FAN BUS Controller

The basic idea behind this project is to have an Atmel CPU read several tempratures, display them on a local LCD, send them to the PC over a serial link and control a fan based on the temperature. If I'm lucky I'll also get the flow meter up and running.

Hardware
1) Atmel MEGA8
2) MAX232CPE serial driver
3) Batron BT22008 HD44780 compatible LCD display
4) Several NTC temperature probes
5) A X9317 digital POT (For the fan controller)
6) A flow sensor of unknown origin

Part 1 - The temperature Sensors
The circuit looks as follows :-

As the values for Vin and R1 are known, and the value of Vout can be measured it is possible to calculate the value of R2(PTC Temperature sensor) using the formular :-


When wired up to an ADC input on the Atmel the signal/temperature looks like this:-


The formular y=-29.225ln(x)+225.8 gives an accuracy of 99.93% for the temperature range 7c to 70c.
Thanks to Excel for calculating the correct value.

The next thing to do is get the flow meter up and running, which shouldn't be too hard as it appears to have a pulsed outout, and the Atmel CPU has an extra interrupt for an external signal.
It should only take afew lines of code to jump to an interrupt handler that increments a variable. And once a second the variable should be reseted.

I think this will be in part 2 - Flow sensor