Known issues:
On PCB following fixes are needed:
- U1 is OPA657U
- U2 is OPA656U
- R4 is 1G
Improvements:
- biasing photodiodes to decrease capacitance
- buffering opamp at analog output
- 12V digital output (low on pulse detection, high on regular state)
- 3 photodiodes instead of 2 for better sensitivity