I thought before my trip that I would like to change the sensor that activates Talkie Toaster from an LDR (light Dependant Resistor) to a SRF05 ultrasonic sensor. In about 2 days I managed to change it over and write the software.
The SRF05 is an ultrasonic detector (as used in my Mini-me Dalek) that can send pulses and then knows distances to an object.
I love the SRF05 it is so easy to work with. Basically you send a pulse and then measure the length of the pulse you get back, divide the answer and you can get your results in metric or imperial based on what you divide by.

I had the most trouble placing it so it was invisible to people looking at Talkie while still peeping out enough that it would catch people in range.