ADCIN giving inconsistent results

[triton99]

A friend of mine had an issue with the cooling fans on one of his cars. He originally asked if I could just wire up a switch so he could turn the fans on/off with a switch. I thought this would work but what if he forgot to turn them on and the car would then overheat. So I decided to look into creating a circuit for him that would monitor the temperature of the coolant and turn the fans on/off respectively.

The coolant sensor on his car is just a thermistor that increases in resistance as the temperature rises. The car ECU provides a voltage divider circuit that then measures the voltage at the sensor. While gathering information about thermistors i concluded that there measured resistance on a graph is non-linear. When using the thermistor in a voltage-divider circuit the graph of the measured voltage becomes more linear. There are different reasons why car cooling fans do not turn on; faulty relay's, ECU's, sensor's, fan's, wiring, etc. The particular model of car he has [2000 Chevy Malibu] is notorious for having this problem. As far as I can see no one has solved the problem directly; it seems to be a problem of the manufacturer.

When connecting a OBDII scanner to the car I am still able to read the coolant sensors temperature. Using a voltmeter, we read the value at the sensor and noted what the temperature was on the OBDII scanner. He told me what temps he wanted the fans high/low to turn on/off. This initially seemed simple enough....so I thought.

Back at my home I decided I would use a PIC12F675 for the project; it was small and had ADC features. I remembered a long time ago I built a circuit for a very simple voltmeter, but it was using a BS2 module. After a little more research, I found a similar example on these forums for reading the voltage on a ADC pin. After a few tests everything seemed to work as it should. The values fluctuated initially, but after I set the VREF to VDD it seemed to stop. Next I etched circuit onto a copper-clad PCB. Now the fluctuation returned.

What I was wanting to know, is there a better way of reading the voltage using an ADC on the 16F675. Here is my code, and I also attached a picture of the schematic. The SEROUT portions will eventually be removed once develpment was complete. They were just used so I could test the circuit while at the car. The fan-relays on the car also have flyback-protection diodes.

Also when I inserted the PICBASIC code it changed the formatting, so everything that was supposed to be capitalized is not anymore.

Thanks, and sorry for the long-winded story.

@ DEVICE pic12F675, INTRC_OSC_NOCLKOUT ' Internal Oscillator
Define OSCCAL_1K 1      ' Calibrate internal oscillator

ADCON0.7 = 1        ' Right justify result
ADCON0.6 = 0        ' set Vref to VDD
ANSEL    = 001000  ' Set AN3 analog, rest digital
CMCON    = 7          ' disable analog comparator

' Define ADCIN parameters
Define  ADC_BITS     10   ' Set number of bits in result
Define  ADC_CLOCK     3   ' Set clock source (3=rc)
Define  ADC_SAMPLEUS 50   ' Set sampling time in uS


sample_size var byte
i                var byte
V_in           Var Word
V_total       var word      
TX             Var GPIO.5
mode          VAR byte
low_fan       var GPIO.2
high_fan      var GPIO.1


mode = 2        '// 2=T9600, 6=N9600
sample_size = 50
v_total = 0
V_in = 0


LOW low_fan
LOW high_fan


pause 500


serout TX, mode, [10,10]
serout tx, mode, [10,13,"------------------", 10, 13]
serout tx, mode, [      "ECT Fan-Controller", 10, 13]
serout tx, mode, [      "------------------", 10, 13]


main:
    gosub read_v
    gosub check_v
    gosub update_display
    goto main


update_display:
     serout TX, mode, [#V_total,13]
     pause 1000           '// slight delay to lower flickering
  Return


check_v:
    if V_total <= 201 then high  low_fan     '/ car test value 201
    if V_total <= 181 then high  high_fan    '/ car test value 191
    if V_total >= 211 then gosub fans_off    '/ turn both fans off
  Return
    
fans_off:
    low low_fan
    low high_fan
    return
    
read_v:
    for i = 1 to sample_size
       pauseus 50
       ADCIN 3, v_in                ' Read channel 3 to V_in (0-1023)
       V_Total = (V_In + V_total)
    next i
    
    V_total = V_total/sample_size       ' average the values
    V_Total = (V_total */ 500)>>2      ' Equates to: (V_in * 500)/1024
  Return
  
  
end