AM Modulating an LED bulb - Page 1
Forum section: electronics / projects

author: skinny
author_rank: Contributor
timestamp: April 10, 2016, 02:25:55 am
content: Hi Everyone,I'm currently trying to AM modulate an LED light bulb. I've taken several apart and finally found a simple bulb to work with. I've attached the schematic. I was hoping to introduce my signaling to modulate the leds between pin 3 of the SM2082D and R4||R5.Before I did this I decided to make sure it would be safe, so I tapped off of each side of the R4/R5 combo with a 10uF 100V capacitor. Then off the second leg of each capacitor I hooked up the ground and the probe lead of my oscope. There was a sine wave on the screen. The sine wave gradually got larger and when it reached approximately 80VRMS, capacitor C2 literally blew up in my face with a ferocity I've never seen a capacitor blow before.After taking some time to open two windows, turn on a fan, and recover my nerve, I'm now glad I didn't hook up my signaling equipment. I'm not entirely sure what happened but would be appreciative of any insight any of you might be able to provide regarding how to modulate this bulb and where I went wrong.My goal is to have the LEDs modulate imperceptively to the human eye (so it still looks like a solid light) and recoverable by use of a phototransistor, photodiode, or small solar panel. I was inspired by seeing people playing with LiFi. My aspirations are not grand but I would like to see some rudimentary data passed or even audio transmitted to play through my speakers.Not having worked with AC power much before, I'm at a loss as to the 4 pin component shown on the schematic. It seems to take in AC voltage and it comes out as 156VDC. The SM2082D is listed as a Single-channel constant current LED driver controller chip. In the datasheet pin 2 is connected to ground. On the bulb's pcb it is not used. However, the heat sink of the SM2082D is connected to the ground plain. After translating the datasheet (thank you google), it says the chip works with both AC and DC voltage. Here is an image from the datasheet:http://www.thinkisemi.com/fckeditor/editor/filemanager/connectors/php/fckeditor/upload/201503/ThinkiSemi_SM2082B_Circuit.jpgThanks for any help you guys can provide!

author: mmagin
author_rank: Frequent Contributor
timestamp: April 10, 2016, 02:46:07 am
content: Quote from: skinny on April 10, 2016, 02:25:55 amAs you discovered the hard way, not "safe".Depending on your local line frequency, a 10 uf capacitor is on the order of 265-318 ohms of reactance. If that 4 pin device is a bridge rectifier (I'm not sure -- based on the circuit it seems like it must be dropping quite a bit of voltage also so that C2 doesn't explode in normal operation), you'd be putting that reactance from live to ground for every other half cycle.You're really going to need to transformer or optically isolate anything you connect to a off-line LED supply circuit like this.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 10, 2016, 02:53:37 am
content: The four pin component is a bridge rectifier. You can google that for more information.Leaving your lamp off gave a problem that "may" not exist with it connected. As drawn and with no LEDs and IC connected you are applying 156 V DC across a capacitor rated at 25V. It could have been worse than your observed results. Are you sure that the circuit did not call for a 250V capacitor? With the lamps and regulator circuit installed there will be a somewhat lower voltage across this capacitor, though I believe it would still be far above a 25V capacitors capability.

author: Buriedcode
author_rank: Super Contributor
timestamp: April 10, 2016, 02:56:27 am
content: That SM2082D looks to just be a constant current sink. Pin 2 *is* connected to ground, because the heatsink tab is also pin 2RexT sets the current for the LED's.I think/hope that 4 pin device is a step down switched mode power supply, or at the very least, a step down transformer. What rating was the cap again? 25V yikes..With 15 white LED's, thats 3.8 * 15 = @57V, along with the drop across the current regulator thats going to be >60V. You'll need a cap of at least 100V.

