The scenario
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.
What is in this drawing
Read the decisions behind it.
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.
More like this