Good ideas with bad execution, or good execution of what should be bad ideas - an analysis of inferior, off-beat or malfunctioning products, and how other people's failures can help us design better stuff.
The Not-So-Bright LED Night Light
In theory, a LED light should last for ten years or so, unlike the typical 4W incandescent lamps used in night lights that seem to burn out in only a few months. But as engineer and founder of the website Discover Circuits Dave Johnson discovered, this Chinese-made unit from Costco (apparently designed by some real dim bulbs) didn’t last but 12 weeks:
“I bought a pack of three of these night lights, made by Elumina Lighting Technologies, for about $15. The unit has a pushbutton switch to toggle between settings (dim and bright) and a CdS photocell that turns off the device during the day.
Inside are three white LEDs, wired in series. When I first plugged the device in, it seemed to emit an acceptable amount of light. But in a short time, the light gradually faded until it was virtually useless.
This has happened to me several times with other inexpensive LED lights. I think some manufacturers from China are using inferior phosphors inside the LED assembly, which fatigue after only a few hundred hours. Opening the thing up, I traced out the circuit and determined it was one that I could easily modify.
I made some component value changes and pulled out the three dim LEDs and replaced them with 10 high quality, super bright surface-mounted units from Osram Opto, which I soldered together into two strips of 5 LEDs each. I paid about $1.00 each for the LEDs. So now for an extra $10 per unit (and therein lies the engineering trade-off!), the light now emits a nice bright white light and should last many years with no risk of someone taking a header down the stairs in the middle of the night!”
View additional product images and the original and Dave’s redesigned circuit , which uses a classic series capacitor method to produce a current limiting LED driver, powered from the AC line.Tags: ac line, night lights, phosphors, product images, pushbutton switch