On Tue, 11 Oct 2011 12:57:11 -0500, "Dave" wrote:
"Robert Baer" 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
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