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selecting a photo transistor?

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Well-Known Member
want to detect daylight so (the PIR triggers the PIC all day long so inserting a timing code to disable the PIR durring daylight) I can disable the PIR.
found two photo transistors but having never worked with them I am asking for suggestions.
the two I am looking at are
Fairchild QSC112 for 18 cents a piece
Orasham SFH309F for 42 cents each
cheap usually comes out expensive so I want something that will work.


Well-Known Member
that I have but ?

how to designate an input as high or low durring daylight then determing darkness as a cDs is a light sensitive resistor?
resistance increases? (decreases?) with light.
I need to disable the PIR in daylight


Well-Known Member
Most Helpful Member
A CDS cell is a low resistance (300 ohms to 2k ohms) in light and a few Megohms in darkness. They take a few seconds to respond.


Well-Known Member
now its a matter which is easier?

a cDs just changes resistance while a photo transistor changes states.
need to configure a circuit for cDs. already have for photo transistor but maybe the photo transistor will consume more current than a cDs.
I even contemplated solar cells.
another option is the compariator in a PIC. I think the 18F1320 has one?? then just use that.
I recall seeing something about ADC using a PIC.
note I am just a newbi with PICs


New Member
A CDS from the PIC pin to +5 V and a 100K resistor from the pin to 0V.

The voltage at the pin will be:


100 K / 100K + 2K * 5V = 4.9 V


100K / 100K + 1000K * 5V = 0.45 V

The PIC will see that as logic 1 and 0.
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Well-Known Member
Would think that .45 is low??

depending on battery voltage.
the photo transistors I found are 18cents each
cDs ???
may do some experimenting.
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