoresistor, but I can t find one that goes anywhere near down to 50 ohms.
The other suggestions you made seem ok, but too expensive. I want to do a few of these.
Anyone have any low cost solutions such as that FET based one you mention? I'm happy to build a small circuit if one is out there, but I cant find one online so far.
The other suggestions you made seem ok, but too expensive. I want to do a few of these.
Anyone have any low cost solutions such as that FET based one you mention? I'm happy to build a small circuit if one is out there, but I cant find one online so far.
uk_caver
Flashlight Enthusiast
Given ambient levels varying between daylight and darkness, what do you want the LED to do at various points on that scale?
For example, with declining light levels, do you want max brightness until the ambient drops below normal room lighting level, and then a fairly quick drop to a lower level, albeit with some kind of gradual dimming, or are you looking for a smooth decline over a wide range of lighting levels?
Do you want to be able to tweak the thresholds and degree of slope once you've built the circuit?
For example, with declining light levels, do you want max brightness until the ambient drops below normal room lighting level, and then a fairly quick drop to a lower level, albeit with some kind of gradual dimming, or are you looking for a smooth decline over a wide range of lighting levels?
Do you want to be able to tweak the thresholds and degree of slope once you've built the circuit?
I'm basically looking for just a switch.
When the PSR gets light, I want the LED to be normal brightness.
When the PSR gets covered, the LED should dim to a reduced brightness.
Would be nice to be able to tweak the switching point, and the reduced brightness level.
When the PSR gets light, I want the LED to be normal brightness.
When the PSR gets covered, the LED should dim to a reduced brightness.
Would be nice to be able to tweak the switching point, and the reduced brightness level.
I remember discussing this on these forums back in about 2002. Cavers have tried this sort of thing from time to time since before there were LEDs powerful enough to really use as lights. When you might be looking at a wall six inches from your face one second and down a 200 foot shaft the next it would be nice to let your lamp do some of the work instead of leaving it to your poor old eyes to cover that enormous dynamic range. Yes you can manually change brightness settings on your light but it gets tiresome.
I only got as far as using a photo-sensitive resistor on the feedback pin of a constant current regulating IC - in fact I had to use two of them in parallel, from memory. Worked OK on my workbench but not very well underground.
I only got as far as using a photo-sensitive resistor on the feedback pin of a constant current regulating IC - in fact I had to use two of them in parallel, from memory. Worked OK on my workbench but not very well underground.
Well, I'm still learning, and feel I don't have enough knowledge or experience to really create a circuit and confidently tell you it will work and be efficient.
There are simple plans all over the web for FET based ckts with high power leds. If it were me, i'd build one of those on mah breadboard, then just think about altering it to suit your needs. I'm sure most give a output depending on the voltage applied to the gate of the fet, a simple one might be a voltage divider, normally just a resistor between the gate and supply of appropriate value, and another going to ground... My way of custom buliding circuits at this point is K.I.S.S.. so to start i'd just pick up a multipack of various CDS cells from electronics goldmine, 2.50$ for 20 asst types, measure one's resistance in the sun on a hot sunny bright day, reduce the gate's resistor by aprx that much, and slap the cell inline with the gate resistor... If I didn't like the "switching points, or scale" I'd try another one, measuring it in light and doing the same thing... Once I found one I liked the scale of, I would measure the resistances in dark/light, and buy a few more individual cells in that region as well as resistors in the gate resistor's region, to tweak it until you find the magic resistance the gets you the response you want. If you can find the scale and response you like, you should be able to make several for a few mere dollars!
*edited because I provided a link to a fet ckt that would be alittle different to tinker with then a reg volt.divider type. Here's the link anyways, this design is smart but would take alittle more patience and you should be comfortable with transistors because you could blow something easier by messing with this one.. Didn't notice until I looked into it; http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/
^ doctaq had mentioned this the other day, if you havn't yet, check out instructables. Lots of amazing knowledge and led stuff there, as well as just about everything else in life from origami to car mods.
