The scenario
This circuit ensures two safety doors are closed before enabling a machine. Each door has a normally-closed switch that illuminates a 4N35 LED; the phototransistors are wired in AND configuration, so the output goes low only when both doors are closed.
What is in this drawing
Read the decisions behind it.
01
Using NC switches for fail-safe operation
02
AND logic with series phototransistors
03
Optical isolation for noise immunity
Industrial machinery with multiple access doors requiring a safe start condition.
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