Detecting light via Photodiode/Phototransistor

Thread Starter


Joined Mar 17, 2009
Hello guys

We are a couple of guys doing our final project in electronics at our school. We want to make a device for a car, that can sense light from other cars and turn down the headlights. We plan to do this by using an LDR, photodiode or photo transistor - the question is just if it'll be able to sense the light of a car more than 100 meters away.
We're also wondering how we should calculate this? I assume it has something to do with lumen and lux, but how exactly would we do?
Another problem is the rear lights. Is it possible to get a photodiode that can read only wavelengths of 600nm and above (for red light) and another one to get the light from the headlights?

Thanks in advance


Joined Apr 20, 2004
You would need to use filters to be able to sense head/taillights.

Such devices were using CDS cells back in the 1950's. One problem you're going to face is highly reflective road signs. If you have enough sensitivity to dim lights when the oncoming car is 100 meters off, then most signs will reflect enough of your high beams to also be detected. I had this problem in a Lincoln a few years ago.

The immediate solution seems to be mounting a sensor low enough to see only headlamps and not signs. That may mean a mount below the windshield, so keeping the sensor free of road dirt becomes a problem. Interesting problem.

For sensors, you might find the stuff at the linked site interesting (and affordable) -


Joined Jan 18, 2008
I would also suggest adding a significant reset delay (100 yds at 60 mph each = 1.7 seconds closing time), say 4 or 5 seconds. It is annoying and may send an unintended message to have on on-coming car blink its lights as irregularities or such in the road may make the sensed intensity vary.