Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: harpo on April 11, 2011, 08:24:24 PM

Title: newb photo cell quetion
Post by: harpo on April 11, 2011, 08:24:24 PM

i have seen photo cells (own a few) resist in the dark but let power flow in the light
are there photo cells that do the opposite?
if so can you send me a link to a shop for them?

Title: Re: newb photo cell quetion
Post by: MikeK on April 11, 2011, 09:13:51 PM

That's how they behave: http://en.wikipedia.org/wiki/Photoresistor (http://en.wikipedia.org/wiki/Photoresistor)

Title: Re: newb photo cell quetion
Post by: vinniewryan on April 11, 2011, 09:40:22 PM

Though if you connect the output of a photocell to the base of a PNP transistor with a pulldown resistor, you've just basically reversed the photocell's output through the transistor to output high when there's no light, and low when there is.

Title: Re: newb photo cell quetion
Post by: Soeren on April 15, 2011, 10:12:15 PM

Hi,

Quote from: harpo on April 11, 2011, 08:24:24 PM

i have seen photo cells (own a few) resist in the dark but let power flow in the light
are there photo cells that do the opposite?

LDR's (if that's what you're referring to have more resistance in darkness than in light and there are none that are opposite that.
How you use that function is up to you however and an inverting circuit will get you the opposite function.

Which circuit is the best depends on what you need, binary action (i.e. light/dark) or a linear varying output.