I am using (attempting to at least) a photodiode to count fringes from a Michelson interferometer. Here is the amplification circuit I am using: Coming out of the op-amp into the comparator I get about 4.2V in a well lit room, 2.17V in total darkness, and 2.33V when the brightest part of the interferometer fringes are hitting it. That range 2.33-2.17 is my problem. I am having a hard time getting the reference voltage of the comparator at a spot where I get a count on every fringe, but don't get extra counts. Is there something I can do in the circuit to make the voltage range between bright fringes and dark fringes bigger? I can't get any more intensity out of the laser, and I have the spot the smallest I can trust it. EDIT: I missed something. I ended up using the comparator to make sure my voltage into the debouncer and pulse counter went high enough and low enough to make counts. Notice that the lowest voltage I got coming out of the op-amp was around 2V. I was under the impression that for this configuration, when there is total darkness (or I pull that photodiode out of the circuit), that the output of the op-amp should go to 0, since the non-inverting input is sent to ground. What's the deal with that?