"John Fields" wrote in message
...
On Wed, 19 Oct 2011 13:05:27 -0500, "Dave" wrote:


"John Fields" wrote in message
. ..
On Mon, 17 Oct 2011 20:41:06 -0500, "Dave" wrote:


"Phil Hobbs" wrote in message
...
On 10/17/2011 06:53 PM, Dave wrote:
"Phil wrote in
message
...
On 10/07/2011 06:36 AM, Dave wrote:
As per my post in SEB, I am trying to use a photo resistor and
sunlight
to
turn off a transistor that would otherwise be conducting like
crazy,
but
can't quite make the leap of comprehension as to how to actually do
this.
Please see attached schematic parts that show more or less what I
am
trying
to do this with. No part number for the photoresistor, sorry. It
drops
from multi-megohms in the dark to single-digit Ohms in the light
and
seems
perfectly capable of carrying the 30 to 50 mA current I am working
with
at
the voltages indicated. Would really appreciate it if someone
could
offer a
hint as to how I should proceed with as few components as possible
(small
circuit board.) I hope that my method of posting the schematic
portion
of
what I a working with is not too obtuse. Couldn't figure out any
other
way
to do it...

Many thanks...

Dave




You're wasting a gross amount of current in that bias network. How
about
a nice MOSFET?

Say a 2N7002, source to V-, drain to series resistor+LED, LDR plus
pot
to
set turn-on level:

(V+)----*----------------*
| |
| R
| R
| R
| R
*---* |
| | ---
| R \ / -----
*- R ----- -----
R |
R | | D
| | |---*
| | |--; 2N7000
*----------J |---*
| | | S
| |
L |
D |
R |
| |
| |
(V-)----*----------------*




Man that was easy. Just had to get the MOSFET. Thank you, Phil
Hobbs.

I spent so much time wrestling with that other configuration, and
this
just
flew together. And damn, but it works nice. Much appreciated.

---
Geez, I feel kind of miffed since my (Jan's) solution would allow you
to rid yourself of the LDR and its associated circuitry.

With that in mind, why would you find Doctor Phil's circuit, which is
wasteful of power and money by having it drive an unnecessary LDR,
superior to mine?

--
JF

Hey John. No offense intended, believe me. Your ASCII description
translated into a schematic where my tiny photocell (I'm guessing that's
what V3 was) was set up as 4 VDC, rather than .4 VDC, which is what it
actually put out. And even then, when I set it all up, it looked like it
ought to work, but it didn't. No idea why, other than that there is
something about the tiny photocell that I don't know. Finally gave up on
using that component. When my Digikey parts came in I put them together
and
they took off. That version worked, so I used it. Now I'm trying to find
a
6V solar cell smaller than the 6"X6" behemoth I currenty have in place.
Will probably go with one from Amazon that is 4"X3" and flexible.

---
No problem, but the circuit I was referring to wasn't the one with the
tiny PV, it was the one with a PNP driving the LED which I posted as
an LTspice circuit list along with two other circuits.

Here it is in ASCII:

. 2N3906
. +-[1N5817]-+---------E C-+
. | | B |
. +-----------|-----+ | |
. | | | | [42]
. |+ |+ [POT]--+ |
. [PV] [BAT] |10k [LED]
. | | | |K
. +-----------+-----+---------+

Notice no LDR and its associated resistor. :-)

Anyway, as long as you have something that works, that's what matters.

BTW, make sure that you get a PV array with the capacity to charge the
battery enough, during the day, so that the LED stays on for as long
as you need it to at night.

--
JF

Hey again. Oh, sorry, I missunderstood. Will have to go back and take
another look at that particular circuit. I didn't know if I had any
2N3906s, and didn't push it past that. Will definetly check it out though.

And yes, I am working to make sure that I am able to charge the batteries
sufficiently to keep things running as long as possible. Thanks for
mentioning that though, and the encouragement.

Take it easy...

D