diff --git a/buttons/main.c b/buttons/main.c
index 859f996..cfe75cd 100644
--- a/buttons/main.c
+++ b/buttons/main.c
@@ -1,21 +1,36 @@
+/*
+ * Control PB5 with the two buttons. One button disables, the other enables.
+ * The board uses interrupts for this.
+ */
 #define F_CPU 16000000UL
 #define __AVR_ATmega328P__
+#include <avr/interrupt.h>
 #include <avr/io.h>
 
-int main(void) {
-  DDRB = 0xff;
-  DDRD = 0;
+// code for INT0
+ISR(INT0_vect) { PORTB |= (1 << PORTB5); }
 
-  PORTD |= (1 << PORTD2) | (1 << PORTD3);
+// code for INT1
+ISR(INT1_vect) { PORTB &= ~(1 << PORTB5); }
+
+int main(void) {
+  DDRB = 0xff; // write to B
+  DDRD = 0;    // read from D
+
+  PORTD |= (1 << PORTD2) | (1 << PORTD3); // pull up D2 and D3
 
   PORTB = 0;
 
+  // enable external interruprs
+  EIMSK |= (1 << INT0);
+  EIMSK |= (1 << INT1);
+
+  // trigger when anything changes
+  EICRA |= (1 << ISC00); // for INT0
+  EICRA |= (1 << ISC10); // for INT1
+
+  sei(); // interrupt magic macro
   while (1) {
-    if (!(PIND & (1 << PIND2))) {
-      PORTB |= (1 << PORTB5);
-    }
-    if (!(PIND & (1 << PIND3))) {
-      PORTB &= ~(1 << PORTB5);
-    }
+    // do nothing
   }
 }