Direct reading of white letter on compass card (for example: the first vertical line of letter [N]orth) of a car accessory compass, and transfom weak torque in usable hight torque reproducing the magnet compass headings in the way of... a gyrocompass (read the position of gyrosphere containing the gyros and transfer the heading value outside for mechanical purposes, ex. synchro-transmitter).

1. The frame oriented to the sun in a clear sky (morning)
2. The frame oriented SW with cloudy sky (afternoon)
If the sky becomes uniform, the system just stops in minimum power consumption (fly over the picture)
The motor of azimuth (vertical axis) and the motor of elevation (horizontal axis) are used in mode "axis fixed, stator mobile".
The first use of gimbal "elevation" is the mounting of a solar panel (evidently more large), it includes the both sensors "azimuth" and "elevation".
object of the experiment: The purpose of the experiment was to evaluate several optical sensors of different types, their arrangements and the servo board input type to be used, to make a solar position sensor (or light source sensor) very simple.

There is really to much UV ! Never look at the sun with the naked eye or through any optical device (fly over the picture)
At the left hand of the "elevation" gimbal the sensors "elevation"
at the bottom in the center of this gimbal the sensor "azimuth".
For tracking the sun the photoresistances mounted "in bridge" seem very effective, one oriented toward the past, the other toward the future, so as not to face the sun in normal operation, the signal of unbalance of the values of brightness initiates the mechanical negative feedback of the servomotors, bringing back the bridges at state balanced.
For clear sky this state is the position of the sun at this time, if the sky is cloudy, the panel is pointed to the most luminous zone (average), which is always better than no orientation at all ...
The best and simple arrangement seems to be: two phototransistors with hemispheric lens under the control of one photoresistance for luminosity correction.

Example of hand wired interface board (60 x 100)

It's not always requested to adjust instantaneous speed of mechanism positioning to the optimal speed requested by target / sensor motion or by reference variation on the entire zone of displacement. The speed can be hight, low or very low.
If a request involves a relatively long and multiple displacements, with or without particular positioning time, in such conditions why to lose power by overheating if only the final position accuracy is relevant?
From this, the following circuit interface suggestion: the servomotor is driven through relays from some adequat, unregulated AC or CC power source, during long motion periods when the speed tracking accuracy isn't required (Note): then the relay switchs off and the motor is driven now in analog mode near and up to the final accurate position. If some new unbalance appears the servomechanism will tend to restore the stability point in his normal analog mode, but if the unbalance reaches the energizing voltage relay the control will return to binary mode.This is possible because the difference between energizing current and hold current relays.
The B1 bridge in CC source gives polarity careless. (a fuse must be added in one bridge input leg) With this small additional circuit, the using of the low power servo board are largely increased without clumsy heatsink under regulators and output transistors, also we get more voltage tolerance. (The choice to use the regulated 12V for the output stages has been done for improve the adjustment stability in controlled levitation due to extreme sensitivity of the flyingmagnet around the balance position.)

Notes:
1) The main points are than the source is adequat to the motor device and stays inside safe low voltage range. Considering the low power involved, it's better to use wall AC/CC or AC/AC adaptor UL/CA/CE and other, certified in conformity with the application.
2) An external board gives ease to implant some necessary functions like:
- Manual emergency stop At the motor level, followed by a reverse direction inpulse (a low power geared servomotor can be "aggressive"...)
- Stop and/or reverse direction at end of useful course
- Stop at dead end, complemented by a short reverse direction inpulse.
Care must be given to the choice and the location of sensor/target in function of type of motion: rotation less than 360°, rotation more than 360°, linear, etc.
It's good to forsee the protection of sensor/target in case of excess travel, especialy during the adjustments time, do not tight, be careful with potentiometer "mono-turn" with effective rotation less than 360° (mechanical stop).