This is a super-simple circuit that you can use to extend the battery life of Christmas lighting strings. Ideal for garden use where it's more convenient than running cables.
It's basically a waterproof box (a Tupperware style box) containing a battery pack and a light sensor based on any general purpose small transistor you can find. I used a 2N7000 FET but you can also use a BC547, 2N3904 or whatever you can find. In this circuit it has to be an NPN or N-channel transistor.
You'll also need an LDR (Light Dependent Resistor) also sometimes called a Cadmium Sulfide photocell. You probably have a scrap nightlight with a cadmium sulfide cell in it already, just ripe for scavenging. In some instances it may actually be cheaper to buy a product with a photocell in it than just the component on its own!
The circuit is simple. A resistor is connected between positive and the transistors base, then the photocell is also connected from the base to the negative. When there's a modest amount of ambient light the photocell conducts and pulls the base down, keeping the transistor off. When it gets dark the photocell goes high resistance and the transistor turns on. You can adjust the turn-on point by varying the resistor value. When the transistor turns on it pulls the LED strings negative to the battery negative and the other terminal for the LED string is connected to positive via a resistor to limit the current. (10 to 100 ohms depending on how bright you want them.)
Battery life is generally very good with this arrangement. And the circuit is great for the long strings of "copper wire" LEDs where two lacquered copper wires have surface mount LEDs tacked along their length and are dipped in resin.
It's basically a waterproof box (a Tupperware style box) containing a battery pack and a light sensor based on any general purpose small transistor you can find. I used a 2N7000 FET but you can also use a BC547, 2N3904 or whatever you can find. In this circuit it has to be an NPN or N-channel transistor.
You'll also need an LDR (Light Dependent Resistor) also sometimes called a Cadmium Sulfide photocell. You probably have a scrap nightlight with a cadmium sulfide cell in it already, just ripe for scavenging. In some instances it may actually be cheaper to buy a product with a photocell in it than just the component on its own!
The circuit is simple. A resistor is connected between positive and the transistors base, then the photocell is also connected from the base to the negative. When there's a modest amount of ambient light the photocell conducts and pulls the base down, keeping the transistor off. When it gets dark the photocell goes high resistance and the transistor turns on. You can adjust the turn-on point by varying the resistor value. When the transistor turns on it pulls the LED strings negative to the battery negative and the other terminal for the LED string is connected to positive via a resistor to limit the current. (10 to 100 ohms depending on how bright you want them.)
Battery life is generally very good with this arrangement. And the circuit is great for the long strings of "copper wire" LEDs where two lacquered copper wires have surface mount LEDs tacked along their length and are dipped in resin.
So is that a breadboard you have rhe components connected to?
I really wish you could remake this using the components because I have no fing idea what you're drawing
I am going to change this , change the LDR for a temperature dependant resistor and put it in/on my fridge, to let me know my fridge is working properly.
Could you make a video you creating this project?
Am I correct to suppose this would not work with a 2x AA cell (3v) string, having insufficient voltage for resistors?
Having watched a ton of your other videos I have been impressed by the reverse engineering explanations you do. Sadly most of it goes straight over my head! This one though I understood, so now I’m on the lookout for more “my level” stuff that I can use to build things. I’m still going to enjoy being mystified and amazed and watching you blow things up and melt them! Great channel. Thanks
Brilliant little project guide… Cheers
I wish I understood this as I really want to make this work for some battery powered LED lights in my little library. It seems like you need a LDR, a transistor, and a resistor, however, in your drawing there seems to be two transistors, one 10-100 ohms and the other 10k-100k? Is that ohms? It would really help to actually see the parts and how they are assembled instead of a drawing for people who are unfamiliar with circuit drawings.
Can the circuit be used with a 12v DC power supply
As its wet n windy outside im tinkering in the garage. ive just made a set of these with my £1.50 set of lights from pound savers…3x AA batterys.. They come wired with a 20ohm resistor. So i changed that for a 39ohm i believe it was and a 150k for the other.. Seems to work ok. Had to use the 150k instead of the 100k because they didnt go out unless it was fairly bright.
I can see lots more of these being made…thanks for sharing clive..
Hi, will any mistakes on wiring can possibly destroy the copper LED lights? also, if i make 3 set of fairy lights, can the LDR handle it all?
I have done a LOT of your projects, and usually get all screwed up trying to convert from the BC547 to the 2N3904. Must be dislepsic or something but I came on a simple solution! It is now easier to just order the BC547's bulk off EBAY, they come just as quickly as the 3904's and there is no longer that pin out confusion for us old farts who's brains are no longer firing on all 8 cylinders (US Motorcars you know!) So now when a quick project comes up like these, I can just use the British version and all is once again right with the world, well until it comes to the damn 240v vs 120v again confusion reigns and i simply skip the project.
Was this before the purple pen ?
I'm wanting to install red LEDs on a elderly lady's gate tied into her solar powered gate opener with a 12v backup battery. I only want the lights to come on at night to make sure the battery doesn't die, will this work connecting to the 12v marine battery???
now watching again 2019. WouldClive comment upon waterproofness (or otherwise) of the led array?
Clive, I'm trying to figure out how to power LED christmas lights with a Bosch 12V power tool battery. The AA battery style lights have too short a run time (180 hrs?) and the power tool batteries I have plenty of. I want to power them for at least a couple weeks. The strings made for AA batteries seem to be smaller lights on shorter strings. I'd like typical strings of at least 50 or 100 lights and be able to run at least two strings. It's been awhile since my circuits class but I understand more than the average lay person. I'm not sure of the LED's specs. What voltage it requires to determine the resistance needed. I also liked the daylight sensor circuit in this video. Can this be incorporated? If I know the specs on the parts, I have no problem building it. I have some pine trees on a hill in some horse trails and thought it would be neat to put red and white lights out there so when riders go through with the snow on the ground, it would be kind of magical.Cheers.
Currently trying to build a similar circuit, I was trying to keep things simple, I like Mosfets, low resistance channels, and less power wastage, no moving parts to go wrong, however, I need to switch around 3-15 watts 12 volts from my 12-volt off-grid supply bank. I do not do lots of electronics, so I am not super clever with it. My test circuit uses a P-channel Mosfet an IRF9540N because I need to switch on the positive, high side of the circuit, however, I want it to function more like a proper dusk to dawn sensor, I am getting feedback issues with the circuit and also threshold point issues when it is not quite dark enough to fully trigger the Mosfet. What I really need is an Hysterisis adding, but I am not sure how to simply achieve this with resorting to more complex electronics. Any ideas anyone?
Or a 2N3055 transistor haha great cuicuit Clive