R1 a 500K pot ■ FIGURE 9
(rheostat), you can
adjust it such that the
motor starts running
when Q1 is partially
shaded.
The way the
circuit works is this:
Q3 is the daylight/
dark detector. In
daylight, the photo
transistor (and CdS
cell) is low resistance,
which allows the motor to run. Q1 is
shaded by an east-west wall for the
elevation servo and by a north-south
wall for the azimuth servo. When Q1
is shaded, the base of Q2 is raised
which closes the relay and starts the
motor running.
When sunlight falls on Q1, the
base is pulled down and the motor
stops. You will want a way to return
the servos to the starting position,
ready for the next day.
An alarm clock could be used for
that or, another CdS cell as in Figure
8. When it is dark, the transistor is
energized, causing the motor to run
backwards until the tracker hits the
normally closed limit switch, which
opens the circuit.
2) It is not the MOSFET resistance
that determines when the lights
come on, it is the voltage output of
the solar panel. The turn-on voltage
of the FET and the value of R2 has a
■ FIGURE 10
minor effect on when the lights come
on, but when the 12 volt solar panel
output gets down to 0.6 volts, it
should be quite dark. You can gain
some by removing R1 but that is only
50 millivolts difference, again minor.
Bottom line, there is not much that
can be done to vary the light level
that determines when the lights
come on.
3) Changing Q1 to a comparator as
in Figure 9 will provide the variability
that you want. The threshold can be
adjusted all the way to zero; in which
case, the lights may never come on.
NIMH BATTERY
CHARGER
QIn the N&V July ‘07 Q&A
column, you provided a
PIC-based charger circuit
for NiMH batteries
QUESTIONS & ANSWERS
■ FIGURE 8
consisting of up to 10 cells.
What limits the charger to this
maximum number of cells? If I
use a power supply with more than
20 volts, can I use this to charge
greater numbers of cells?Per your
description, I would change the
charge current by a bit because
new cell capacities are always being
introduced and, of course, the sense
voltage divider (R4, R5) would also
need adjusting.
— Dave Carpenter
August 2008 31