[PIC] 3 wire Fan tachometer

[anishpsla]

hi friends,

I want to design a RPM counter for 3 wire CPU fan. For that, I design circuit using PIC16F84 as counter. The counter is working perfectly. But when I try to connect the 3rd wire to PIC, I am getting constant 3000 RPM in display. There is no change even the fan is stopped by hand. Here is the code for PIC16F84A using MikroC. When I try to add the pull-up resistor, the reading changes to zero.

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unsigned int fan_rpm;
 
sbit LCD_RS at RB0_bit;
sbit LCD_EN at RB1_bit;
sbit LCD_D4 at RB2_bit;
sbit LCD_D5 at RB3_bit;
sbit LCD_D6 at RB4_bit;
sbit LCD_D7 at RB5_bit;
 
sbit LCD_RS_Direction at TRISB0_bit;
sbit LCD_EN_Direction at TRISB1_bit;
sbit LCD_D4_Direction at TRISB2_bit;
sbit LCD_D5_Direction at TRISB3_bit;
sbit LCD_D6_Direction at TRISB4_bit;
sbit LCD_D7_Direction at TRISB5_bit;
 
char *RPM = "0000 RPM";
 
void Display_RPM(unsigned long num){
  RPM[0] = num/1000 + 48;
  RPM[1] = (num/100)%10 + 48;
  RPM[2] = (num/10)%10 + 48;
  RPM[3] = num%10 + 48;
  Lcd_Out(2,5,RPM);
 }
 
void main() {
     OPTION_REG = 0b00110000; //Timer0 Config
     TRISA = 0x01;  //PORTA Configure
     TRISB = 0x00; //PORTB Configure
     PORTB = 0x00;
     
     Lcd_Init();        // Initialize LCD
     Lcd_Cmd(_LCD_CLEAR);             // CLEAR display
     Lcd_Cmd(_LCD_CURSOR_OFF);        // Cursor off
     
     while(1)
     {
       TMR0=0;
       Delay_ms(1000);
       fan_rpm = TMR0*120;
       Display_RPM(fan_rpm);
       Lcd_Out(1,3,"RPM Counter");
     }
}

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[johnjoe]

Can you post a schematic, and you should use pull-up res because the output of the fan is an open-collector.

 

[anishpsla]

When I put pull-up resistor to the pin, the reading will become zero. Even higher value pull-up will make the count zero. I tried all the combination of circuit I got from internet. The circuit have nothing special, the 3rd wire is connected to RA4 of PIC16F84A. The PIC will work as a counter. The code is already posted.

 

[johnjoe]

Long time ago I used PICs, so please excuse if i'm wrong: should trisa=0x01 be trisa=0x10?

 

[anishpsla]

you are correct. Then still I have problem.

 

[johnjoe]

Maybe check with an oscilloscope your fan signal and also check the voltage levels of your signal.

 

[FvM]

Constant display of 3000 RPM seems to indicate 50 Hz hum but no tachometer signal at the counter input. Looks like a hardware problem.

 

[johnjoe]

Post a schematic anishpsla.

Did you corrected the value of 3000rpm because of the 1:2 prescaler of the timer.

 

[anishpsla]

Maybe check with an oscilloscope your fan signal and also check the voltage levels of your signal.

I have no oscilloscope to confirm fan have tacho signal.

Constant display of 3000 RPM seems to indicate 50 Hz hum but no tachometer signal at the counter input. Looks like a hardware problem.

You are correct. It's caused the crap Chinese adapter. I tried with battery, 3000 RPM in display gone. But still there is no RPM Count in LCD.

Did you corrected the value of 3000rpm because of the 1:2 prescaler of the timer.

1:2 prescaler is already corrected in program. Please see 41st line of the code.

 

[johnjoe]

Please, post a schematic, maybe hand-drawn or just the connection between PIC and fan with associated voltages.