原创 Mending the flashlight

I typically can't help myself when it comes to fixing things, especially electronics.

My better half returned from a bargain-hunting day with a wind-up flashlight and charger she found for a dollar in a thrift store. She said she wanted a flashlight for an upcoming camping trip and a charger for her cell phone that she could use out in the wild. My wife loves a bargain, so she was pretty pleased with herself and asked me to help her hook up the charger. Unfortunately, she was disappointed when nothing seemed to charge. I had to investigate; I hate things that don't work. After some exploration I discovered the output was only 3V. Maybe that was why it was found in the thrift store. In her typical style she threw the offending item in the garbage, rolling her eyes in frustration.

As I said, I can't help myself when it comes to fixing things so I picked it out of the trash and had a little look to see if I could diagnose the problem. I thought I could add a boost circuit to get it to 5V, but I didn't have the parts. I added it to the mountain of unfinished projects that used to be my desk.

Eventually, the day came when I couldn't ignore Mount Electronica any longer, and I embarked on an expedition to the desk. Not long into the tidying session, I discovered an LED light bulb I had bought for the kitchen a while ago. I had made the rookie mistake of not checking the voltage that was required and picked up a 12V bulb instead of the one I needed. It had joined the pile and now it distracted me from my chore. I decided to strip out the 1W LED from the bulb for another project. Inside I found an MC34063 IC. Checking the datasheet, as we geeks do religiously, I identified it as a Step-Up/Down Regulator that was being used to step down 12V to 3V.

I started work on transforming it to a step-down straight away. Lady Luck was smiling on me that day as the datasheet had an example, which meant that all I needed to do was pick the right resistors. The circuit needed to change completely, so out came the breadboard. After some cavalier soldering to mount a few of the surface mount parts, I was up and running. I'd only used two resistors extra to the original parts from the bulb. After the initial success I just had to cram it into the flashlight and I was done.
On cracking open the flashlight I realised that I didn't have much room to spare. The breadboard simply wouldn't fit. Undeterred, I decided that a little PCB design was needed. I knocked together a single-sided board and mounted it on top of the existing PCB in the flashlight, placing the components to fit inside the curved case as best I could. I had no idea if it would actually fit or whether this last hurdle would be the one that tripped me.

With a PCB etched and all of the components soldered, I fired up the converter for one final test before hacking it into the case. It worked like a charm, first time! The voltage out was as close to 5V as I was going to get: Now it was time to get it into the case. Unfortunately, Lady Luck was looking the other way. The case just would not close. A closer inspection gave me some hope. The PCB could potentially shed some excess weight. I took a deep breath and sanded down the PCB as close to the components as I dared and trimmed the wires as short as they could possibly be.

I gave it another try. The case closed. It fit perfectly. All that was left to do was connect the switch and the winder, and I was on the home stretch. Then came the moment of truth: Would it actually charge? With the cell phone connected, I cranked the handle, building up speed and hoping for the best. Success! The charge light came on. I had done it. But it wasn't all good news. The flashlight wasn't quite what it once was, as the boost converter was draining the capacitor. It now needs a crank before every use. It's a small price to pay, though. One day I may do a rewire, but for now, my work is done. I had transformed the charger that didn't make the grade into something that actually worked. Sadly, it was a little late for the original camping trip by the time I had finished.

Brice Harris is a father of three, husband of one, and he enjoys seeing things work. Together they raise backyard chickens and make maple syrup.

He submitted this article as part of Frankenstein's Fix, a design contest hosted by EE Times (US).