This past week I picked up a couple of cheap LED nightlights at Marden’s ($1.99 for a two-pack) and had to take them apart, of course, as I am a self-respecting geek, and because I was at one time mildy obsessed with running LEDs off line power. I guess that hasn’t quite worn off.
This all has led me to two conclusions:
- American consumers are suckers
- Chinese are either idiotic electrical engineers or brilliant social engineers, or both
Actually, both of those are not entirely deserved, nor totally fair, but read on for the full tear-down.
The nightlight in question is a cheap made-in-China LED model with a light sensor which turns off the nightlight during the day. The idea is to save you electricity, and this is why we buy them, thinking “I’m going green! Not only do I have low-power LEDs, it’s only on when the room is dark!”. This is the “American consumers are suckers” part because light is indeed only on when it’s dark, but still consumes about the same amount of power.
That’s actually part of why I bought these – I read somewhere online once about cheap neon nightlights with the photocell which actually consume more power when the light is off than when it’s on and wanted to see if these were any different. (Wish I could recall where that was – somewhere in the depths of the internet. I still have the neon nightlight I bought as a result.)
The Technical Explanation
Here’s the schematic:
Everything to the left side of LED and D3 (and C2) is power supply; the transistor (Q1) and resistors (R3 & CDS) on the right are the light-sensing part. Basically, when light hits the photocell (CDS) its resistance decreases, turning on Q1 and shunting current around the LED – and still consuming power in the process. The difference in current is negligible, as you can see from the photos below. Current draw in full light (with LED off) was 14.7mA, 14.5mA or so in dark, with the LED on, and about 14.35mA after I removed Q1, R3, and the CDS cell from the board.1
Electrically, it’s a ridiculous design. It uses three extra parts (and corresponding extra circuit layout and more complex case design) to consume the same amount of electricity regardless of whether or not it’s lit. That’s the social engineering part and the American Consumers Are Suckers part – the device appears to behave just as we expect it to but in reality does nothing of the kind. Furthermore, most of us have no way of knowing so the designers can get away with it.
And is that important? I could argue it is, generally, but it’s difficult to do in this case when leaving this nightlight plugged in year-round costs a couple quarters.2
1 The neon nightlight, for reference, consumes about 3.4mA but is mostly useless for illumination of anything beyond about six inches. A standard 4W incandescent nightlight uses about 33mA. I have not converted current measurements to power because I suspect the LED lights have a comparatively low power factor and I don’t have a good way to measure or calculate.
2 Rated maximum power consumption of 0.3 W*24 Hours*365 days=2628 W-Hours per year, or 2.628 KW-hours, each of which costs about $0.15 according to the CMP Home Energy Calculator, accessed 2012apr28 – rates at the bottom of the page.