Optoisolators

• Problem: motors are very electrically noisy
• Solution: isolation -- no electrical connection between logic and power
• Relay is already isolated
• FET works by Vgs allowing Ids -- "s" must be common?
• FET is on "power" side of circuit
• FET gate is driven not by logic output, but by phototransistor output
• Phototransistor is turned on by shining LED onto it
• LED lit by logic
• Only photons travel from logic side to power side; no noise comes back
• Inexpensive, in DIP package, with 1-4 optoisolators per package

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