Hardware changes to improve sensitivity

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

Initial version of working hardware

Known issues:

On PCB following fixes are needed:

• D1 is BPW34
• U1 is OPA657U
• U2 is OPA656U
• 2p capacitor is connected in parallel with R6
• 820k resistor is connected between U2 out and BNC socket.