The question seemed innocent enough. What type of bulbs do I need for those lanterns? The lanterns in question were a set of five solar-powered models that my wife Lorraine purchased at Costco about a year ago for $89. This wasn’t her first request for that information, but I admit I don’t always pay attention to requests like these. For some reason, this time I listened.
So I’m going over the possibilities in my head, trying to remember what the bulbs in these lanterns looked like. I remembered that they used a candle-like bulb of some sort, but I wondered about the dc nature of this solar lantern setup. Finally, Lorraine presented one of the unlit lanterns to me.
I looked at the bulb inside and it looked strangely glasslike—not like any lamp bulb I’ve ever seen. I turned the lantern upside down and noticed two LEDs at the top part of the lantern (see the figure). “Neat,” I said to her. “This is not a lightbulb at all. It’s just reflecting the light from these LEDs.”
“What’s an LED?” she said, ready to go to Home Depot to pick up a few. At this point, I’m thinking, I’m not talking about electronics enough at home, pointing out things like LEDs in myriad gadgets in the house. But I have a penlight that I got at an electronics event, took apart, and left in my desk.
I retrieved all the pieces—LED, batteries, and case—and showed them to her. This is an LED, and this is how it works. But this one is red and the ones in the lanterns are white. She was very happy to gain this knowledge. If I still had kids in my house, they would have loved this simple demo as well.
I looked more closely at the lantern and noticed some Philipshead screws. Good, I thought, this is serviceable. I took the top of the lantern off and exposed three AAA rechargeable batteries that were coated with rust.
“Here’s the problem,” I said. My first thought was to replace the batteries with new rechargeables since I had three on hand. but after a quick cleaning, I realized that the batteries were still fully charged.
I sprayed all the rust on the batteries and contacts with my favorite rustbuster, WD-40, and then used a Black & Decker rotary tool with a grinding tip to get down to the bare metal. I closed the lights and did a test. The LEDs shined brightly, and my wife had a big smile on her face.
It reminded me of how I felt as a kid when I would ask one of my uncles to check out my “broken” transistor radio. He would poke around a bit, find a cracked solder joint, stick something into the radio to put pressure on the joint to stay in place, and have the radio playing in no time flat. Those kinds of experiences were pretty much the inspiration for me to go into electrical engineering.
But the LED story doesn’t end there. There were two broken lanterns. The second one had the same battery problem, but a cleaning didn’t work this time. Now, I had to remove a couple more screws to get at the electronics inside.
The printed-circuit board (PCB) was about the size of a stamp and had two transistors, two LEDs, and a few capacitors and resistors. One of the LEDs had a lead that had rusted away and broken off.
I remembered that I had purchased a white LED several years ago in hopes of putting together a simple circuit to develop an idea I had for a novel medical device. I never pursued it, but the white LED was still sitting in my junkbox. I desoldered the broken LED and replaced it with the new one. It didn’t produce as much light as the one that was still in there, but the two together glowed almost as brightly as the original duo.
TO THE WEB!
Something else about the circuit intrigued me. If you visit electronicdesign.com on any given day and check out the top 20 articles, you will invariably see “Build a Smart Battery Charger Using a Single-Transistor Circuit” by Ejaz ur Rehman in the top five (ED Online 1823).
This particular lantern seemed to use two transistors to charge the AAA batteries. Immediately, I thought that there might be a way to reduce some cost from the lantern’s bill of materials. My other thought throughout this whole exercise was the amazing ability of designers to come up with novel ways to employ LEDs in simple circuits.
Years ago I attended an editorial gathering at a $6 million dollar home in Los Angeles. The guy who owned the home made his fortune by creating the LED buttons and pins that made a big splash at trade shows and in retail stores some years back. It occurred to me that LEDs are still offering all sorts of possibilities to clever design engineers. I may even revisit the LED circuit that I had contemplated years ago.