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I once purchased a cheap FM transmitter that had a USB port for charging your device. Super cheap. $5, I think. It sounded like crap, so I decided to open it up

I once purchased a cheap FM transmitter that had a USB port for charging your device. Super cheap. $5, I think. It sounded like crap, so I decided to open it up and see how they managed to build it so cheap. The first indicator that it might be super cheap was the fact that it would transmit on two frequencies; if I recall, those were 88.5MHz and 106.2MHz. "That's strange", I thought, "most North American FM radios won't tune an even-numbered frequency (the .2)." Once I got inside, I realized the magic. The FM transmitter section was based around a 17.7MHz crystal. 88.5MHz was the 5th harmonic, and 106.2MHz was the 6th harmonic. There was a single transistor that was used as part of the oscillator. "OK", I thought, "that explains the bad sound quality." Then I started looking closer at the PCB and discovered a few parts that were missing. The scariest was in the USB power section: instead of a 5V regulator (as indicated on the board), there was instead a jumper directly from input to output. That is, it was pumping the noisy, spiky, 13.8V from the car's ignition system directly into the USB port. Luckily, I didn't ever actually plug anything into that port, but it could have gone miserably bad...

vectran 15y ago
Interesting point. I've seen plenty of customers bring in damaged iPhones with damaged motherboards or charging ICs who have been using their devices with non-branded chargers sourced from the likes of eBay. Most people seem to be happy saving a few dollars on a charger for an expensive phone! Its great to see an in depth article in to the design and the cost-cutting. (Source - I work at an iPhone repair company)
hwillis 15y ago
really? Not even a voltage divider? That would kill anything plugged into it right quick
- dfox 15y ago
  I would not be surprised that most USB devices will actually survive that without any damage. It's usually not possible to use USB's roughly 5V directly (because it's not actually reasonably exact 5V and modern logic often uses lower voltages) and thus most devices contain some kind of switchmode power supply (linear regulators essentially cannot be used due to strict input current requirements of USB specification) that tends to be designed to work up to input voltages of tens of volts.

vectran 15y ago

Interesting point. I've seen plenty of customers bring in damaged iPhones with damaged motherboards or charging ICs who have been using their devices with non-branded chargers sourced from the likes of eBay. Most people seem to be happy saving a few dollars on a charger for an expensive phone! Its great to see an in depth article in to the design and the cost-cutting. (Source - I work at an iPhone repair company)

hwillis 15y ago

really? Not even a voltage divider? That would kill anything plugged into it right quick

dfox 15y ago
I would not be surprised that most USB devices will actually survive that without any damage. It's usually not possible to use USB's roughly 5V directly (because it's not actually reasonably exact 5V and modern logic often uses lower voltages) and thus most devices contain some kind of switchmode power supply (linear regulators essentially cannot be used due to strict input current requirements of USB specification) that tends to be designed to work up to input voltages of tens of volts.