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OfflineChuangTzu
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Circuit that drops voltage from Vsupply to ~0 as light levels go from 0-->ambient
    #15712932 - 01/24/12 06:32 PM (12 years, 6 days ago)

I'm trying to figure out the simplest way of lowering the voltage that a component sees as light levels increase.  The component should get almost supply voltage in darkness but should see almost almost 0V when illuminated with bright light.

I've brainstormed a few options using opamps and light dependent resistors, but it's hard to get the scaling right with those.

A digital solution would be easy but much more complex, so let's stick to analog options. 

This is for a battery-powered device so options that dump current to ground are non-optimal (that was my LDR voltage divider solution, but it didn't scale right anyway...)

If this could be done with 1 or 2 components, all the better...


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Invisiblekoraks
Registered: 06/02/03
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Re: Circuit that drops voltage from Vsupply to ~0 as light levels go from 0-->ambient [Re: ChuangTzu]
    #15714771 - 01/25/12 03:12 AM (12 years, 6 days ago)

How big is the load on 'the component'? I.e. how much power must your circuit put out?
Also, I assume you need a circuit that will gradually drop the output voltage as the ambient light intensity increases, right? How linear does the relationship between ambient light and output voltage that need to be?

Quote:

ChuangTzu said:
I've brainstormed a few options using opamps and light dependent resistors, but it's hard to get the scaling right with those.



Still, that would be one of the simplest solutions out there. You could substitute the LDR with a regular LED (yes, LEDs can sense light as well as emit it.)

Quote:

A digital solution would be easy but much more complex, so let's stick to analog options. 



It needn't be very complex if you use a small micro-cpu like one of the ATtiny models. Using this route makes it easier to achieve a linear relationship between light and voltage even if a non-linear sensor is being used.


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OfflineSeussA
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Re: Circuit that drops voltage from Vsupply to ~0 as light levels go from 0-->ambient [Re: ChuangTzu]
    #15714830 - 01/25/12 03:37 AM (12 years, 6 days ago)

Sorry for the low quality.  It is early and I haven't had coffee yet, but will this work?



Choose RBE such that:
RBE < VBD/Id
RBE > VBE/[IP - Vcc/(RL × hFE)]

Edit:

Oops, I missed: "This is for a battery-powered device so options that dump current to ground are non-optimal"


--------------------
Just another spore in the wind.


Edited by Seuss (01/25/12 03:50 AM)


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OfflineChuangTzu
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Re: Circuit that drops voltage from Vsupply to ~0 as light levels go from 0-->ambient [Re: koraks]
    #15715109 - 01/25/12 07:30 AM (12 years, 6 days ago)

Quote:

koraks said:
How big is the load on 'the component'? I.e. how much power must your circuit put out?




About 1-3 watts.

Quote:


Also, I assume you need a circuit that will gradually drop the output voltage as the ambient light intensity increases, right? How linear does the relationship between ambient light and output voltage that need to be?






It can be approximately linear, or not that linear.  It doesn't matter really as long as it isn't too close to a step function.


Quote:

It needn't be very complex if you use a small micro-cpu like one of the ATtiny models. Using this route makes it easier to achieve a linear relationship between light and voltage even if a non-linear sensor is being used.




I was hoping there would be just a couple components I could slap together to accomplish this, but I'm starting to think a PWM solution might actually be the best. In that case, going digital is probably actually simpler...


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OfflineChuangTzu
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Re: Circuit that drops voltage from Vsupply to ~0 as light levels go from 0-->ambient [Re: Seuss]
    #15715116 - 01/25/12 07:34 AM (12 years, 6 days ago)

Quote:

Seuss said:
Oops, I missed: "This is for a battery-powered device so options that dump current to ground are non-optimal"




Yeah, the first circuit that came to mind was a variable voltage divider.  Besides being impossible to scale properly, after drawing it I realized it could potentially draw more current when the component is "off" than when it's "on"...


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Invisiblekoraks
Registered: 06/02/03
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Re: Circuit that drops voltage from Vsupply to ~0 as light levels go from 0-->ambient [Re: ChuangTzu]
    #15715150 - 01/25/12 07:49 AM (12 years, 6 days ago)

Quote:

ChuangTzu said:
I'm starting to think a PWM solution might actually be the best. In that case, going digital is probably actually simpler...



I think a simple micro-CPU approach is going to be the best choice. You can easily make adjustments to the software without having to adjust the hardware design.

If you want to use discrete components only, then I'd recommend downloading LTSpice and working out a circuit that'll work for your application.


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