Electronic – Using Raspberry Pi to read open collector outputs from tally connector

The 74LS05 is an open collector inverter, therefore it can only output a logic zero on the line (sinking it to the ground), but never a logic one. The line is basically at 1 by default (because of the 10k pull-up resistors) and can be driven to zero either by the inverter output or by the PIC.

Unfortunately, the open collector nature of the 74LS05 is not explicitly indicated in the schematics (although it is possible, generally by putting an asterisk or an open-collector symbol ⎐ near the output of the gate), thus rendering it more difficult to understand.

The LED has a voltage drop of 2v and a runs well on about 10-15ma, but how do I handle the capacitor in parallel?

The capacitor won't have any effect on the LED current after the circuit has been "on" for a few microseconds, so you don't need to worry about it when calculating the resistor value.

do I need a resistor between the optocoupler and B as the video would suggest?

You don't need a resistor.

You don't need to limit current through the optocoupler --- it won't draw any more current than what's stimulated as photocurrent by its LED (driven by the 15 V AC)

You aren't using the signal at the optocoupler's 'C' pin as a logic signal, so you don't need it to pull to a low voltage when the opto is active.

A current path into the 'B' pin could speed up the operation of the phototransistor, but for this kind of application, you don't need nanosecond response times.

It looks like there's a pathway, when the optocoupler is closed and the transistor is allowing current from E to C, that directly connects the 3.3v source to the GPIO pin without a resistor on the route (via the transistor) - is that a problem? Wouldn't it cause too much current to the GPIO pin?

Assuming you have the GPIO configured as an input (and this should be the power-up default), it will have a high input impedance, and won't allow more than a few microamps to flow into it.

A few k-ohm resistor would likely not change the behavior any and would protect you from overcurrent in the event you accidentally program the GPIO as an output and the driven value conflicts with the value generated by this doorbell circuit.