Light Sensor to be used in fog

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elwyn

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Hello, new to the forum, currently at college for a apprenticeship and we have been tasked in designing and making a project, mine is for a light to come on during low visibility. My current idea is a form of light sensor using maybe a LED that switches on a light when the visibility of the LED is reduced.

How ever, i cant seem to find a DIY kit/circuit or sensor for the job, i wondered if anyone on here had an idea, circuit or link to where i could find them.

Alternatively i would be more then happy for other ideas

thank you
 

hi,
One way that is used is to have light path between two points.
The light source is a IR emitter and the receiver is a IR photo diode.
The project requires lenses and a collimator for thr receiver.

This method is used on motorways for fog warning systems.

How far apart do want to have the light emitter to detector.?
 
well i need it to be used in a car as thats where the idea has come from, but for the project i really only need it to work so could be any distance just to show the idea works, so lets say for example up to 300mm from emitter to detector?
 
elwyn said:
well i need it to be used in a car as thats where the idea has come from, but for the project i really only need it to work so could be any distance just to show the idea works, so lets say for example up to 300mm from emitter to detector?
Click to expand...

A 300mm path isnt going to give a big change in light intensity, unless its a real 'pea souper'...
 
what would be the best distance? just saying that size so i can show its good for automotive use
 
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Just throwing another idea out there, why not use a humidity sensor, since that's when fog happens (very high humidity). It's a bit more expensive (actually after checking its only 10$ US), but I think it would be more accurate? Perhaps the response of a sensor isn't fast enough (I've never used one).

Temperature Sensors

Humidity and Temperature Sensor
 
Try googling 'fog detector'. There's plenty of useful hits on the first page.
https://www.electro-tech-online.com/custompdfs/2009/10/FD-310A.pdf
**broken link removed**

Anyway, it seems IR backscatter is common for a single unit 'fog detector', using modulated light. There will be problems if other objects (e.g. cars) come in the path of the light; e.g. if you're up the rear of a white car, then you'll get a lot of backscatter (e.g. sensor will think it's thick fog) and your fog lights will come on. Also, ambient light will affect the characteristics of the IR detector and will likely need to be compensated for.
 
elwyn said:
cheers for that, these wouldn't be affected by rain would they?
Click to expand...

Rain is 100% humidity, so I assume so, you could always ignore the sensor when humidity is above 90% or something (some set point). The bonus is you do not need to worry about how it is strapped down, the ir led must be facing the sensor and perhaps vibrations and such will throw that off.
 
cheers for the ideas guys, i have googled fog detectors when i took up the project but main problem is most kits are for weather stations/motorways which are fixed and most are in the £1000s where as im looking to make a kit that you could get from a motorfactors or on the options sheet for a car.

Have also looked at where it could be placed since i have taken in to account that other road users could throw the sensor out including vibrations caused from the car.

Might have to buy a few sensors and have a play, would i need something to transfere the signal to say a light switch? sorry if i lack knowledge on electronics, i'm more mechanical by trade
 
Depends which idea you plan on using, IR or Humidity?

You will need some sort of logic processor, either analogue or digital.
 
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If you point the detector up you shouldn't have to worry too much about detecting objects (except perhaps overpasses).
To minimize ambient light effects you could use a light filter plus if the beam is modulated, only AC signals will be detected.
 
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