On Fri, 07 Oct 2011 07:26:02 -0400, Phil Hobbs
wrote:

On 10/07/2011 07:13 AM, 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-)----*----------------*





Oh, man. Something to WORK with. THANK YOU! Much appreciated...

Dave



De nada. If you're planning to use that circuit for real, you might
want an 0.1 uF cap from gate to source. That'll make it less twitchy
and less vulnerable to electrostatic discharge.


Phil has already answered the hard part but for the posting schematics
part, try either AACircuit or LTSpice.

AACircuit is available over at http://www.tech-chat.de/aacircuit.html
A rendering of your right-hand snippet would look like (view with a
fixed-pitch font):

2N3904
V- o-------o--------- ----------.
| ^ / |
| --- |
| | .-.
.-. | | |
| | | | | 160
120 | | | '-'
'-' | |
| | |
| | |
o----------' |
| |
| |
| |
.-. -
| | ^ -
680 | | |
'-' | NTE 30045
| |
| |
V+ o-------o----------------------'
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Stylized but understandable -- and easier than trying to do it in a text
editor or Usenet client's window.


LTSpice is one of several free apps provided by Linear Technology
http://www.linear.com/designtools/software/ It is a full Spice
simulator but it's also used around here to pass the schematics as ASCII
netlists.

AACircuit is typically best for small snippets, and nice since they are
directly viewable. LTSpice for the larger layouts.

Version 4
SHEET 1 880 680
WIRE 128 -32 -16 -32
WIRE 208 -32 128 -32
WIRE 384 -32 304 -32
WIRE 128 48 128 -32
WIRE 384 48 384 -32
WIRE 128 160 128 128
WIRE 256 160 256 32
WIRE 256 160 128 160
WIRE 128 192 128 160
WIRE 384 208 384 128
WIRE 128 336 128 272
WIRE 128 336 -16 336
WIRE 384 336 384 272
WIRE 384 336 128 336
SYMBOL res 112 32 R0
SYMATTR InstName R1
SYMATTR Value 120
SYMBOL res 112 176 R0
SYMATTR InstName R2
SYMATTR Value 680
SYMBOL res 368 32 R0
SYMATTR InstName R3
SYMATTR Value 160
SYMBOL npn 304 32 M270
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL LED 400 272 R180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
TEXT -56 -32 Left 2 ;V-
TEXT -64 336 Left 2 ;V+

--
Rich Webb Norfolk, VA