diff --git a/src/main.c b/src/main.c
index 25b1f6f..f33dc7e 100644
--- a/src/main.c
+++ b/src/main.c
@@ -11,9 +11,14 @@
 
 /* global variables */
 volatile uint8_t pwr_ok;
+volatile uint8_t fan;
+volatile uint16_t tachometer = 0;
+volatile uint8_t timer2_ovf = 0;
+
+const uint16_t TIMER2_PRESCALE[8] = {0,1,8,32,64,128,256,1024};
 
 /* PWR_OK interrupt */
-ISR(PCINT0_vect) { /* PCINT1 is actually triggering PCI0 Interrupt Vector */
+ISR(PCINT0_vect) { /* PCI0 Interrupt Vector for PCINT[7:0] */
 	if (pwr_ok!=(PINB&(1<<PWR_OK))) { /* did the PWR_OK pin state changed */
 		pwr_ok = PINB&(1<<PWR_OK); /* save new state */
 		if (pwr_ok) {
@@ -24,6 +29,25 @@ ISR(PCINT0_vect) { /* PCINT1 is actually triggering PCI0 Interrupt Vector */
 	}
 }
 
+/* fan tachometer interrupt */
+ISR(PCINT1_vect) { /* PCI0 Interrupt Vector for PCINT[14:8] */
+	if (fan!=(PINC&(1<<FAN))) { /* did the FAN pin state changed */
+		fan = PINC&(1<<FAN); /* save new state */
+		if (fan) {
+			tachometer = timer2_ovf*256+TCNT2; /* save time */
+			TCNT2 = 0; /* reset timer 2 */
+			timer2_ovf = 0; /* reset timer 2 overflow counter */
+		}
+	}
+}
+
+/* timer 2 interrupt used to measure fan tachometer */
+ISR(TIMER2_OVF_vect) { /* timer 2 overflow interrupt verctor */
+	if(pwr_ok) { /* only measure tachometer if power is ok */
+		timer2_ovf++; /* increase time 2 overflow counter */
+	}
+}
+
 static void ioinit(void)
 {
 	/* configure power */
@@ -37,7 +61,22 @@ static void ioinit(void)
 
 	/* configure peripherals */
 	DDRB &= ~(1<<IR); /* IR receiver is input */
-	DDRC &= ~(1<<FAN); /* FAN is input */
+
+	/* configure FAN */
+	DDRC &= ~(1<<FAN); /* FAN (PC5/PCINT13) is input */
+	fan = PINC&(1<<FAN); /* save state */
+	PCIFR &= ~(1<<PCIF1); /* clear interrupt flag */
+	PCICR |= (1<<PCIE1); /* enable interrupt for PCINT[14:8] */
+	PCMSK1 |= (1<<PCINT13); /* enable interrupt for PCINT1 */
+	/* use timer 2 to measure the tachometer */
+	/* use normal mode */
+	TCCR2A &= ~((1<<WGM20)|(1<<WGM21));
+	TCCR2B &= ~(1<<WGM22);
+	/* /256 prescale timer */
+	TCCR2B |= (1<<CS22)|(1<<CS21);
+	TCCR2B &= ~(1<<CS20);
+	TIFR2 &= ~(1<<TOV2); /* clear timer 2 overflow interrupt flag */
+	TIMSK2 |= (1<<TOIE2); /* enable timer 2 overflow interrupt */
 
 	/* configure channels (used for powering LEDs using an nMOS) */
 	DDRC |= (1<<CH1_1)|(1<<CH1_2)|(1<<CH1_3)|(1<<CH1_4)|(1<<CH1_5); /* CH1_x is output */
@@ -52,7 +91,7 @@ static void ioinit(void)
 
 	/* configure LED (on PD6/OC0A) */
 	DDRD |= (1<<LED); /* LED is output */
-	/* use phase correct PWM ode (because fast PWM is always on for at least 1 cycle) */
+	/* use phase correct PWM mode (because fast PWM is always on for at least 1 cycle) */
 	TCCR0A &= ~(1<<WGM01);
 	TCCR0A |= (1<<WGM00);
 	TCCR0B &= ~(1<<WGM02);
@@ -89,13 +128,13 @@ int main(void)
 				}
 				break;
 			case 'a':
-				PINB |= (1<<nPS_ON);
-				printf("power supply: ");
+				printf("switching power supply ");
 				if (PINB&(1<<nPS_ON)) {
-					puts("off");
-				} else {
 					puts("on");
+				} else {
+					puts("off");
 				}
+				PINB |= (1<<nPS_ON);
 				break;
 			case 's':
 				printf("power: ");
@@ -207,6 +246,17 @@ int main(void)
 				}
 				printf("increasing LED: %d\n",OCR0A);
 				break;
+			case 't':
+				if (tachometer) {
+					uint16_t prescale = TIMER2_PRESCALE[TCCR2B&((1<<CS22)|(1<<CS21)|(1<<CS20))];
+					if (prescale) {
+						uint32_t speed = ((60*F_CPU)/(prescale*(uint32_t)tachometer))/2;
+						printf("fan speed: %lurpm\n",speed);
+					}
+				} else {
+					printf("fan off or too slow\n");
+				}
+				break;
 		}
 	}