I bought a set of these about a year ago:
http://cgi.ebay.com/ebaymotors/SECUT...Q5fAccessories
They sound an alarm if the tire goes below 28# and a red light tells you which tire (if you don't mix them up).
I tried them on a leaky bicycle tire that normally holds 65# of air. All I can say is that they work...... for $25........ that's all wanted.