900 MHz Receiver

I hate throwing things out that aren't broken. Not only does it add to the landfill but it just seems like such a huge waste. Take this little project from 2008 for instance.

Years ago, I bought a set of 900 MHz stereo headphones. These were branded by Emerson but I've seen lots of others over the years. I used them in my office, and they worked - had plenty of range. They had one really serious problem - they were just plain uncomfortable. Despite all the electronics to make the sound decent, the design didn't handle the comfort factor. The more I wore them, the less I wanted to. In the end, I stopped using them entirely.

Well, I was all set to get rid of them when it occured to me that I can re-purpose this as a wireless transmitter/receiver pair. While the sound is not perfect - there's some amount of hiss - it's not bad for casual use. What if I could use this in a powered speaker for outdoor use? Or maybe hook up the transmitter to a PC playing MP3s and listening over the receiver in another room?

I tossed all sorts of ideas around for some time, but many of them required me to build something else to go with it. In the end, I decided to simply re-package the receiver into something useful. I can always append to it as needed in the future. So here's what things look like today. First the transmitter.

The transmitter was left untouched. This alien looking device comes with a wall-wart and a 1/8 inch stereo cable designed for a stereo headphone jack. It also has a small cord to recharge the batteries in the old headphones - this is now unused. The receiver looks like this:

The box is just a plastic case from my local Radio Shack. On the rear is the power cord and an antenna from my parts box. The front panel has a 2 red LEDs (signal lock and power), an 1/8 inch stereo output jack and a power switch. The bottom has four stick-on rubber feet. The device was originally designed to drive 32 ohm headphones. In this configuration, it's more than capable of driving the input an amplifier. Now for a view of the interior:

The are three obvious parts. On the upper right is a small 6.3v 300mA transformer. On the bottom right is a small power supply. This LM317 based supply takes the AC power, converts it to DC and reduces it to about 2.5v. The original headphones were powered by two 1.2 volt NiCd batteries housed on the left ear cup and would recharge via the transmitter. I simply matched the voltage of those batteries.

The guts are on the left. The oval shaped circuit board used to sit on the right ear cup. The two LEDs were originally located on the PCB; they are now epoxied to the front of the box. The volume and tuning are preset and I've given no access to them from the outside. From testing, this has not been a problem. I can always adjust the tuning on the transmitter side if necessary; and the volume is set near max.

So that's it for now. I've tested the setup and it works well. I still need to put it to actual use but that's another chapter for another time.

07-January-2010

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