Re: LDR / photo resistor - increasing resistance with increasing light???

From: John Popelish (jpopelish_at_rica.net)
Date: 10/20/04 

Date: 20 Oct 2004 10:04:30 -0700

myothermailishotmail@gmail.com (mike) wrote in message news:<f6382c10.0410192250.7f7a9a34@posting.google.com>...
> wow, thanks for the response. here is exactly what i want to do:
>
> i live in a basement w/o windows, and i want to rig inside light so it
> coincides with the outside light. i was figuring i could run an LDR
> placed outside, to a dimmable ballast in my basement apt.
>
> i have an electronic dimming ballast, made by the now defunct JRS
> Technology (#C232120RS501). it has a dimming control by way of a 0 to
> 10v class 2 circuit, 0.5mA output. it is 0-10v out, and by connecting
> a 0-100k ohm potentiometer to this circuit i can dim the connected
> flourescent bulbs no problem-- the greater the resistance, the greater
> the V, the greater the resulting brightness.
>
> however, what i want to do is connect an LDR in such a way that a
> greater amount light hitting the LDR makes the bulbs brighter. this
> means that in order to get brighter bulbs, i need a greater voltage
> accross this circuit, which means i need greater resistance connected
> to this circuit (i think, right?)
>
> i have an LDR (from a radioshack multi-pack) connected in series (with
> a 0-100k trimmer/potentiometer) to this 0-10v circuit, and it does
> what i want it to do, only in reverse. that is, as lighting
> conditions on the LDR get brighter, the dimmer makes the bulbs get
> dimmer.
>
> this makes sense since an LDR decreases resistance as more light hits
> it. what i need is increasing resistance with more light. or a
> circuit that simulates this.
>
> if this looks familiar, i posted 6 weeks ago before i had any idea
> what i was talking about (thanks to those who offered suggestions):

You can make a voltage divider with a light dependent resistor in
series with a fixed resistor that has the LDR in either the pull up or
pull down side of the divider. If you have a 10 volt supply available
(a wall wart, perhaps) or if this voltage is available from the
ballast, you can experiment with the LDR and fixed resistor replacing
the potentiometer. Varying the value of the fixed resistor shifts the
light level that produces about half light output. 

-- 
John Popelish

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