LED Dimmer - Page 1
Forum section: electronics / projects

author: jawa_master
author_rank: Contributor
timestamp: March 14, 2015, 10:14:40 am
content: Hi.recently i got myself a computer with one of them fancy side windows, and now i'm going to pimp it up.I have bought a three color led strip. (the inputs are: {12 v} {red} {blue} {green}i thought it would be cool to have a front-panel with three potentiometers, controlling each color to get any color i want.but as you probably know, dimming a LED light is NOT a piece of cakecould some of you be as kind as to draw a circuit for dimming the three color strip?

author: void_error
author_rank: Frequent Contributor
timestamp: March 14, 2015, 10:46:14 am
content: Dimming a LED strip is easy. Two ideas come to mind: 1. TL494-based dimmer 2. Opamp based dimmer. The second one occupies less space but works at a lower frequency. Give me some time and I'll come up with something.It'll be based onthis. 5 opamps will be required, or 3 dual opamps.

author: SeanB
author_rank: Super Contributor
timestamp: March 14, 2015, 11:06:38 am
content: Bigclive.com has some ready made RGB colour control kits he sells.http://www.bigclive.com/knob.htmThis is all the details you need to roll your own, or just buy the kit from him shipped to wherever that black flag country you live in is.

author: Psi
author_rank: Super Contributor
timestamp: March 14, 2015, 11:24:42 am
content: how much current do you need to switch?3x LM317 adjustable vregs with pot control could work if the current is under an amp but you will want to put them on a heatsink.

author: ajb
author_rank: Super Contributor
timestamp: March 14, 2015, 02:00:49 pm
content: Simplest solution would be a microcontroller. Read the pots with the ADC, and dim the LEDs via PWM. In arduino this would be three lines of code. You'll need three MOSFETs sized for the current required by your LED tape with logic level gat thresholds (which isn't hard with a 5V micro).Once you have an mcu in the mix, if you want to get fancy you've got the option of doing presets or automatic fades, or anything, really.

author: jawa_master
author_rank: Contributor
timestamp: March 14, 2015, 02:03:07 pm
content: I would like a circuit that didn't require me to go shopping.ergo: no rare, specific or weird parts.is it possible to do this with a regular potentiometer and a 555?

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2015, 02:22:02 pm
content: Quote from: void_error on March 14, 2015, 10:46:14 amThat will work very well. The integrator is better being an op-amp and the rest comparators.If linearity isn't important the integrator can be omitted and only four comparators are needed.

author: DanielS
author_rank: Frequent Contributor
timestamp: March 14, 2015, 02:47:01 pm
content: Quote from: ajb on March 14, 2015, 02:00:49 pmA microcontroller sounds awfully complicated for low-power LED dimming, even more so if you are reading external pots. The microcontroller option becomes more interesting if you want to do fancy stuff like non-static colors and brightness.For basic dimming, I would simply use a resistor in series with the LED strips to limit absolute maximum current, a 2N3904, a 2k pot with end terminals across C-E and the base connected to the pot's wiper terminal. It doesn't get much simpler than that.

author: ajb
author_rank: Super Contributor
timestamp: March 14, 2015, 03:34:53 pm
content: LED strips will already have current limiting resistors built in, and since the required current varies with the length of the strip, current control isn't really ideal. Especially with RGB tape, you wind up getting different dimming curves due to the lower Vf of the red LEDs as compared to the green and blue. PWM is really the better way to go.Of course, since the OP doesn't want to have to buy anything, all of this is moot without knowing what the OP has on hand.You could certainly do this with a 555 timer. You'll still need a suitable drive transistor. Googling "555 PWM driver" brings up a fair number of useful results.

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2015, 06:18:07 pm
content: If you want to go for an MCU the you might as well interface it with your PC via USB or RS323 and write some software to control it from the computer.What parts do you have? Have you got a quad comparator such as the LM339?Here's what I was taking about before. I know it has duplicate designators. I copied it from a single channel circuit and couldn't be bothered to renumber it.

author: Cloud
author_rank: Regular Contributor
timestamp: March 14, 2015, 06:49:49 pm
content: For dimming led strips I am using the dimmer with 555 from EEVblog video but instead of a power PNP transistor I am using BC557 where collector is on +12V and emitter goes to a gate of N-Mosfets. If you want I can send you schematics and PCBs

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 14, 2015, 09:52:25 pm
content: Here's a schematic that uses a 555 timer. It was taken pretty much as-is from Colin Mitchell'sTalking Electronicswebsite. The only difference is the use of a MOSFET to drive the load. You probably won't need something as beefy as an IRF510 but you get the idea. (An N-channel MOSFET in a TO-92 case should work.)You would need three of these--one for each color.The only real problem with the circuit is that it can't get to either 0% or 100% duty cycle but it will probably get close enough for your application.

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 15, 2015, 01:02:46 am
content: Quote from: Cloud on March 14, 2015, 06:49:49 pmI forgot about that video.Dave uses pretty much the same circuit that I posted. He uses a BJT and drives from the high-side. Sounds like you'll need to drive from the low-side but, other than that, you can likely use the same circuit.Here's the video...

author: Psi
author_rank: Super Contributor
timestamp: March 15, 2015, 01:51:07 am
content: Quote from: jawa_master on March 14, 2015, 02:03:07 pmI don't really recommend doing it this way, but if a low part count is important to you and this PC has a printer port (LPT) you could easily do it in software on the PC.The only external hardware needed would be 3x logic level N-channel mosfets. That's it.One per LED string with the gate connected to a output on the printer port.Downside is your leds will only work once your in windows and have the app running.And you might get some slight flicker if windows is busy doing other stuff and timing is a bit off.And you of cause need to write a tiny bit of code to pulse the port and vary duty cycle

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2015, 11:40:03 am
content: The 555 timer circuit works quite well but the duty cycle can't go down to zero so with LEDs you'll probably notice them glowing dimly, rather than turning completely off.The comparator solution can turn the LEDs completely off and for three channels it actually uses less parts than the 555 timer which would need three ICs or two if you use a 556.A more compact option is to use a Schmitt trigger oscillator, although it still has the shortcomings of the 555. The 74HC14 has six of them and three can be configured as oscillators and the other half used as buffers.

author: Prime73
author_rank: Regular Contributor
timestamp: March 15, 2015, 12:11:37 pm
content: What would be the best approach to use two buttons to change LEDs brightness ( + / - ) ? My guess with 555 it's not that trivial as with the pot and 555 ?

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2015, 12:33:55 pm
content: Quote from: Prime73 on March 15, 2015, 12:11:37 pmThat could be done by adding a digital potentiometer to the 555 circuit but the easiest way is with an MCU.As I said before you could interface it with the PC via RS232/USB and control it with software. One of the main advantages over a parallel port is the MCU could dim the LEDs, regardless of whether the OS or program is running. The program could send a command to the MCU when it's closed to save all the settings to flash, then it will remember them and start up when the PC is rebooted. The software would only be used to change the settings.