# Question for those in the know...

Started by 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
|          |
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
> |          |
> 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
>

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!