I was planning to see if I can make a UV meter using an EPROM. The idea is that ultraviolet light erases an EPROM. When a fully programmed EPROM (all zeor's) is exposed to uv light I should clearly see the number of bits (one's) increasing.
So I have built a small device that counts the number of bits (one's) in an EPROM. I had guessed that the speed of increase of bits would certainly be a measure of the UV intensity.
But...
When exposing the EPROM to uv-light I expected a smooth and stable increase of bits, but that didn't happen. In stead, after some time of exposure to uv-light the bitcount begins to vary because of the bits (all Floating Gate devices) become unstable.
Knowing that the bits become unstable, I covered the EPROM window with aluminium and watched what would happen.
As I expected the bitcount still kept varying (I measure 64k bit 4 times per second). I have a built in threshold of 50, and everytime 2 measurements have a difference of 50 of more I show a flash on an LED.
The device is flashing seemingly random between 20 to 60 flashes per minute. However it is not compltele random, because I see patterns over long periods of time (days).
Clearly, some effects help to flip the very unstable bits, but I don't know what it is that makes them flip. So my question is: What is it that I measure?
Can anyone give me a clue?