At work we use a lot of environmental monitors. I wanted something similar for home. I saw these DHT-11 Sensors on ebay pretty cheap. there are wider range and higher resolution version available but I wanted a few of them cheap.
I decided to do a simple serial interface so it would work easily with Linux, Pic's, Windows etc..
Step 1: Prototype
I had a bunch of PIC16F73's left over from a project so I used them.
I opened the serial to USB adapter's case and ran a wire from the USB power pin to pin 9 on the serial port. This is actually the ring indicator input but this way I didn't need an external power connector.
The hex file attached uses a simple menu interface good for testing and a raw data feed command for using in scripts.
Step 2: Final Design
I saw these nice time bulkhead boxes on Electronics Goldmine. I used them in my Instructables on Balun's too.
I used a mill to cut the holes for the serial port and sensor but a Dremel or drill and file will do just as well.
Step 3: Schematic and Code
I used a 10Mhz ceramic resonator instead of the 10MHz crystal and two 12pf caps. I just happened to have a bunch of those around and its quicker to wire on a protoboard
Step 4: Communicating With It
Its pretty simple to talk to. I use 2400 Baud to help with slow devices or long cable runs.
The screen shot above was from Zterm on a mac.
The commands are simple and can be upper or lower case:
V - Version (4 line info header)
A - Ascii Reading Output (2 lines in plain English)
T - Temperature Reading (always 3 digits)
H - Humidity Reading (always 3 digits)
B - Both Reading separated by a comma
all readings are zero padded for fixed length. This makes it easy to read with some languages. All lines are terminated by a CR and LF. If the sensor fails the English commands will say "Sensor not Responding" and the data reads (T-H-B) will return 3 dashes in place of the reading.