Datasheet

Jon Waddington
32
'' Show the temperature
tc <<= 16 ‘shift left 16 bits
if (tc < 0) ‘check temperature sign
text.str(string("-")) ‘display minus sign
||tc ‘convert sign
tc >>= 16 ‘shift bits back
t := tc >> 4 ‘discard lower 4 bits
text.dec(t) ‘display temperature
text.out($B0) ‘display degrees sign
text.out($43) ‘display ‘C’
3.6 External Temperature Sensor
The external temperature circuit is powered by 4 AA batteries, giving 6V to the input of
the 3.3V voltage regulator. The temperature sensor and XBEE are both powered by 3 .3V.
The temperature sensor used in this circuit is an MCP9700A. This IC was chosen due to
its analogue output which can be connected directly to one of the XBEE’s analogue input.
The MCP9700A’s output voltage varies by 10mV for every 1°C change in temperature.
The voltage on the XBEE’s analogue input is sampled every 4 seconds and transmitted to
the receiving XBEE.
3.6.1 XBEE
Two XBEE Series 2 modules were used in the Information System, a receiver and a
transmitter. They are designed to mount into a socket, allowing the modules to be
removed with ease to place them into development kits to allow the parameters to be
modified.
The schematic in figure 3.6.1.1 shows the remote temperature circuit diagram. It can be
seen that the output voltage from the MCP9700A is connected to the analogue input,
AD0, of the XBEE.