// declcare
@property (assign) NSTimer *repeatingTimer;

// implements
- (IBAction)startRepeatingTimer:sender {

 

    NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:0.5

                              target:self selector:@selector(timerFireMethod:)

                              userInfo:[self userInfo] repeats:YES];

    self.repeatingTimer = timer;

}

- (IBAction)stopRepeatingTimer:sender {

    [repeatingTimer invalidate];

    self.repeatingTimer = nil;

}