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Light Intensity Level Switch

KFriend

Nov 28, 2016
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Hello,
I'm looking for help in designing a device that can be aimed at a light source and measure the current level of light. This light level will be used as a threshold to monitor an increase in light intensity. If the level of light is exceeded by (x) amount, I need to activate a 12vdc relay output. The light level threshold needs to be set by pushing a button.

I've looked around, but haven't seen anything similar to this that s commercially available. If there is such a device I would appreciate directions on where to purchase. If not, I'm willing to build and experiment myself.

Thanks.....
 

AnalogKid

Jun 10, 2015
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How bright is the light?
How far away is it?
Is the light level changing slowly? This can cause relay chatter unless you add hysteresis.
Is this indoors, outdoors in weather, exposed to caustic fluids or high temperatures?

ak
 

Alec_t

Jul 7, 2015
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Is the device a fixed distance from the light source?
 

KFriend

Nov 28, 2016
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How bright is the light?
How far away is it?
Is the light level changing slowly? This can cause relay chatter unless you add hysteresis.
Is this indoors, outdoors in weather, exposed to caustic fluids or high temperatures?

ak
The distance is about 35 feet. Lights change fast (LED Floods). Outdoors but never in the rain. No fluids of any kind. Ambient temps less than 120F.
 

AnalogKid

Jun 10, 2015
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So far, sounds like a fairly straightforward problem. One thing - are you familiar with the concept of hysteresis in electronic circuits? If not, you should read up on it. You have described what you want the circuit to do as the light level increases, but not what to do when it decreases.

ak
 

Alec_t

Jul 7, 2015
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The distance varies from 30-40 Feet from the source.
Assuming an inverse square law for irradiance as a function of distance, that implies a 16/9 variation in the measured light level which will have to be allowed for. Light beam shape/focussing would determine how close to an inverse square law the function actually was. How will distance change be monitored?
How will you distinguish between wanted light from the source and unwanted ambient light?
 

KFriend

Nov 28, 2016
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The receiver would be aimed in the direction of the light source and would then read the light level available at that time. Then if the light intensity increases by a set ammount, it would trigger an output.
 

AnalogKid

Jun 10, 2015
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So something needs to store one light level so it can be held and compared with a later light level? If so, stored for how long, and what tells the receiver when to capture a new reference level?

al
 
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