 Attiny85v
 - If not enough light SLEEP.
 - Turns on by high on pin interrupt. 
-  The pin is attached to the voltage divider that measures the resistance of the “up” LDR. 
+  The pin is attached to the voltage divider that measures the resistance of the LDR. 
   When dark, the voltage is less than the voltage required to register as HIGH.
 - Measures the battery voltage. 
 - Every minute (may need to reduce this) sends data over RF 

  //wdt_reset();

  // @todo Only signal master wake condition if not in SPI mode.
  if (ldr_val < THRESHOLD_DARK) {
    // Too dark
    // Let atmega328p know it should be asleep
    digitalWrite(PIN_WAKEUP_MASTER, LOW);
    

#define LED_DEBUG 0 
#define PIN_TX 3 // PB3

#define PIN_WAKEUP_MASTER 1 // PB1 Connected to the atmega328 external interupt pin.

#define PIN_WAKEUP_SELF 4   // PB4 / pin 3 / LDR attached
//#define PIN_LDR 2           // PB4 / pin 3 / A2 - NB: ANALOG pin 2 not digital pin 2

//#define THRESHOLD_DARK 200 // A reading below this is considered dark.

#define I2C_SLAVE_ADDR 100

