Air & Ground Sensor

It is aniticpate that up to 20 of these sensors will be attached to a board with Solar Power & Battery, a Microcontroller, and Zigbee node.

Auxiliary circuitry which can power up one of these sensors at a time will be useful in maintaining battery power and increasing operation time.

First for development purposes we should created a PCB which has all the necessary sensors and routing which can be wired over to an AT91SAM7S Development Board. It should containt the following parts:

• 7x Maxim 1-wire DS1821 Temperature sensor. In the final ruggedized "Pole" these can be spaced equidistintally to measure variations in temperature across a small distance. 3 spaced approximately 6 inches apart at ground level and below (-1', -0.5', 0'), and 4 spaced about a foot apart (1',2',3',4' up). It will be necessary to use the extra enumeration and match ROM commands to cycle through these sensors to get all of the readings. These sensors are $10 in small quantities, $4.77 @ 100, and $2.62 @ 1000 + so quite cheap. On the PCB they can be placed close together. What is important here is that we're able to develop the code necessary to muliplex multipled DS1821s on one wire
• One Maxim DS2450. It should also have a header to connect the Watermark soil moisture sensor, and appropriate blocking capacior as mentioned in the documentation. Since it is a resisitve sensor, we will need to apply a known voltage across the sensor and another known resistance value so that the resulting voltage will be proportional to the Soil Moisture. This will also be multiplexed in with the other 1-wire devices.
• One Chipcap Temperature and Humidity Sensor. Even in low quantities these are $20 (vs $50 for the maxim ibutton). This will be on a separate 1 wire bus, at least on this development platform.

Later the system will add : 

• Solar Power and Battery
• microcontroller
• Xbee Module. 
• Power Circuits (Probably a transistor triggered by the multiplexor circuit) for turning individual sensor On/Off to save power
• a Multiplexer to allow communications to iterate between multiple sensors to save pins (each sensor will require 1 pin for the 1-wire devices, and 1 pin for the Chipcap device) and control power pins etc.