[dragontamer]

> Couldn’t you just hook up a light sensor, temperature sensor, etc and take the last few decimal places of the reading for a random number

Heat-entropy is of course one of the best and truest sources of random number generation. But a temperature sensor is far more complicated than what you actually need.

All resistors vary their resistance by temperature. This is called Johnson Nyquist noise (https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise). Effectively, every resistor you have on the board is generating white noise.

The question is how to cleanly separate the white noise out, amplify it to measurable levels, and then how to feed that into a computer. Various "white noise generators" trace their true entropy to heat noise (ex: Intel's RDRAND assembly instruction has an oscillator which likely varies due to circuit-level heat noise).

I bet that the voltage across any 10 MOhm resistor would be very noisy, and that could probably be a source of noise for any hardware generator design. The issue with MOhm level resistors is that you start to vary the resistance with physical interaction (a human is in the single-digit MOhm region: so if a human touches the circuit board, the circuit may drop its resistance down to 5MOhms or less, which could affect your circuit design very severely).