diff --git a/timer/main.c b/timer/main.c
new file mode 100644
index 0000000..5614053
--- /dev/null
+++ b/timer/main.c
@@ -0,0 +1,61 @@
+/*
+ * Control PB5 with the two buttons. One button disables, the other enables.
+ * The board uses interrupts for this. PB5 will quickly oscilate between on and
+ * off to make cool sounds with a summer.
+ */
+#define F_CPU 16000000UL
+#define __AVR_ATmega328P__
+#include <avr/interrupt.h>
+#include <avr/io.h>
+#include <util/delay.h>
+
+#define MINUTE 60 * 1000
+const int TIME = 45 * MINUTE;
+
+int volatile reset = 0;
+
+int alarm();
+
+// code for INT0
+ISR(INT0_vect) { reset = 1; }
+
+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; // pull down port B
+
+  // 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) {
+    _delay_ms(TIME);
+    alarm();
+  }
+}
+
+int alarm(void) {
+  int sound = 0;
+  while (!reset) {
+    PORTB |= (1 << PORTB5);
+    _delay_us(sound);
+    PORTB &= ~(1 << PORTB5);
+    _delay_us(sound);
+    sound++;
+    if (sound > 1000) {
+      sound = 0;
+    }
+  }
+  reset = 0;
+  PORTB &= ~(1 << PORTB5);
+  return 0;
+}