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Use a Atmel ATTINY26
as a serial port based fan controller for a PC. Tracy’s machine is very noisy but has built in sensors for the CPU and case temperature. Use a PC app to do closed loop control on the temperature. Run it hot-ish as the cooling effect increases with the temperature differential so the hotter it runs the slower the fan needs to run.

Use a FET for the fan drive. The reset case on the micro will be full noise. Could also use the on-board ADC and a resistor divider to sense the back EMF when the PWM is off to get an idea of fan speed.

It should be very cheap - the micro is $6.20 and there is _very_ little support circuitry. Instead of using the CPUs sensor, you could add 1-wire or similar temperature gauges directly to the Atmel but at significant additional cost.

For measuring the vertical temperature gradient cheaply at different stations across about a heactare of land. A heactare is 10000 m^2 or about 100×100 metres. You could use a set of 1-wire temperature sensors in a local cluster. Use a 433MHz radio link and coding similar to this wireless mouse trap and a battery pack to give greater than six month life.

It would need something to collect the information as you don’t want a laptop to be on all of the time. The individual stations could keep transmitting a rolling few days of data or use something like MRTG’s methods of collating data. Or keep something low power on all of the time that also decodes the protocol and provides a serial interface.

Basic costs:

• Transmitter ~$4.20US
• Dallas 18B20 sensor - $2.57US
• Atmel ATTINY or similar - $5NZ

Mechanicals including a waterproof box and antenna, if any, would double the cost.