This page describes the design, implementation, and usage of the second generation of the Presto board, designed to support peripherals, radio control, and emergency stop for mobile robots.
Presto v2 introduces:
https://info.bannerengineering.com/cs/groups/public/documents/literature/159857.pdf
The TL50BL Banner Light Tower series is designed around the PLC ecosystem, which can make this a little confusing moving from a simple Arduino LED circuit. You just need to keep in mind that the switch in these diagrams is where the transistor goes.
The PC817 optocoupler is used to isolate the external light circuit from the main board. The BC337 NPN BJT was chosen due to its robust characteristics, but a 2N3904 could potentially suffice given low current.
R1 regulates current for the PC817 LED, calculated based on an assumption of a forward voltage of 1.2V and a forward current of 7mA.
R2 ensures saturation of the BJT. The PC817 has a collector-emitter saturation voltage of ~0.1-0.2V and the BC337 base-emitter on voltage is 1.2V. The required current was determined to be 6.25mA with a forced beta h_FE of 20 and assumped I_C from the light being 125mA. With a desired 6.25mA and expected voltage drop of 3.6V given a 5V input, R2 is determined to be around 576 Ohms.