Parts List: All resistors are 1/4 watt, 5%, unless otherwise posted. R1 = 75 Ohm R2 = 10K trimmer pot R3,R5 = 33 Ohm R4 = 20K trimmer pot R6 = 40 ohm, 1 watt R7,R8,R9 = Photo resistors (LDR) C1 = 0.02uF, polyester (or equivalent) C2 = 0.1uF, ceramic D1 = 1N4004, general purpose silicon diode Q1 = 2N4401, NPN, general purpose transistor Q2 = TIP120, NPN, darlington Ry1 = Relay, 7 to 9V MOT1 = Motor, any type from 1.5 to 12 volts DC Circuit Description: The Solar Tracker uses a combination of three photo resistors (not photo-transistors) or LDR's (Light Dependent Resistor) R7, R8, and R9 to ensure that the circuit will follow the sun during the day, but not look for it at night. The LDR's are approximately 150 ohms in full sunlight, and about 5K in the shade, but that is not absolutely critical. R7 is mounted in a "well" with a narrow slit so that sunlight falls on it only when the photo resistor is pointed directly at the sun. If so, R7's resistance value drops. R7 and potmeter R4 form a voltage divider at the base of darlington Q2. When R7's resistance is low, Q1 will remain switched off. When the sun swings a little westward, R7 will no longer be in direct sunlight, causing its resistance to go up and in turn raising Q1's base voltage, turning on the darlington Q2. That, in turn, closes the relay Ry1 and activating the small, low-torque hobby motor (MOT1). The motor then turns slowly (R6 limits maximum current to the motor and keeps it from running too fast), putting LDR R7 again in direct sunlight; Q1's base voltage then drops and the tracker stops. This sequence is repeated for as long as the sun moves across the sky. R8 is mounted on the outside of the "well" so that it receives a wide angle of full sunlight. When the sun is shining, R8's resistance is low, keeping Q2 turned off, and allowing the tracker to act as describe, without interference. But if the sun "slips" behind a cloud, R8's resistance goes 'high', producing a forward bias on the base of Q2 which turns it 'on' and sinks the base of Q1 to near ground so that Q1 remains 'off'. That immobilizes the tracker drive and also keeps the drive (read motor) shut-down during the night. LDR R9 is the "dawn" sensor. It is mounted on the back of the solar tracker. When the tracker stops at sunset, pointing towards the West, R9 is pointed to the East. When the sun rises again the following morning and shines on R9, its resistance goes low, turning Q2 off and allowing Q1's base to go 'high' which in turn activates the relay Ry1 and causing the tracker to swing back to the east; the starting position. Make sure to mount your LDR's or phototransistors etc., in side a short black tube to eliminate any sort of straylight. The 'dawn' sensor LDR R7 may not require one, you have to experiment yourself. It depends on the location of where the sun tracker is placed. In regards to the motor, you can use anything you like depending on the torque your system needs. I experimented with a variety of motors varying in size and power from 6V to 24V, all were working fine with this circuit diagram. As for the relay voltage, the relay is only on momemtarily each time while tracking the sun, hence I specified 7 to 9V but the supply voltage is 12V. Keep that in mind.