Nuts and Volts - August 2008

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.