As I’ve described in Unbelievable Prices, I bought a crappy chinese MP3 player to use as a “true” sound source (instead of the function generator I use while early-testing my audio projects) for testing my audio circuits without having to worry about accidentally shorting 12V into my iPod’s headphone jack or something like that while probing around.
Yesterday it arrived and as it was expected, it’s the best of Shenzhen. Horrible plastics and build quality, the buttons are super stiff, and overall a shitty product as it was expected to be, but since I’m only going to use it for testing, I don’t care. Here are some pictures of the “beautiful” thing:
As you can see it’s a typical chinese product. The LiPo battery has no markings, except for a weird XI logo, it doesn’t look like a protected pack and the flimsy wires that connect it to the main board can snap off at any second and short the thing out.
Right next to what looks like the main processor, which sadly I couldn’t find any information about it, there’s a very nicely heat-shrunk clock crystal. On the center of the board you can see a generic 4871 audio power amplifier, and on the left side there’s a BK1080 FM receiver IC.
On the other side of the board all you can see is the horrible LCD and the shittiest buttons you can buy in the Shenzhen market.
This week I’ve been experimenting with a very simple and cheap project for wireless transmissions, a lightwave AM transmitter and receiver based on Scott’s design, which was based on VK2ZAY’s design. In my final design I’ve increased the base biasing resistors to decrease the size of the coupling capacitor and also used a darlington transistor to get more current gain.
The transmitter is pretty straight forward, the input modulates the current passing by the LED, which modulates the intensity of light, if you’ve designed any class A amplifiers in the past you surely know how it works. The receiver is just a simple transimpedance amplifier, which is amplifying the signal quite a bit (~56x gain) since the transmitter will usually be a bit far from the receiver. You can do the same with a op-amp, but I much prefer a discrete circuit for these simple things.
You can put a buffer stage with a darlington emitter follower on the output of the receiver so you can drive a speaker directly. Something that I would recommend is to add a small (10x gain maybe?) pre-amplifier for the transmitter, that way you’ll get a bit more signal if you’re source isn’t very loud, specially if you want to drive some high power LEDs, since you have a lot of current headroom with those.
If you want to experiment with different values in a simulation, here is the LTspice schematic. The best way to choose the best LED + photodiode combination to maximize the range is to build some breakout boards that you can plug different LEDs and photodiodes until you have the perfect combination.
Since I’m building a bunch of audio circuits lately and I usually have to test them using a “normal” sound source (after extensively testing it with my function generator), I decided I should buy the cheapest chinese crap MP3 player just to protect my phone or any audio source that I’m using. The last thing I want is to short something or accidentally apply a (relatively) high voltage to one of the audio pins, or do anything that could damage my device.
So, I quickly went to MercadoLivre, the brazilian version of eBay, and searched for the cheapest ($3 multimeter quality) MP3 player available, and sure enough, this popped up. It always amazed me how cheap they can produce this sort of crap, but seriously, $6.31 with taxes and all for a device with a shitty color display and all? That’s just insane, they are probably assembling millions of those to be able to produce them at that price.
So, in a nutshell, I’ve bought this piece of junk which will happily suit my needs, since it’s so fucking cheap that it can be considered disposable.