Some time ago, while browsing eBay for a simple LED controller to rejuvenate my retro bedroom lamp, I came across this touch-LED PWM module. To be honest, while it was a complete module and it made an overall good impression, the documentation made me a bit wary. Looking closely at the little circuit board, I noticed that it’s based on a single-channel DC LED control touch chip SGL8022W (from Sigma Micro), which is pretty much what I expected.
According to Sigma Micro, SGL8022W is a single-channel touch chip for LED lightness regulation. This chip can control the on/off state of LED light and regulate its lightness on a continuous range, which could be used on incandescent and halogen. As SGL8022W has an EFT value over ±2 KV, the touching sensitivity and response time of SGL8022W proves to be fine, even under the interference of mobile phones that exist in the near field. See the pin assignment and description of SGL8022W in SOP 8 package. The chip has a relatively wide range of input voltage, which could be chosen optionally within 2.4 to 5.5 Vdc.
The circuit surgery of the module is pretty simple because I have the hardware schematics and I’m an experienced hardware design engineer. However, looking at the module, I quickly found that the circuitry is somewhat different from the original application example included in the datasheet. Besides that, two “aliens” are there: one diode and one resistor, plus the “power on” LED and its current limiter resistor!
It’s clear that the diode works as a “wrong input polarity” guard (keep in mind that this inserts the inherent 0.7-V drop because it’s not a Schottky diode). That’s great, but what’s the role of the next surplus component? Lack of insight forced me to replicate the original circuitry of the FC-106 module, which is shown below.
As we can see in this schematic, the component is a 36-Ω (36R) resistor connected between the Vcc rail and common Vcc points of the mode selection jumpers T1 and T2. I think that this one is probably for additional circuit protection. Furthermore, note that the touch sensitivity can be tweaked by modifying the value of the sampling capacitor between pin 2 (VC) and pin 4 (GND) of SGL8022W. The 100-μF capacitor (C1) indicated in the original application circuit is not used here — see its footmark between the chip and diode in the module under test.
Now to the mode selection; i.e., the four mode options determined by the configuration of mode selection jumpers T1 and T2 (as examined by me):