 void loop()
 {
   
-  int ldr_val = analogRead(PIN_LDR);
+  int ldr_val = digitalRead(PIN_WAKEUP_SELF);
 
   digitalWrite(LED_DEBUG, HIGH);
   

#define LED_DEBUG 0 
#define PIN_TX 3 // PB3

#define PIN_WAKEUP_MASTER 2 // PB2 (SCK) Connected to the atmega328 external interupt pin & SCK.

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

#define THRESHOLD_DARK 600 // A reading below this is considered dark.

#define I2C_SLAVE_ADDR 100


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 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 
