The scenario
An industrial safety mat uses a force-sensitive resistor (FSR) to detect foot or pressure presence; when pressure is applied, the FSR resistance drops, raising the transistor base voltage and turning on the LED indicator.
What is in this drawing
Read the decisions behind it.
01
Use NPN transistor as threshold switch
02
Add capacitor C1 to debounce mechanical pressure transients
03
Place LED and current-limiting resistor in collector path so transistor saturates cleanly
Reuse this pattern for any pressure or force sensing indicator where a simple discrete threshold switch is preferred over a microcontroller.
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