Simple Fan Controller

This is one of those see-a-need, fill-a-need type of projects. A while back I was struggling with my cable modem and wireless router. I have the cable modem, the router, and the telephone modem all co-located on the same area. It is a small area on our workroom which happens to share the printer as well. The printer is more accessible and visible (for obvious reasons), but the modems and the router are not. As it happens, the telephone modem seems to be pretty robust; but the cable modem and router (mainly the modem) are quite the pre-madonnas. After a week of continuous use (especially during summer), those two boxes get quite toasty (the cable modem is king though). Once this happens the whole network crawls to a stop until eventually it locks up. The solution: power down the modem (and sometimes the router), let it rest (cool down) for a few minutes and try again. In reality this screams: get a new (better) modem, and perhaps a better router. But there is one point which also became obvious: the location I'm using is less than optimal (airflow-wise) and I need to improve on the airflow.

So in came the simple fan controller. What it was meant to do was monitor the space/cavity temperature and apply forced cooling (i.e. external fan) to the modem and/or router as required. To do this a PIC MCU would sense the temperature of the space/cavity and enable the fan as necessary.

A few things came to mind. First, we needed a way to make sure the fan current was being monitored for potential stalls and/or power supply issues. Second, we wanted to measure the temperature and only turned the fan on when the temperature was above a certain limit.

Well, as all other great plans; once you start executing it, things change quite fast. So what happened, even though monitoring the current to prevent stalling and monitoring the temperature are worthy goals; we were over-complicating the main goal. The main objective was to lower the temperature of these devices using forced air (i.e. a fan). But we added the current and temperature sensing as nice things to have, but not necessarily a requirement.

As it turned out, the temperature monitoring was a bit misleading. I managed to monitor the temperature alright; but the temperature change (measured as ambient near the devices) did not vary significantly. More importantly, the limits during summer and winter were quite different. Also, I discovered that regardless of season, if I turned he fan on and off a timed intervals, that was sufficient to keep the temperature under control. So I go back to what is really needed. Temperature sensor is on the design, but it's really not used right now.

The current monitor suffered a similar fate. Even though that one is really useful, for the first proto I decided it was not worth the extra effort to accommodate this feature. Again the current sensing is there, but it's not currently used.

Ultimately the following schematic shows what I built.


There is a simple current sense circuit (resistor), and a temperature sense; but these are not implemented yet. There is a simple on/off transistor to the fan which allows to enable/disable the fan; and/or apply speed control (if desired).

Below is a sample of the code (Oshonsoft Basic) which operates this controller. Interestingly, the simple enable/disable o the fan at timed intervals has been sufficient.

'languer (2011)

'Pin Allocation:

	'PIN# Main_Fn        Secondary_Fn

	'GP0 -> RS232_OUT    ICSP-DAT

	'GP1 -> VREF         ICSP-CLK

	'GP2 -> FAN_MON

	'GP3 -> MCLR

	'GP4 -> T_SENSE

	'GP5 -> FAN_CTRL



'General Configuration

Define CONF_WORD = 0x33c4  'for internal 8MHz osc



'Oscillator/Clock Configuration for internal 8MHz osc

Define CLOCK_FREQUENCY = 8

OSCCON = 0x71



'Variable Declarations

'>>I/O

Symbol rs232_out = GP0  'rs232 output

Symbol adc_vref = GP1  'voltage reference input

Symbol fan_sense = GP2  'fan current sense Input

Symbol temp_sense = GP4  'temperature sense input

Symbol fan_ctrl = GP5  'fan control output

Const _trisio = %11111110

'>>Constants

Const fan_off_time = 120  'time fan is off, in minutes

Const fan_on_time = 5  'time fan is on, in minutes

'>>Variables

Dim _true As Bit

Dim _false As Bit

_true = True

_false = False



'Main Program

main:

WaitMs 2500

Call init()

While _true

	Call wait_minutes(120)

	Call fan_enable()

	Call wait_minutes(5)

	Call fan_disable()

Wend

End                                               



Proc init()

	AllDigital

	TRISIO = _trisio

	Serout rs232_out, 9600, "Starting...", CrLf

End Proc                                          



Proc fan_enable()

	'enable fan

	Config fan_ctrl = Output

	Low fan_ctrl

	Serout rs232_out, 9600, "FAN On", CrLf

End Proc                                          



Proc fan_disable()

	'disable fan

	Config fan_ctrl = Input

	Serout rs232_out, 9600, "FAN Off", CrLf

End Proc                                          



Proc wait_minutes(time As Byte)

	Dim cnt As Byte

	cnt = 0

	While cnt < time

		WaitMs 100

		'WaitMs 60000

		cnt = cnt + 1

	Wend

End Proc