(sorry mods! i goin edit crazy!)
:thumbsup:
There are simple plans all over the web for FET based ckts with high power leds. If it were me, i'd build one of those on mah breadboard, then just think about altering it to suit your needs. I'm sure most give a output depending on the voltage applied to the gate of the fet, a simple one might be a voltage divider, normally just a resistor between the gate and supply of appropriate value, and another going to ground... My way of custom buliding circuits at this point is K.I.S.S.. so to start i'd just pick up a multipack of various CDS cells from electronics goldmine, 2.50$ for 20 asst types, measure one's resistance in the sun on a hot sunny bright day, reduce the gate's resistor by aprx that much, and slap the cell inline with the gate resistor... If I didn't like the "switching points, or scale" I'd try another one, measuring it in light and doing the same thing... Once I found one I liked the scale of, I would measure the resistances in dark/light, and buy a few more individual cells in that region as well as resistors in the gate resistor's region, to tweak it until you find the magic resistance the gets you the response you want. If you can find the scale and response you like, you should be able to make several for a few mere dollars!
*edited because I provided a link to a fet ckt that would be alittle different to tinker with then a reg volt.divider type. Here's the link anyways, this design is smart but would take alittle more patience and you should be comfortable with transistors because you could blow something easier by messing with this one.. Didn't notice until I looked into it; http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/
^ doctaq had mentioned this the other day, if you havn't yet, check out instructables. Lots of amazing knowledge and led stuff there, as well as just about everything else in life from origami to car mods.
(sorry mods! i goin edit crazy!)
:thumbsup:
Last edited:
I was thinking/talking about this is class today...
Honestly you almost HAVE to have "that friend" with an oscilloscope, or logic circuitry knowledge... Converting any given value (resistance, current, or voltage) to 0-5v is a snap for logic circuitry people.. And it's not expensive. The ADC's or voltage comparater's I mentioned (logic stuff) are really cheap, like 30 cents... Your main problem is converting the ambient light to a suitable scale... Like, a photoresistor at 1174kohms at dawn = ??? A phototransistor giving current of 15mA at sunrise = ???...
It's such a specific request that you have to have to knowledge yourself or of somebody else... or atleast a PIC to do the work for you.. If you've looked everywhere and can't find a high power version for the small LEDs, maybe you should link me to a version you found completely suitable, for the low power version... I could look at that, and in my spare time as well as mention it to classmates, maybe I could come up with a circuit that could "amplify" the output of the low power circuit to something suitable for 100mA leds.. I'm thinking just a emitter follower setup with adequately specced resistors and a power transistor feeding a fet gate after your provided circuit could probably work.
Honestly you almost HAVE to have "that friend" with an oscilloscope, or logic circuitry knowledge... Converting any given value (resistance, current, or voltage) to 0-5v is a snap for logic circuitry people.. And it's not expensive. The ADC's or voltage comparater's I mentioned (logic stuff) are really cheap, like 30 cents... Your main problem is converting the ambient light to a suitable scale... Like, a photoresistor at 1174kohms at dawn = ??? A phototransistor giving current of 15mA at sunrise = ???...
It's such a specific request that you have to have to knowledge yourself or of somebody else... or atleast a PIC to do the work for you.. If you've looked everywhere and can't find a high power version for the small LEDs, maybe you should link me to a version you found completely suitable, for the low power version... I could look at that, and in my spare time as well as mention it to classmates, maybe I could come up with a circuit that could "amplify" the output of the low power circuit to something suitable for 100mA leds.. I'm thinking just a emitter follower setup with adequately specced resistors and a power transistor feeding a fet gate after your provided circuit could probably work.
wyager
Flashlight Enthusiast
- Joined
- Feb 10, 2010
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Not expensive... just difficult. Sorry, but the fact is, unless you know your way around a microcontroller this kind of stuff is out of your scope. That's an important lesson about electronics it took me a while to learn.
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