Question for those in the know...

[Dave]

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

 

[o pere o]

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

 

[Dave]

"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 ....

 

[Rich Grise]

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