Dave
Guest
Fri Oct 07, 2011 10:10 am
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
Guest
Fri Oct 07, 2011 10:46 am
On 10/07/2011 12:10 PM, Dave wrote:
Quote:
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
Guest
Fri Oct 07, 2011 11:15 am
"o pere o" <me_at_somewhere.net> wrote in message
news:j6ml9s$ql9$1_at_dont-email.me...
Quote:
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
>
Guest
Fri Oct 07, 2011 3:15 pm
On Fri, 7 Oct 2011 05:10:05 -0500, "Dave" <db5151_at_hotmail.com> wrote:
Quote:
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
Guest
Fri Oct 07, 2011 9:47 pm
Dave wrote:
Quote:
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
