allanhurst:
You'll get a random load of transitions...
... and the digital inputs of the arduino can count at up to 8MHz
To be honest I don't like thresholding. It automatically reduces the measured entropy rate by a decade as you down sample from 10 bits to 1 bit. You then just have to reassemble all the bits into bytes to transmit them over Ethernet, and you still have to process them again to make them truly random. Simply thresholding a noise source into bits won't make cryptographic strength random numbers.
Also, I'm not sure that 8MHz is anywhere near the practical bandwidth of the pentodes I have. I think that we're looking at audio frequencies. I can't easily test /trouble shoot 8MHz frequencies but I can hear audio. Plus I now have my super dooper PC oscilloscope to see it too 