author: skinny
author_rank: Contributor
timestamp: April 10, 2016, 03:48:25 am
content: Thanks for your input guys. Just a few points of clarity.I misprinted the voltage range on C2. It's actually 200VDC and I've corrected it up above.From measurements with a working bulb I've found that the LEDs are dropping a little over 10VDC each, so by the end of the chain it has dropped nearly all of the 156VDC.The circuit I have shown here is from a commercial LED light bulb. It is a Utilitech 60W Equivalent (really 9W) light bulb.http://www.lowes.com/pd_740057-43921-YGA03A41-A19-9W-830_0__?productId=50423206All of the components shown on the schematic are native to the light bulb. The capacitors I added to attempt a measurement are not shown on the schematic.I have taken the bulb apart and am powering it with clips connected to a stripped power lead.mmagin - I see now that my method of hooking up the oscope caused the cap to explode. Thanks for the insight.Buriedcode - Thanks for reminding me that the heatsink is the ground pin.

author: German_EE
author_rank: Super Contributor
timestamp: April 10, 2016, 08:45:59 am
content: You may find the following link of use:http://www.bluehaze.com.au/modlight/

author: orolo
author_rank: Frequent Contributor
timestamp: April 10, 2016, 05:43:01 pm
content: Be very careful when connecting your oscilloscope to a floating circuit: the DC at the other side of the bridge rectifier is floating above a wave of mains voltage. It is quite possible that your oscilloscope ground probe shorted one side of the resistors to mains ground, while the other side charged the capacitor until boom.https://youtu.be/xaELqAo4kkQ

author: skinny
author_rank: Contributor
timestamp: April 11, 2016, 01:26:53 am
content: I am positively sure that's what happened. It was quite a magnificent shot of magic smoke that poured out.My next approach will be to place the secondary winding of a 10k CT audio transformer between the last LED and the current sink. On the primary side I'll introduce some sort of audio and see if that works. Hopefully there won't be too much destruction.However first I need to go buy more bulbs.

author: orolo
author_rank: Frequent Contributor
timestamp: April 11, 2016, 07:09:49 am
content: Please, be careful. If you are studying how to modulate a LED, why don't you use a PSU or battery powered LED at this stage?

author: skinny
author_rank: Contributor
timestamp: April 11, 2016, 12:10:43 pm
content: Quote from: orolo on April 11, 2016, 07:09:49 amI've already modulated an LED and a laser. I've also built a receiver for both. Now I'm stepping up to the bulb. My AC circuit analysis is crap. This is forcing me to learn. I've no doubt they'll be a few more light bulb carcasses along the development trail.

author: orolo
author_rank: Frequent Contributor
timestamp: April 11, 2016, 02:16:09 pm
content: Ok. If you are to work with mains voltage, consider using an isolation transformer: you can rig one with two back-to-back mains transformers. An isolation transformer is a must for this kind of work. You could also try other types of LED bulbs. The ones you are using aren't the safest or more reliable around: I've seen some clones of yours recently, after a friend brought me several that failed shortly after installation. Funnily enough, he bought them from a "serious" shop. And never forget that mains power is lethal and powerful, nothing like the average PSU or battery: for example, if you shunt the constant current IC with the secondary of a transformer, the whole ripple (about 1 amp, 80V peak to peak, if I'm right) of the unloaded LEDs will flow through it. If you are lucky, the transformer will saturate and the LEDs will blow up. If you aren't, that wave will carry over to the primary, and fry whatever is on the other side. At least try some spice simulation before implementing the circuits, to get a grasp of what may happen, and what voltages you are working with.Good luck, and be safe.

author: mikerj
author_rank: Super Contributor
timestamp: April 11, 2016, 02:59:09 pm
content: Quote from: skinny on April 11, 2016, 01:26:53 amAny transformer you use must have sufficiently rated isolation voltage otherwise you are playing with a time bomb.Also note that there is likely to be a significant level of ripple at 120Hz, so bear this in mind if this is within your modulation frequency range.

