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Question for those in the know...

Started by Dave October 7, 2011
Hate to be obtuse, but its been nearly 40 years since I was in school, and I 
am now out of practice.  Any idea as to how I could use a photoresistor (and 
sunlight) to turn off a transistor that would otherwise be conducting like 
crazy?  Please see my post in ABSE for the particulars of what I am working 
with.  Many thanks for any help...

Dave


On 10/07/2011 12:10 PM, Dave wrote:
> Hate to be obtuse, but its been nearly 40 years since I was in school, and I > am now out of practice. Any idea as to how I could use a photoresistor (and > sunlight) to turn off a transistor that would otherwise be conducting like > crazy? Please see my post in ABSE for the particulars of what I am working > with. Many thanks for any help... > > Dave > >
Without looking at your posts, to turn off a transistor with sunlight you first have to be able to get a "low" voltage when sunlight is present and "high" voltage when dark. Since an LDR exhibits a low resistance in sunlight, this voltage divider does exactly this: Vcc | R | o----- Vout = PhR/(PhR+R)*Vcc | PhR | GND When dark, PhR is very high (PhR>>R) and the circuit is almost equivalent to a) Vcc-----R-----x In sunlight, PhR is low (PhR<<R) and the circuit is almost equivalent to b) Vcc*Phr/R-----Phr----x Now, your design goal is that the transistor turns on when connected to circuit a and turns off when connected to the circuit b. Specific values depend on the values of PhR. A typical LDR may exhibit 1K or less in the sun and 1M when dark. Then, R between 10K and 100K should work ok. Vcc Vcc | | | load R | | C o----x--- B | E PhR | | | GND GND Pere
"o pere o" <me@somewhere.net> wrote in message 
news:j6ml9s$ql9$1@dont-email.me...
> On 10/07/2011 12:10 PM, Dave wrote: >> Hate to be obtuse, but its been nearly 40 years since I was in school, >> and I >> am now out of practice. Any idea as to how I could use a photoresistor >> (and >> sunlight) to turn off a transistor that would otherwise be conducting >> like >> crazy? Please see my post in ABSE for the particulars of what I am >> working >> with. Many thanks for any help... >> >> Dave >> >> > > Without looking at your posts, to turn off a transistor with sunlight you > first have to be able to get a "low" voltage when sunlight is present and > "high" voltage when dark. > Since an LDR exhibits a low resistance in sunlight, this voltage divider > does exactly this: > > Vcc > | > R > | > o----- Vout = PhR/(PhR+R)*Vcc > | > PhR > | > GND > > When dark, PhR is very high (PhR>>R) and the circuit is almost equivalent > to > > a) Vcc-----R-----x > > In sunlight, PhR is low (PhR<<R) and the circuit is almost equivalent to > > b) Vcc*Phr/R-----Phr----x > > Now, your design goal is that the transistor turns on when connected to > circuit a and turns off when connected to the circuit b. Specific values > depend on the values of PhR. A typical LDR may exhibit 1K or less in the > sun and 1M when dark. Then, R between 10K and 100K should work ok. > > Vcc Vcc > | | > | load > R | > | C > o----x--- B > | E > PhR | > | | > GND GND > > Pere
Why, *thank you* Pere. Between your post and that of Phil Hobbs, I think I'm set. Many, many thanks. Dave
>
On Fri, 7 Oct 2011 05:10:05 -0500, "Dave" <db5151@hotmail.com> wrote:

>Hate to be obtuse, but its been nearly 40 years since I was in school, and I >am now out of practice. Any idea as to how I could use a photoresistor (and >sunlight) to turn off a transistor that would otherwise be conducting like >crazy? Please see my post in ABSE for the particulars of what I am working >with. Many thanks for any help... > >Dave >
http://www.evilmadscientist.com/article.php/nightlight Simple cheap.... AND it uses a photo transistor which would be immune to degradation from high humidity or water leaks. But I'd use a mosfet ....
Dave wrote:

> Hate to be obtuse, but its been nearly 40 years since I was in school, and > I > am now out of practice. Any idea as to how I could use a photoresistor > (and sunlight) to turn off a transistor that would otherwise be conducting > like > crazy? Please see my post in ABSE for the particulars of what I am > working > with. Many thanks for any help...
Sure! http://www.google.com/search?hl=en&source=hp&q=photoresistor+transistor+circuit Have Fun! Rich