On Fri, 07 Oct 2011 06:59:06 -0400, Phil Hobbs wrote:
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-)----*----------------*
I couldn't find the original post, but if you're driving 20mA to an LED
you should only need a mA (at most!) into the base of a BJT. So you
could almost use Dr Hobbs's circuit above with an NPN, or use something
like
V+ o----------------o------------.
| |
| |
.-. .-.
| | R2 | | R5
| | | |
'-' '-'
| |
| |
.---------o |
| | |
| | V -
| .-. -
| | | R3 | D1
| | | |
| '-' |
| | |
| | |/
| o----------| Q1
| | |
| | |
.-. .-. |
~ | | R1 | | R4 |
~ | | | | |
'-' '-' |
| | |
| | |
| | |
V- o -----o---------o------------'
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
Set up R2 and R3 to bias the transistor on, with R4 to lend a bit of
authority pulling the base down. When R1 starts to conduct the junction
of R2 and R3 doesn't have to get all the way down to 0.6V in order to
turn Q1 off fully.
Come to think of it, you could do this all with a jelly-bean comparator,
and add some hysteresis to boot. Then D1 would snap on and off, instead
of shivering on and shivering off as the light changes.
--
www.wescottdesign.com