I'm trying to trigger a GPIO on an ESP8266 when something moves past in about a 4-5m range (e.g. car). So I thought about using an ultrasonic sensor but the distance while rated at 4m probably is limited to about 2m. IR/laser range sensors are about 1m exact. I have nothing that could reflect the signal on the other side. I can't use anything mechanical (i.e. ground based sensor). And I need it to be fast to trigger (within 50ms).
Any ideas out there of things I could use? Potentially I could have something reflecting a beam but that would need to be at least 5m away.
Cheers & Thanks!