diff --git a/stm32/aioc-fw/Src/led.c b/stm32/aioc-fw/Src/led.c
index 1cbee73..00e6cdf 100644
--- a/stm32/aioc-fw/Src/led.c
+++ b/stm32/aioc-fw/Src/led.c
@@ -1,5 +1,75 @@
 #include "led.h"
 #include "stm32f3xx_hal.h"
+#include <assert.h>
+
+uint8_t LedStates[2] = {0, 0};
+uint8_t LedModes[2] = {LED_MODE_SOLID, LED_MODE_SOLID};
+uint8_t LedLevels[2] = {LED_IDLE_LEVEL, LED_IDLE_LEVEL};
+uint16_t LedCounter = 0;
+uint16_t LedCounterPrev = 0;
+
+#define LED_SET1(x) LED_TIMER->CCR3 = (512 - ( ((uint32_t) (x)) & 0xFF ))
+#define LED_SET2(x) LED_TIMER->CCR4 = ( ((uint32_t) (x)) & 0xFF )
+
+void LED_TIMER_IRQ(void)
+{
+    LED_TIMER->SR = (uint32_t) ~TIM_SR_UIF;
+
+    for (uint8_t i=0; i<2; i++) {
+        uint8_t mode = LedModes[i];
+
+        switch (mode) {
+        case LED_MODE_SLOWPULSE4X:
+        case LED_MODE_SLOWPULSE3X:
+        case LED_MODE_SLOWPULSE2X:
+        case LED_MODE_SLOWPULSE:
+            if ( !(LedCounterPrev & 0x200) && (LedCounter & 0x200) ) {
+                /* Rising Edge  */
+                LedLevels[i] = LED_FULL_LEVEL;
+            } else if ( (LedCounterPrev & 0x200) && !(LedCounter & 0x200) ) {
+                /* Falling Edge */
+                if (LedLevels[i] != LED_IDLE_LEVEL) {
+                    /* Only reset colors and mode when a pulse has been carried out */
+                    LedLevels[i] = LED_IDLE_LEVEL;
+                    LedModes[i] = mode > LED_MODE_SLOWPULSE ? mode - 1 : LED_MODE_SOLID;
+                }
+            }
+            break;
+
+        case LED_MODE_FASTPULSE4X:
+        case LED_MODE_FASTPULSE3X:
+        case LED_MODE_FASTPULSE2X:
+        case LED_MODE_FASTPULSE:
+            if ( !(LedCounterPrev & 0x40) && (LedCounter & 0x40) ) {
+                /* Rising Edge  */
+                LedLevels[i] = LED_FULL_LEVEL;
+            } else if ( (LedCounterPrev & 0x40) && !(LedCounter & 0x40) ) {
+                /* Falling Edge */
+                if (LedLevels[i] != LED_IDLE_LEVEL) {
+                    LedLevels[i] = LED_IDLE_LEVEL;
+                    LedModes[i] = mode > LED_MODE_FASTPULSE ? mode - 1 : LED_MODE_SOLID;
+                }
+            }
+            break;
+
+        case LED_MODE_SOLID:
+            LedLevels[i] = LedStates[i] ? LED_FULL_LEVEL : LED_IDLE_LEVEL;
+            break;
+
+        default:
+            assert(0);
+            break;
+        }
+    }
+
+    /* LEDs are connected anti-parallel */
+    LED_TIMER->CCR3 = (512 - (uint32_t) LedLevels[0]);
+    LED_TIMER->CCR4 = ( (uint32_t) LedLevels[1]);
+
+    /* Advance counters */
+    LedCounterPrev = LedCounter;
+    LedCounter = (LedCounter + 1) & 0xFFF;
+}
 
 void LED_Init(void)
 {
@@ -27,5 +97,9 @@ void LED_Init(void)
     LED_TIMER->CCR3 = 0;
     LED_TIMER->CCR4 = 512;
     LED_TIMER->EGR = TIM_EGR_UG;
+    LED_TIMER->DIER = TIM_DIER_UIE;
     LED_TIMER->CR1 |= TIM_CR1_CEN;
+
+
+    NVIC_EnableIRQ(TIM4_IRQn);
 }
diff --git a/stm32/aioc-fw/Src/led.h b/stm32/aioc-fw/Src/led.h
index ddf8f74..0ca80ba 100644
--- a/stm32/aioc-fw/Src/led.h
+++ b/stm32/aioc-fw/Src/led.h
@@ -4,15 +4,31 @@
 #include "stm32f3xx_hal.h"
 
