On Sun, 9 Oct 2011 23:15:44 +0200, "petrus bitbyter"
wrote:


"John Fields" schreef in bericht
.. .
On Fri, 7 Oct 2011 05:36:05 -0500, "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...

---
While Petrus' suggestion to use the PV array as the sensor is
excellent, it has the drawback that setting the switching point and
getting a nice "snap" into the LED is difficult, if not impossible,
using a single transistor.

Using a comparator capable of driving a 50 mA load solves both
problems at once.

..
..
..

I considered the use of a comparator though not the LT1011 but the LM393 or
similar. FAIK more popular, almost everywhere available and maybe cheaper as
well.

---
Indeed, but incapable of handling, by itself, the 50mA load current
the OP was asking for.

The LT1011 is an improved LM311 clone and I used it because it was in
Linear's library, but an LM311 would work just as well for about 1/4
the price, qty 1.
---

The next thing could be a two transistor Schmitt trigger. I ever calculated
every component of it and I can still do so. It's just a matter of setting
the threshold and the hysteresis to the required values.

---
Post a schematic, please?
---

But the one transistor solution struck me for it's simplicity, low component
count and light effect. So when the (sun)light is fading, the LED starts to
glow faintly to come to full brightness when it grows really dark. Next
morning the other way around if there's enough energy left in the battery
that is.

---
Yup!

--
JF