LED flashing error code

Hans hans.ridder at gmail.com
Sun Feb 25 21:46:28 GMT 2018

Just to close the loop here...

I replaced avalanche transistor with an ebay 3904, and it didn't seem to fix the blink error (stuck at zero) that started this whole thing. I didn't have time to dig into it at that time and set it aside.

When I picked it up again, I was getting ~100mV of noise (compared to ~40mV before), which looked reasonable compared to a new OneRNG I've since received. However it was still failing. Probing around the circuit, comparing it to the new unit, the signals all looked reasonably the same to the eye. When comparing the old and new units under the microscope I noticed R11 (base pullup on the amplifier) on the new unit is 1M ohm instead of the 470k on the schematic(s) and old unit.

With that in mind, using the stats/measure functions on the scope, it seems the "average peak" levels (+ and -) were shifted maybe 10 mV or so between the two. Hard to say, it's "glitchy" to get those kinds of numbers. While probing around, it even manged to "work" (solid LED) a few times but I couldn't reproduce it. I hacked a 1M leaded resistor in place of of the 470k and it worked solid, though I was unable to make it fail again with a parallel resistor on that (even down to about 100k). Too much external noise?

Bottom line, the seemingly undocumented R11 change seems to have helped. It seems clear the circuit as currently designed is too sensitive to the base current in the amplifier transistor or other component variation, and needs a comparator or perhaps a second gain stage to ensure solid zeros and ones to the input to the microcontroller. No time to mess with those ideas for the moment.

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