 #define LED_TIMER       TIM4
+#define LED_TIMER_IRQ   TIM4_IRQHandler
 #define LED_GPIO        GPIOB
 #define LED_GPIO_PIN1   GPIO_PIN_8
 #define LED_GPIO_PIN2   GPIO_PIN_9
 
-#define LED_IDLE_LEVEL  0.1
-#define LED_FULL_LEVEL  1.0
+#define LED_IDLE_LEVEL  25
+#define LED_FULL_LEVEL  255
 
-#define LED_SET1(x) LED_TIMER->CCR3 = (512 - ( ((uint32_t) ((x) * 255)) & 0xFF ))
-#define LED_SET2(x) LED_TIMER->CCR4 = ( ((uint32_t) ((x) * 255)) & 0xFF )
+typedef enum {
+    LED_MODE_SOLID = 0,
+    LED_MODE_SLOWPULSE = 4,
+    LED_MODE_SLOWPULSE2X = 5,
+    LED_MODE_SLOWPULSE3X = 6,
+    LED_MODE_SLOWPULSE4X = 7,
+    LED_MODE_FASTPULSE = 8,
+    LED_MODE_FASTPULSE2X = 9,
+    LED_MODE_FASTPULSE3X = 10,
+    LED_MODE_FASTPULSE4X = 11
+} Led_Mode;
+
+extern uint8_t LedStates[2];
+extern uint8_t LedModes[2];
+
+#define LED_SET(i, onOff) { __disable_irq(); LedStates[i] = !!(onOff); __enable_irq(); }
+#define LED_MODE(i, mode) { __disable_irq(); LedModes[i] = (mode); __enable_irq(); }
 
 void LED_Init(void);
 
diff --git a/stm32/aioc-fw/Src/main.c b/stm32/aioc-fw/Src/main.c
index 20c31e2..bf343c1 100644
--- a/stm32/aioc-fw/Src/main.c
+++ b/stm32/aioc-fw/Src/main.c
@@ -66,10 +66,11 @@ int main(void)
 
     LED_Init();
 
+    LED_MODE(0, LED_MODE_SLOWPULSE3X);
+    LED_MODE(1, LED_MODE_SLOWPULSE3X);
+
     USB_Init();
 
-    LED_SET1(LED_IDLE_LEVEL);
-    LED_SET2(LED_IDLE_LEVEL);
 
     while (1) {
         USB_Task();
diff --git a/stm32/aioc-fw/Src/usb_serial.c b/stm32/aioc-fw/Src/usb_serial.c
index 1c6837b..f0fadc2 100644
--- a/stm32/aioc-fw/Src/usb_serial.c
+++ b/stm32/aioc-fw/Src/usb_serial.c
@@ -16,6 +16,7 @@ void USB_SERIAL_UART_IRQ(void)
             int32_t c = tud_cdc_n_read_char(ITF_NUM_CDC_0);
             assert(c != -1);
             USB_SERIAL_UART->TDR = (uint8_t) c;
+            LED_MODE(1, LED_MODE_FASTPULSE);
         } else {
             /* No char left in fifo. Disable TX-empty interrupt */
             __disable_irq();
@@ -29,6 +30,7 @@ void USB_SERIAL_UART_IRQ(void)
         if (tud_cdc_n_write_available(ITF_NUM_CDC_0) > 0) {
             uint8_t c = USB_SERIAL_UART->RDR;
             tud_cdc_n_write(ITF_NUM_CDC_0, &c, 1);
+            LED_MODE(0, LED_MODE_FASTPULSE);
         } else {
             /* No space in fifo currently. Pause this interrupt and re-enable later */
             __disable_irq();
@@ -132,19 +134,19 @@ void tud_cdc_line_state_cb(uint8_t itf, bool dtr, bool rts)
     if (dtr & !rts) {
         /* PTT1 */
         HAL_GPIO_WritePin(GPIOA, GPIO_PIN_1, GPIO_PIN_SET);
-        LED_SET2(LED_FULL_LEVEL);
+        LED_SET(1, 1);
     } else {
         HAL_GPIO_WritePin(GPIOA, GPIO_PIN_1, GPIO_PIN_RESET);
-        LED_SET2(LED_IDLE_LEVEL);
+        LED_SET(1, 0);
     }
 
     if (!dtr & rts) {
         /* PTT2 */
         HAL_GPIO_WritePin(GPIOA, GPIO_PIN_0, GPIO_PIN_SET);
-        LED_SET1(LED_FULL_LEVEL);
+        LED_SET(0, 1);
     } else {
         HAL_GPIO_WritePin(GPIOA, GPIO_PIN_0, GPIO_PIN_RESET);
-        LED_SET1(LED_IDLE_LEVEL);
+        LED_SET(0, 0);
     }
 }