switching a relay when a voltage detector detects no voltage

[mnqobilozo]

i am currently making a UPS for a school project. im using a pic16f877a and a LM35 sensor to monitor the UPS temperature. i have been able to do that but now i have a problem with additionally switching a relay if the output of the UPS is zero detected by a voltage detector. the voltage detector input is PIN_B2 and the relay output is PIN_B4.

here is the program

[color=orange]/*
PIC16F877A and LM35 Based Temperature Monitor

MACHI M.C
20403715
*/

#include <16f877a.h>
#device adc=10 // Set ADC resolution to 10Bit
#fuses XT,NOLVP,NOWDT,NOPROTECT
#use delay(crystal=4000000)
#use rs232(baud=960,xmit=PIN_C6,rcv=PIN_C7,ERRORS)
#include <lcd.c>

#define LOAD PIN_B7
#define THRES 70.0 // load switching threshold in Celsius
#define OFF 50.0 // fan stops when temparature reaches value in degrees
#define ADC_OFF 0 // ADC off
#define RELAY PIN_B4
#define R2 PIN_B2
#define R1 PIN_B1
#include <stdlib.h>
int16 digital_reading; // ADC resolution is 10Bit, an 8Bit integer is not enough to hold the reading
float temp;



void main()
{

/* ADC Initialization */
setup_adc(ADC_CLOCK_INTERNAL); // initialize ADC with a sampling rate of Crystal/4 MHz
setup_adc_ports(RA0_ANALOG); // set PIN_A0 as analog input channel
set_adc_channel(0); // point ADC to channel 0 for ADC reading
delay_ms(1); // ADC module is slow, needs some time to adjust.

disable_interrupts(GLOBAL); // DISABE ALL INTERRUPTS.
SET_TRIS_B(0X0F); //set RB0,RB1,RB2 & RB3 as inputs, RB4,RB5,RB6 & RB7 as output.
output_low(RELAY); // relay initialy off
if (input(PIN_B2)==0)
{
output_high(PIN_B4);
}
else if (input(PIN_B2)==1)
{
output_low(RELAY);
}

/* Peripherals Configurations */
lcd_init(); // Turn LCD ON, along with other initialization commands
output_low(LOAD); // the load is initially OFF
lcd_gotoxy(1,1); // point LCD cursor to col1 row1
lcd_putc("Temperature is:"); // print on LCD
{
while(1) // infinite loop
{
digital_reading = read_adc(); // capture current temperature reading
delay_us(100); // 0.1ms delay for ADC stabilization
temp = digital_reading * 0.4883; // convert reading to Celsius
lcd_gotoxy(1,2); // point LCD cursor to col1 row2
printf(lcd_putc,"%2.1f C",temp); // print value on LCD

if(temp>=THRES)
Do
{
output_high(LOAD); // Control Load
}
while (temp>=OFF);
else output_low(LOAD);
delay_ms(1000); // 1 second delay between readings
}
}
}[/code][/color]

[Ttelmah]

Start by not using colour, and using the code buttons properly. Trying to post the code in orange has just resulted in a mess....

Then you have a fundamental problem about using a digital input to use this. A logic input only sees a '0' when it is below 0.6v, and is indeterminate between 0.6, and about 2.4v. However, the peripheral that can do what you want, is the comparator. This can be programmed to switch an output at a specific voltage from the internal voltage reference module, and once setup, does it for you.....
You'd need to change the pins being used, but it will be the better solution.

Now, I've pointed out some problems with the existing code in the comments. Some parts are probably wrong, but you have several things being 'not used', such as the 'OFF' threshold value, so the logic is not totally clear: