Three Way LED Bulb Gives Up Its Secrets

You’ve probably seen three-way bulbs. You know, the ones that can go dim or bright with each turn of a switch. [Brian Dipert] wondered how the LED version of these works, and now that he tore one apart, you can find out, too. The old light bulbs were easy to figure out. They had two filaments, one brighter than the other. Switching on the first filament provided some light, and the second gave off more light. The final position lit both filaments at once for an even brighter light.

LED or filament, three-way bulbs have a special base. While a normal Edison-base bulb has the threaded part as the neutral and a center contact for the live wire, a three-way bulb has an extra hot contact ring between the threaded part and the center contact. Obviously, a compatible LED bulb will need this same interface, but will work differently inside.

Inside the LED, [Brian] found two rings of LEDs that took the place of the filaments. He was able to identify all the ICs and devices on the board except one, an MT7712S. If you can read Mandarin, we think this is the datasheet for it.

We weren’t sure what [Brian] would find inside. After all, you could just sense which contacts had voltage and dim the LEDs using PWM. It probably wouldn’t take any less circuitry. LED lighting is everywhere these days, and maybe they don’t all work the same, but you have to admit, using two strings of LEDs is reasonably faithful to the old-fashioned bulbs.

Sometimes LED bulbs are different depending on where you buy them. We were promised LED bulbs would never burn out. Of course, they do, but you can usually scrounge some LEDs from them.

18 thoughts on “Three Way LED Bulb Gives Up Its Secrets

  1. I had some CFL lamps and now LED that are used on a regular 2 pole Edison socket that change brightness just by quick On/Off/On action. Every time I do this, it selects one of 3 brightness levels.

    1. Many decades ago, I used to have a little disk that went between the bulb and socket that did this. I also had one that turned off the light after 15 mins, unless you flicked the switch to turn it off. I wish I could find more of these, but I’ve never seen them since.

        1. Close, but these weren’t built into the bulb; they were a little disk that was sandwiched between the bulb and the hot contact in the socket. It’s possible they were made by Philips, but I don’t remember the “IQ” labeling. There was also one that functioned normally, but if you do the rapid off-on-off flicker, it would make the bulb blink around 1-2 Hz.

          1. If you watch the video you can see that they weren’t actually a Philips product, but resold by them. I think the video actually mentions a product like that, but i’m not going to rewatch the video now to see.

        2. (Technology Connections is a really terrific channel!) As a general comment here, I am thrilled with LEDs. I’ve been designing and building decorative light fixtures and lamping them was always the toughest part. Because…HEAT! Even CFLs gave off a lot of heat. I would have given anything to have LED bulbs back then. Best wishes to all HACKADAY friends-! :)

    1. Hot Air station on the board from the oposite side of the Led until it gets Hot enought to remove the Led, also its Better on lamps that has alluminum boards for the Leds, unless you already have other lamps to reuse the leds doesnt worth the trouble.

      1. Also, reflowing LEDs is hit or miss for long term reliability in my experience. The mfg’s publish reflow curves for a reason. Doing it three times triples the heat the chip is subjected to, which drastically increases its failure rate.

        Much better to spend the $0.05-$0.45/led and get factory fresh versions with a known spec sheet

    2. I started harvesting the good leds out of dead led bulbs a few years ago, so far I have a small vial with ~50 leds just waiting for a project to be used on. Easiest way to pull the leds is to use a small hot plate to heat up the bottom of the metal core pcb the leds are soldered to and once up to temp just pick them right off with tweezers quickly.

        1. Valid point.

          I date back to a time when scavenging parts was a rite of passage, and a group of hackers at MIT achieved a bit of fame for inventing a “wave desoldering” technique to speed up rescue of 7400- family logic chips, so I hate the thought of throwing out working parts from an unreparable device… But, yeah, LEDs aren’t scarce yet.


          I am anticipating passing peak plastic before too many more decades, and that’s going to put a heck of a dent in manufacturing…

    3. You could try modifying the power supply with a lower current so that it doesn’t overheat and shut off. I’d remove and bridge the dead led though so no unexpected behavior is going to happen :)

  2. If you were sold something that with a special feature took 5 or 10% of the life or utility off you probably would not buy it. Those 3 way bulbs roughly halved their life with their on off on again switching. Have a mood light and a reading light separately. Easy. Light is exponential but those linear wattage sizes were not realistic. Triac dimmers? Buzzzzz. The proto conservative and ham senator Barry Goldwater wanted to ban them.

    1. In the pre-triac-dimmer days, 3-ways made sense for folks who didn’t have space, or ready cash, for an additional or multi-bulb fixture. Yes, you paid for the convenience, but convenience is, ultimately, the single feature most shopped for and the ongoing cost wasn’t unreasonable.

      And now 3-way fixtures exist as heirlooms, and folks mostly don’t want to rewire them, so 3-way LEDs to maintain the traditional behavior are a thing. Nothing wrong with that if you want to keep great-gran’s table lamp “unchanged” modulo going to LED.

      Tools for tasks. If it’s the wrong tool for your task, that’s fine but if there wasn’t anyone who wanted it, it wouldn’t still be on the market.

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