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. The whole
thing goes into a mushroom lawn ornament, with the LED inside the mushroom
cap to shine down on the fairy sitting underneath at night. Gonna look nice
between the sage and Mexican Mint Merigold, which is a weed down in Mexico,
but grows great in Texas and tastes just like Tarragon (which absolutely
will not grow in Texas.)
Dave
You're welcome. As the wise man said about beer, 2N7000s are "Good for
what ails ya, and if nothing ails ya, it's good for that too."
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net