author: skinny
author_rank: Contributor
timestamp: April 11, 2016, 11:37:35 pm
content: Quote from: mikerj on April 11, 2016, 02:59:09 pmI was hoping this might do the job.http://catalog.triadmagnetics.com/Asset/SP-66.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: April 11, 2016, 11:55:57 pm
content: There's also the phosphor response time to consider - white LEDs generally use a blue LED die with a yellow phosphor or bicolour phosphor mix to convert much of its light output to yellow or red and green to provide a reasonably balanced white. However, depending on the phosphor the response can be quite slow, and a pronounced after-glow may exist so a generic white LED optimised for lighting applications may have a very limited freqquency response compared to a similar monochromatic LED.

author: skinny
author_rank: Contributor
timestamp: April 12, 2016, 12:00:25 am
content: Quote from: Ian.M on April 11, 2016, 11:55:57 pmWould the response be too low to accommodate audio frequencies?

author: Buriedcode
author_rank: Super Contributor
timestamp: April 12, 2016, 12:37:49 am
content: I would take a wild stab at the response time of white phosphor-based LED's at about 5-15ms. That'll be off time, which limits the bandwidth somewhat to <10kHz for data, but if you're modulating it with an analogue signal, then it should be fine. But note that, analogue audio modulating an LED bulb will cause the brightness to noticeably change with the beat, unless the modulation depth is very low. And then if you're receiving analogue from a photo-diode, you could have to filter out the mains hum, and also probably any other mains powered source, which will have signals in the audio band. Oh and tv remotes.As for digital, I can't see you getting enough data through the system for audio. Looks like you'll need to go RGB if you want a 'white' light. I am unsure of any commercial devices, specifically doing 'white' with R,G,B, but there are plenty of 'cheap colour changing ones (that come with the crappy remote control). I'm going to buy one just really to take it to bits, use the remote and controller for a low voltage lamp, and see just how bad the construction is.I was thinking of doing something similar as I would like some half decent wireless headphones without being tethered to bluetooth (fine for PC, tablet, phone, but not for.. guitar practice!) and quickly came to the conclusion it would probably be a custom build for the bulb.

author: skinny
author_rank: Contributor
timestamp: April 12, 2016, 12:59:31 am
content: Quote from: Buriedcode on April 12, 2016, 12:37:49 amRight now I'm going to shoot for analog. I've already been able to to modulate normal white, discrete LEDs with no visible change in the light. The circuit was battery powered and recovery was accomplished with a phototransistor.The 60Hz hum does concern me. I imagine I'll need to throw in a filter of some sort. For this first run I'll just be happy if I can modulate the bulb without melting something down or blowing something up....again.

author: mikerj
author_rank: Super Contributor
timestamp: April 12, 2016, 12:46:58 pm
content: Quote from: skinny on April 11, 2016, 11:37:35 pmI think you will be disappointed. Check out the DC resistance - at a minimum it will drop so much voltage that the LEDs won't work properly, and it may get hot enough to melt in doing so.

author: skinny
author_rank: Contributor
timestamp: April 13, 2016, 12:59:52 am
content: Quote from: mikerj on April 12, 2016, 12:46:58 pmYour are absolutely right. I forgot that there's 10k across the primary and secondary. Well this is plan B. Looking at an optoisolator.http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=194335772&uq=635960878370139365

author: Ian.M
author_rank: Super Contributor
timestamp: April 13, 2016, 01:12:21 am
content: Try connecting the transformer secondary with a series capacitor to block DC between pins 2 and 3 of the SM2082D. Current injected at pin 3 should directly add to/subtract from the LED current.However this *may* cause instability so YMMV.

author: skinny
author_rank: Contributor
timestamp: April 13, 2016, 01:51:44 am
content: Thanks for the suggestion. I'll give it a try. Also, what is YMMV?

author: Ian.M
author_rank: Super Contributor
timestamp: April 13, 2016, 02:38:46 am
content: YMMV