"Dave" wrote in message
ica...

"Phil Hobbs" wrote in message
...
On 10/11/2011 04:38 PM, Dave wrote:
"Jim wrote
in
message ...
On Tue, 11 Oct 2011 12:57:11 -0500, wrote:


"Robert wrote in message
...
Dave wrote:
Okay, I haven't had time to get by the candy store, so I still don't
have
the MOSFET recommended, and am still tinkering with this project out
of
my junkbox. Problem is, it's not working right, and I can't figure
out
why. Details a the photoresistor (and every photoresistor I have)
DROPS it's resistance in sunlight, rather than raising it, so the
transistor conducts when exposed to sunlight only. I replaced the
photoresistor with a device that seems to be a tiny photocell behind
a
window (with leads, and sized similarly to the photoresistor) which
provides .4VDC in normally bright light, installed in polarity
opposite
to that which would assist Q1 in conducting. My calculations
(obviously
flawed, but I can't figure out where) tell me that it should
counteract
some of the voltage resulting form the 4.2K/680 voltage divider, and
keep
Q1 from conducting in normal sunlight. My numbers say there should
be -.7VDC from the voltage divider, and the photocell shouldmake
that -.3VDC, which should turn Q1 off. Only it doesn't seem to work
that
way. Q1 conducts regardless. What am I doing wrong? I have
verified
the actual voltages at the base of Q1 (the juncture of the voltage
divider) and coming from the tiny photocell. Only nothing changes.
Could the problem be the miniscule current coming from the tiny
photocell? I somehow thought voltage was voltage, and current
didn't
matter...

Attached is my schematic of the current configuration. Haven't
bothered
to convert it to LTSpice, as I have no idea how to incorporate a
solar
cell, sunlight, or the photocell on the PC board.

Ignorantly yours...

Dave





WELL!!!
It is a known fact, a result via the laws of physics, that photo
resistors increase their conductivity when illuminated; to expect
otherwise is not exactly sane or an educated "deduction".
Just use a voltage divider to the gate / base, placed in an
appropiate
position (i sure ain't gonna do that "work" fer yah).

Okay, so please explain to me how a voltage divider will change it's
values
according to whether it is day or night, and turn the transistor on at
night
while turning it off during the day. Look at the schematic. I've
biased
the transistor properly I believe, it's getting things to respond to
daylight I am having trouble with.

Thanks,

Dave



Put the photo-resistor across the base emitter junction of the
transistor. Then a bias resistor to plus (NPN assumed). Twiddle bias
resistor value as you try from dark-to-light until you get the value
right. Strong light will keep the Vbe below 0.7V, and the transistor
will not conduct.

...Jim Thompson
--
Umm, I *think* I see a problem with this solution. If I'm correct, it
will
spend it's daytime hours conducting *instead* of Q1, which will drain my
batteries rather than allowing them to charge. Then when the lights go
out,
they will not be able to support conduction of Q1 or the LED. Please
correct me if I'm wrong. It doesnt' look like a real solution though...

But thanks,

Dave



So order some MOSFETs from Digikey for 42 cents apiece in quantity 1 (or
5 cents in quantity 2000), and get on with it. Even Radio Shack has
IRF510 MOSFETs for two bucks. It really isn't this hard!

Cheers

Phil Hobbs

Well, I could only find the surface mount devices at Digikey, and my local
Radio Shack doens't stock anything more than standard general purpose NPN
and PNP transistors. I know where to get what I need, I just have to get
by there (severely limited in my potential for getting about town.) But
you're right, it's not that hard, I just have to pick up what I need.
Could have done it today, but I spent my time putting a new wax seal on
the toilet and doing laundry. Will try to get it Thursday. In the
meantime I keep fiddling with junk box paraphernailia. Thought I had it
with the mini photocell. Still can't figure out why that one doesn't
work. sigh

Dave


Well Phil, I owe you an apology. My local Radio Shack DOES stock that
MOSFET. They have two of them, and I am going to walk down and get them.
Thank you. It was knowing what to search for that apparently made the
difference. This place has been such a frustration to me in the past I
almost didn't give them a chance. But they do have two. Thank you for
clueing me in to what to look for.

Dave