How about this? Preparation: Pick some measurement interval in which you'd expect a bunch of clicks, e.g. a minute. Count how many clicks you get in that inter

How about this? Preparation: Pick some measurement interval in which you'd expect a bunch of clicks, e.g. a minute. Count how many clicks you get in that interval. Then take 138% of that time and divide it by the number of clicks you counted. That will be the interval for your timer. RNG: Tune the counter so it rolls over approximately once per time interval you've computed above. We'll take two measurements per interval, b0 and b1. Every time the counter rolls over, reset b0 and b1 to zero. When a click occurs, set b0 to one if the counter is less than half its maximum value, otherwise set b1 to one (if multiple clicks occur, the respective bit just stays at 1). When the counter rolls over, check b0 and b1. If they're equal, do nothing (discard the measurement). If they're different, take b0 as a random bit that you've just created (discard b1, and then reset b0 and b1 for the next round). You can then combine multiple successive bits into random numbers of the width that you want.