I'm a newbie at designing at the nanopower level, hence my question. In this circuit, I have plastic fiber optical inbound signal that drives D1 photodiode, whose dark current is extremely low (tens of nanoamps). It will be an RS-485 like signal up to 56Kb maximum.
With no fiber optic signal, I need the quiesce current of all components to be as low as possible to maximize battery life. The N-Channel enhanced mode MOSFET BSS138 is used as a switch to drive two outputs: 1) a fiber optic LED used as a repeater, and 2) a UART RxData and external interrupt line into a PIC16LF MCU. R10 was chosen as a Very Weak Pull-up to minimize quiescent current at around 600 nA.
I'm also unsure if my attempts to use the single BSS138 for dual purposes like this is going to work. The MCU UART needs a high signal (RS-232 mark) with no optic signal present. The IF-E96E datasheet suggests it requires many mAs to conduct, so unsure how that's going to factor in.
So, am I on the right track here or heading into the ditch? If it's the ditch, how can I keep component costs and quiescent power at a minimum here?
In particular, when JP2 is not installed, taking the photo LED out of the circuit, will 600 nA be enough current to create flow and voltage drop across the BSS138? How low can I realistically go with R10 and still expect reliable operation?
Many thanks. Rick