A deep dive into squeezing more speed and reliability from low-cost optoisolators, particularly the 4N35 series. Covers three key factors affecting bipolar transistor switching speed: base-emitter capacitance, base-collector (Miller effect) capacitance, and base-collector saturation. Practical circuit techniques include adding a base-emitter resistor (~100kΩ) to speed up turn-off, a Schottky diode clamp (BAT54) to prevent saturation, a speedup capacitor on the LED input, and an output buffer stage using a 2N3906 PNP transistor to reduce phototransistor output swing. Also includes a cautionary engineering story about the hidden costs of over-optimizing for price too early in a product's development cycle, using a failed battery charger project as the example.
