상황
Two laser beams across the road detect a vehicle; the start sensor triggers a 555 monostable, and if the stop sensor closes before the pulse ends, the vehicle exceeded the speed threshold and an alarm lamp lights.
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01
Use a 555 monostable to create a fixed time window
02
Model optical sensors as pushbutton switches for schematic clarity
03
Drive an alarm lamp with a BJT switch gated by the stop sensor
Reuse for any two-point speed gate or presence timer where a fixed detection window and threshold comparison are needed.