First, read this post in order to understand the basics of interrogating the “Normal” Dallas One Wire chips.

The DS1821 requires a different programmer.

I bought mine from a Dr.Anderson

Breadboard Diagram

No reason you could not buy a BasicStamp SX, and load this program in.

Mine came as a breadboard rather than a Through-hole board, but it works good.

I used Xp ;( bummer… to get the thing up and running.

A raw Xp installation needs two drivers from this ZIP file to hit the FTDI chip.

Copy and extract it to some reasonable directory.

Plug the Basic Stamp in and when XP goes a’ shoppin’ for a driver, browse to that directory.

Carry on with Peter Anderson’s tutorial to set the thresholds.

Here is a sample circuit