Hello,

For a project I am attempting to make a solar tracker that tracks the sun throughout the course of a day from east to west. I will have circuitry to sense the sun and then use a motor and linear actuator to adjust a PV panel to face directly at the sun.

The circuitry uses photoresistors (along with an LM339 comparator) that will change resistance depending on the amount of sunlight. Therefore, when the West sensor sees more light than the East, it will have less resistance and allow for more current to flow and activate a motor relay to turn the panel towards the West. When the panel is directly under the sun, both sensors will see equal light and the panel is not moved. At the end of the day, when both panels see equal darkness (high resistance), the panel will move back to the East.

I found several schematics online for the control circuitry but am having trouble getting the design to simulate correctly (using MultiSim). I can configure the circuit to work correctly for one instance (i.e. pointed directly at sun or with the West seeing more sunlight), but it does not work when I change resistances.

Basically, when the circuit sees equal light, the photoresistors will have a value of approx 10k. In order for the panel to stay where it is, the voltage at the base of each of the transistors needs to be high (the actual motor relays will connect to the collector of the xistors).

Similarly, the base voltage of the EAST_DRIVE xistor needs to be high when the WEST resistor is 10k and the EAST is 100k (dark) so that the panel will turn towards the West.

The problem I am seeing is that since the outputs of U3D and U3B are tied together, that node will only be high when both outputs are high. For whatever reason, I cannot get the values to come out right.


I am attaching a jpg of the schematic as well as the MultiSim file (in zip folder) incase anyone can maybe run the simulation and see if they can find out what is wrong? I have been racking my brain trying to troubleshoot this.

Thanks!!

Originally Posted by BrownOut

Then "East" relay energizes, and the safety feature prevents the "West" relay from energizing. Safety featuer could be made much better, I'm sure.

Thanks for your input, sorry about the delayed response.

I guess I am a little confused. If the base of the transistors is High, then the multimeter attached to the collector sees a Low voltage. It was my understanding that I want a Low value in order to keep the relay, which will be where the multimeter is, from activating. Thus, when I want the panel to turn towards the West, I want a High value on the multimeter XMM1 and a Low value on the multimeter XMM2. Is this correct?

Originally Posted by BrownOut

If your relays are modeled by the 10K ohm resistors, then it's just the opposite. You have one side connected to VDD, so it takes a "Low" on the collector side for any current to flow, and energize the relay.

Ahh, that makes perfect sense. Thanks!

On another note, I am having a hard time figuring out the purpose of the 1M feedback resistors (I did not create this circuit originally). Taking them out doesn't seem to affect the output node on the comparator. Do you know what they might be there for, especially when their value is so high?

Originally Posted by BrownOut

It's called "hysterisis" and it prevents the output from oscillating at the switch point. Leave them in.

Ok. Any specific reason why U3D doesn't need one?

Originally Posted by BrownOut

I'm not crazy about the way U3D is connected. I would probably not connect the way it's in your drawing. But without at least simulating it, I would't feel comfortable telling you to change anything. IMO, you can connect a 1M feedback or not.

I believe it is set up that way so that both relays can never be on at the same time. Do you have any suggestions as to a better configuration?