Cars regularly exceed the speed limit in our street


Measure each car's speed and clearly display the speed to the driver


  1. Measure the time the car takes to cover a set distance (e.g. 25 meters) to calculate the speed
  2. For this I'd set up two sensors and calculate the difference between the timestamps recorded between the car passing the first and second sensors -> then convert into km/h
  3. Display the result on an LED screen


A. What sensors should I use here? Photoresistor with laser diode, digital or analog IR sensor, ...

B. How can I best cover the distance between the sensors and connect them to the Arduino? Long wire, wireless, connect two Arduinos, ...


  • radar is used to measure speed of cars. google found sierzega.com/en-us/about-us/our-products-services/…
    – Juraj
    Aug 29, 2020 at 12:56
  • 1
    Thanks @Juraj. I posted a question regarding a 150$ project on an Arduino forum so you send me a link to buy 2000$ ready-made systems... Sorry, not very helpful at all. The point is to build stuff! :-)
    – Uralan
    Aug 29, 2020 at 13:11
  • @Uralan your post says nothing about a price limit
    – jsotola
    Aug 29, 2020 at 15:58
  • Okay 👌 that's true.
    – Uralan
    Aug 29, 2020 at 15:59
  • eevblog.com/forum/projects/…
    – jsotola
    Aug 29, 2020 at 17:55

1 Answer 1


Why 25meters? If you place the two sensors 1 meter apart, you only need 1ms accuracy to get km/h (assuming a 50km/h speed limit). The Arduino can handle 1ms resolution with ease.

I'd probably go with 3-5 meters, so you don't need your setup to be that precise (i.e. distance and alignment of the two sensors). Lowering the distance also makes it less likely for two cars, from both directions, to simultaneously be inside your sensor area.

For the sensors, the biggest problem is the legal one. Having objects on/near the road could cause some liability issues. The laser solution is probably the least likely to cause this issue. Though I'm unsure about having a display next to the road.

You could go with laser and photo-transistor on one side of the road. And a reflector/mirror on the other side. I'd probably go with an IR laser, so it's invisible. You'd have to shield the photo-transistor as much as possible from ambient light, so a sudden cloud won't trigger it. You probably still need to do some filtering in software.

There are radar modules available, but I think they are pretty expensive.

Another option is a Laser range sensor, like this one from sparkfun

  • Thanks! Good points! I will probably go for IR laser curious to see how well it'll all work...
    – Uralan
    Aug 29, 2020 at 13:12
  • You need 3 ir/laser emitters, mirrors and phototransistor sensors to get the car's direction and some reliability. Good luck with the legal issues. Aug 29, 2020 at 15:34
  • You can also modulate the laser, similar to how tv remotes modulate the signal, so other IR light sources don't interfere. You could maybe even use IR receivers, instead of phototransistors.I don't have a great solution on how to align the system, so the laserbeam hits the sensor. Maybe some crappy phone camera that doesn't have an IR filter, so you can see where it's pointing.
    – Gerben
    Aug 29, 2020 at 16:09
  • The alignment issue would be solved by using a corner reflector instead of a mirror. See also Retroreflector. Aug 29, 2020 at 19:37

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