LED strip circuit - Page 1
Forum section: electronics / beginners

author: christos10
author_rank: Contributor
timestamp: March 11, 2014, 04:16:59 pm
content: hello,i found a circuit that makes a LED fade in and out..i want to replace the LED with a LED strip..the problem is that these led strips i have are at 12 volts...how can i make this circuit to operate in 12v so the 2 LEDs i will replace with the LED strips work at 12 volts?http://3.bp.blogspot.com/-8dh2sGOARuM/TqAbenpiOnI/AAAAAAAABEA/mZ70J74VUXI/s1600/fading+led.jpg

author: gregallenwarner
author_rank: Regular Contributor
timestamp: March 11, 2014, 04:25:54 pm
content: An N-Channel MOSFET on the low side of your LED strip will allow you to switch a high voltage circuit on and off using a low voltage signal at the gate.

author: christos10
author_rank: Contributor
timestamp: March 11, 2014, 04:28:10 pm
content: so i wont replase any transistors?

author: gregallenwarner
author_rank: Regular Contributor
timestamp: March 11, 2014, 04:29:42 pm
content: I assume you'd be replacing the NPN with the MOSFET, but as the IC in your schematic is unlabeled, I have no idea what it is and cannot therefore be certain. Is it fading the LED linearly or using PWM?

author: christos10
author_rank: Contributor
timestamp: March 11, 2014, 05:53:36 pm
content: fading..one more question if you can answer it..ill power it with 12 volts not 9volts.. remove the leds and resistors and put the led strips intead...will this work?

author: gregallenwarner
author_rank: Regular Contributor
timestamp: March 11, 2014, 06:40:48 pm
content: Quote from: christos10 on March 11, 2014, 05:53:36 pmFading via linear current regulation or via PWM? What is the IC in your schematic?A MOSFET acts as a simple switch. It has very low on resistance, so it won't contribute to much voltage drop in your LED strip circuit. As long as the LED strip is rated for 12 volts with its own built in current regulation, or if you provide adequate current regulation with a limiting resistor, this should work as expected. Note that it's common to use this method for PWM fading, so if your circuit was doing linear fading through current limiting, it'll be a bit trickier. PWM is more common for LED fading though.

author: christos10
author_rank: Contributor
timestamp: March 11, 2014, 06:56:55 pm
content: i dont know if its a fading or pwm..i found it on youtube..title said fading led..the curcuit i posted was, when one of the leds was fading in the other fade out,alternately like his vidbut with fade in and out..if its easier to make it in pwm then can you recomend me a circuit at google?im using 2 led strips(diferent color)

author: Torrentula
author_rank: Regular Contributor
timestamp: March 11, 2014, 06:59:26 pm
content: Code:What is the IC in your schematic?#Based on the familiar pin connections (2 shorted to 6, 4&8 are +9V) I think it's safe to assume that this is a 555 based astable multivibratorso fading is done using PWM.It's genrating a slow square wave to switch the transistors on and off. Because the circuit uses a PNP for the blue LED and an NPN for the red LED, the LEDs are alternating.To switch higher currents you may want to use P and N channel MOSFETs as already suggested. Make sure to calculate the correct drop resistor based on the 12V and the current your LED strip needs.Cheers,Elia

author: christos10
author_rank: Contributor
timestamp: March 11, 2014, 07:02:15 pm
content: Quote from: Torrentula on March 11, 2014, 06:59:26 pmthx for your help

author: djococaud
author_rank: Regular Contributor
timestamp: March 13, 2014, 01:13:52 pm
content: You can use this schematics...Nothing to program, juste standard opamps.Replace R6 with a potentiometer if you want to adjust the breathing speed...(you will want to simulate this circuit, I can't remember if the values are correct)This circuit simply fades in and out a led strip (in PWM mode)...

author: christos10
author_rank: Contributor
timestamp: March 13, 2014, 10:19:57 pm
content: Quote from: djococaud on March 13, 2014, 01:13:52 pmi can do that but its only for one led strip..the other circuit i posted was for 2 ,if i use a n-channel and p-channel mosfet it shoud work with hight current..i measured the current of each led strip and was around 500mA,or i can use for each led strip 2 transistors in darlington pair to get more current(not sure about this)

author: djococaud
author_rank: Regular Contributor
timestamp: March 17, 2014, 07:07:30 am
content: Sorry to be late to answer, I don't have the time to read the posts as often as I want...With my circuit, you can drive as much Led Strip as you want (assuming the behave the same), just take a mosfet that can handle the current...For simplicity, I would recommand to use a N-Channel Mosfet, on the low side (much simpler to drive).If you want to have different Strip behaviors, feel free to simulate the circuit and see what each section does (you can copy paste sections easily, they are quite independant).If you want your second led strip to behave the same as the first, but inverted, you can double the right section (U1, R9, M1) and swap the opamp inputs (the PWM will be inverted).Have a nice day, and btw, good luck !

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2014, 07:14:36 pm
content: The circuit is not using PWM. Using N-channel MOSFETs won't work because the turn on voltage is too high.The two transistors form emitter followers that buffer the voltage on the 220µF capacitor, which will be a sawtooth shape.The power can be boosted by adding a power transistor of the opposite polarity to each side, forming a Sziklai pair.http://en.wikipedia.org/wiki/Sziklai_pair