> Any high-gain electronic amplifier whose input is connected to a resistor, or for a higher signal level, to a diode, will produce copious amounts of random noise at its output. If the output is converted with a comparator to digital, you have a good source of random bits.

These are so fun. One time I was able to get a device to replay an entire WPA2 handshake by triggering a signal generator with its reset line and feeding the output to its rng ADC. Good parlour trick that one.