I wouldn't do it (at least, not unless I was being slap-dash in a quick temporary something-or-other). Howvever, if you're sure you'll never misconfigure your Pi or your physical wiring you could use the builtin pullups and omit all the resistors on the output side of teh opto-isolator. This delivers accurate differential measurements on reference. Some people may even refer to them as options. IsoVu Probe Technology virtually eliminates common mode interference using optical isolation. If you add your own external pullups it's easier to predict what happens in circumstances like this, If you rely on the chip pullup then the protection resistor gets in the way of teh voltages, because it acts as a voltage divider with the pullup on the chip (if that's a simple resistor, or behaves as one). You may refer to it as an optoisolator, photocoupler, optical coupler, optical isolator, or just optocoupler. I woudl use a bit higher than 1k, but teh value isn't very critical. In the circuit of Figure 1d, the initialresistan. A simple way to avoid these problems is to use an AOIin place of a mechanical switch. Sudden changes in signal cancause the speakers to thump and damage may occur if the speaker isunderdamped. These are mainly used in free space optical system as the system maintains the polarization of the source. More precisely, they exhibit a relatively low propagation loss in one direction, but a much higher propagation loss in the other direction. Polarization dependent isolator: A polarization dependent isolator is composed of mainly input and output polarizer along with Faraday rotator. Optical isolators (also called optical diodes) are devices which transmit light in one direction but not in the opposite direction. This is the 'protection resistor', and limits teh currents in such a case. Mechanical switching of low level audio signals requires the use ofswitches with precious metal contacts. An optical isolator is majorly classified into the following categories: These are mainly categorized on the basis of their polarization characteristics. Without the 1k you'll get high currents into (or out) the GPIO. Definition: devices which transmit light in one direction but not in the opposite direction. Through control over the hydrophilicity of a simple wick. The 1k in the posted schematic protects the Pi in case the optoisolator drives its output high and the Pi is accidently configured to drive the GPIO low (ie you set it to an output and drive it low) or vice-versa. very intriguing properties and applications in photocatalysis and magneto-optics. You don't absolutely need a pull up / down resistor or protection resistor on the output of teh opto-isolator (input to teh Pi).
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