Use a single diode as a temperature sensor with Arduino ICU
Our LED light-sensing experiments lead to an interesting observation: When these loggers are left running overnight they still produce readings because reverse-bias ‘leakage-current’ eventually triggers the Interrupt Capture Unit (ICU) – in the absence of any light. The speed of this self-discharge depends on the ambient temperature. If you deliberately cover an rgb LED with heat shrink, the different color channels have different rates of thermal decay
More details on Underwater Arduino Data Loggers blog.