Introduction: Dual Axis Transistorized Solar Tracker
This design uses 16 transistors, 8 per board, for the pan and tilt axes.
The frame is made from 25mm² aluminium tube joined with 90° Connect-It corner pieces and M6 threaded stainless steel rod is used to rotate both axes.
This design was intended to be portable and as such it is mounted on a small stand, easily erected during power cuts etc. Using radio control servos (JR NES-507) and a small ni-cad 600ma rechargeable battery allows approx 2~3 days runtime before the battery needs a recharge.
Step 1: The Electronics
The idea of using transistors to drive the motors was sourced at the Redrok energy site. I used the "LED5S5V Simplified LED low power tracker", its a long web page, use Ctrl + F in your browser to find the spot, or direct link here Led5S5V.
The max current using the suggested 2n2222 transistors is about 100ma, a bit on the small side for my intended motors and so I used some BD634's and 245's I harvested from an old telex machine. These are capable of 1A and will easily drive my RC servo's and small hobby motor.
I'm using rechargeable RC ni-cad batteries for the initial testing phase, but I imagine the LM7805 5V TO220 style voltage regulator would be able to drive both the pan and tilt axis from the panel itself.
I used a 5mm Clear Ultra Pure Green, 20° due to the high, 1.7V bandgap voltage. It is this voltage of the 2 leds back to back which is summed and the difference amplified by the 8 transistors which then feeds the motor, so a high as possible voltage is needed.
The bandgap voltage is easy enough to check, connect a meter to the led leads and point the led at the sun, the wide angle greens (120°) were a disappointing 0.45V.
The attached files were created using the free ExpressPCB software, comprising a basic schematic and my final pcb layout.
Some components are missing from my schematic so its best to work off the original at the Redrok site, led5s5v schematic
Step 2: The Tilt Axis
Given that the maximum degree of movement for the tilt axis could be 90°, although it is a bit less in summer and a lot less in winter, I decided to use a short length of threaded rod for the drive mechanism.
I wanted both the motor mount and the shuttle mount to rotate to allow for the changing arc of the panel frame when swinging from vertical up to horizontal. The motor which rotates a M6 stainless steel threaded rod was positioned under the panel to keep it out of the rain, and was mounted on a hinge in order to rotate. The shuttle side through which the threaded rod travels, was mounted to a bearing and bolted to the frame.
The panel swings freely on 2 small hinges attached to the upright arms.
I used 25mm square tubing for the upright arms as well.
Hacksaw cuts on each corner allowed me to fold over the soft aluminium so that the hinges could be pop riveted.
Step 3: Tilt Axis Swiveling Shuttle
For this axis the threaded rod can be rotated for linear motion which means that the nut has to be somehow held captive, similar to a garage door opener, and rotate with the changing arc at the same time.
I squeezed a nut in each end of a section of 12mm ss tube (10mm inside dia) leaving a waist in the middle which holds both nuts captive when the threaded is screwed in.
Next a hard drive bearing (1360ZZ) was fitted into a copper pipe end cap with a ss hose clamp fastening the M6 nuts to the bearing assembly. This was topped off with a M6 machine screw through the bearing center and fastened to the frame upright.
Step 4: Tilt Axis Motor Drive
A rc servo mounted on a hinge is used to rotate a threaded rod, here is how I've done it.
- A custom adapter is made from soldering a M6 Tee nut to a large washer.
- A servo horn is bolted to the adapter plate, no the plastic horn isn't symmetrical but the flange centered well in the washer for marking the holes.
- the small pcb is removed from inside the servo, the feedback pot must be left because its shaft is used for the main gear.
- the motor is connected directly to the tracker board.
- A hinge is aligned to the servo and frame and glued to the servo case.
- M6 threaded rod is screwed into the Tee nut and cross drilled for a retaining clip.
Step 5: The Panning Axis
Alas we cannot use a simple straight rotating rod for this axis because we have more than 180° of rotation. Some pondering will also reveal that the straight rod will yield less than 180° travel and suffer from too much flex at full extension.
However we can rotate the nuts and keep the rod captive, to this end the threaded rod can be curved and mounted solidly to the central shaft.
- Threaded rod curved using 3x 6203 bearings, 2 mounted on one vise jaw, 1 on the other and rod sawed back and forth while tightening the vise. (the bearing surface flattens the tops of the thread which can be cleaned up by chasing the threads with a M6 die.)
- A fixed center shaft for mounting the rod circle.
- A motorized shuttle driving the nuts anchored to the upright frame.
Geometry tripped me up, the center of the pan arc was different to the center of the shuttle travel arc. I originally didn't want to weaken the frame by drilling a hole in the center of the horizontal arm, so I offset the central shaft and clamped it to the outside of the revolving ss tube, seemed like a good idea at the time.
The new solution called for the central shaft to be centred and some pondering yielded a workable solution, pity I didn't think of it the first time around.
Anyhow, not exactly a train smash, the electronics were built in 2007 so I don't appear to work in a linear fashion. :)
Step 6: Panning Drive Motor Shuttle Assembly
This assembly consists of a motor driving a worm gear which is loosely mounted on the aluminium square tube frame so as to be a sliding fit to accommodate any out of round of the threaded rod ring.
- A bracket was made using a 180mm length of perforated Xbox chassis.
- Bearing retainers in the form of 20mm copper pipe end caps were soldered to the bracket while it was straight.
- 627ZZ bearings (22mm od 7mm id) were a press fit in the copper caps.
- Another m6 tee nut was used, this time glued to a plastic gear, tee nut was filed to fit inside the bearing id, keeping the motor worm to gear clearance consistent.
- A PET bottle was heat shrunk to provide a dust/rain cover for the motor/gear assembly.
Step 7: Final Tips and Tricks
I use a wooden lollipop mounted perpendicularly on the panel as a visual indication of how well the tracking is going, initial adjustment of the leds is required to center the bead's shadow on the mount block.
Gusting winds are a problem requiring you to stake down the stand legs to prevent the panel from being blown over.
I have yet to see if the tilt drive rod interferes with the pan ring, guess I'll have to wait for midday on summer equinox which is 22nd December.
The center tube is a force fit in the tripod stand, which leaves me with the option of wall mounting at a later stage.