Solar tracker using photodiode: circuit design

Question

My aim is to differentiate between two different levels of illuminance. I would like to know when one photodiode is in shadow and other in direct sunlight. So I don't really need the level of lux but the difference in lux.

I have come across circuits like this, but instead, I would like to have this in differential mode utilizing both the photodiode in one amplifier. Also, I will be reading the output on Arduino so if possible I would like to avoid the amplifier prefer doing taking the difference in digital. But I guess directly reading across the diode won't be a good idea either as the voltage range is very small (0-0.5 V) and so the ADC resolution won't be good.

So, overall, I am trying to find a schematic for either:

1. A direct ADC of each photodiode with sufficient resolution so that I can take difference digitally.

Or

2. A differential amplifier with range suitable for ADC.

Sorry if you feel I haven't tried enough on my own. In that case please just leave a guide point and I will try to design the circuit.

EDIT

The reason I want to use photodiodes and not LDR is the nonlinearity of variation of the resistance with lux. In my experiment with LDR I first measure the resistance at ambient (~50k ohm) and then threw a flash on it for a few seconds, but when I removed it I found that resistance does not go back to the same value (stays around 10k ohm for minutes). It is kind of a Hysteresis non linearity with LDR. While for photodiode I felt that readings were consistent. Please try to take this too in consideration.

EDIT 2 So, considering the solutions suggested below and keeping in mind the time and components availability constraints I finally implemented the LDR and baffle mechanism. It worked out all well and so I wanted to share this video with you all Solar tracker-Light follower. Thanks a lot.

Answer 1

A simple way to do this is with differential measurement:

R1 and R2 are two LDRs (light dependent resistors). A light baffle causes the light on them to be equal when the unit is pointed at the sun. When off to one side, one LDR gets more light and the other less. The voltage between them is then a measure of centerdness, with the middle value being centered. The A/D in the microcontroller reads this value and drives the motors appropriately.

You need one pair of LDRs for each axis.

For extra credit, figure out how to do this with just 3 LDRs. This is possible if you can drive nodes high, low, or measure their voltage only the fly. Fortunately, that is easy to do with a microcontroller.

Added about LDR variations

It was pointed out in a comment that two LDRs will not have exactly the same resistance at the same light level. This means that the voltage won't be exactly the center value when the unit is pointed directly at the sun, which should make the resistances equal.

This doesn't matter much to most applications because:

1. The light baffle system is set up so that the maximum change in brightness on a LDR as a function of angle is when pointed directly at the sun. Imagine each LDR half in shadow and half in sunlight, for example. Only a small angular change will shift this to all shadow or all sunlight. Even a few 10s of percent mismatch in the resistance will be swamped by only a small angular change from directly aligned. At any other angle, you don't care. You just need to know which side of correctly aimed the system is.

2. The unit can be calibrated to the voltage actually measured when directly pointed at the sun. This becomes the center reference for the algorithm.

   If the two sensors have different non-linearities, not just overall scale, then the center value is a function of brightness. In that case, calibrate at full sun, since that's when you care most. Put another way, it's not so important to point directly at the sun when it's behind clouds anyway.

3. If the purpose is to keep something like a solar panel pointed at the sun, note that small deviations from exactly aligned cause very little change in the power hitting the panel. The power due to misalignment falls off with the cosine of the error angle. That is basically 1 for reasonable error angles anyway.

   For example, it takes a 8° aiming error to drop just 1% in power hitting a flat surface. 5° error results in only 0.4% drop.

Answer 2

Some decent answers, but many not addressing the desire to determine the balance-point by determining the difference between two photosensors by calculation.

The raw difference circuit is easy, but has serious problems on an overcast day, where op-amp voltage offsets seriously influence the balance-point. One is never sure if bright sunlight is equally exciting two photo diodes, or if a dull sky is weakly exciting the two photodiodes.
So how to use two photo diodes (in one circuit) to determine both light magnitude and illumination difference? Have simulated the following circuit, but haven't built it, so component values likely have to be changed to suit photodiode characteristics. Pulse output has characteristics easily measured by a logic input of a microcontroller. The pulse "high" period is influenced by one photodiode, while the pulse "low" period is influenced by the other photodiode. When both are illuminated equally, pulse high period equals pulse low period. Strong sunlight will give shorter pulse periods, and weak light gives longer pulse periods, so pulse frequency gives an indication of sun strength. A microcontroller with internal clock-counter easily finds frequency and individual pulse-high period and pulse-low period.
Use a rail-to-rail op amp having very low bias current. Many CMOS op amps are appropriate. A digital comparator could be used as well, if it has a logic-type output that actively pulls "high" as well as "low":

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

I made a light tracking device for my senior project for my B.S.EET degree. It consisted of two photo-voltaic (PC) cells, a light tight box, 2 A to D converters, and a self written computer program that monitored the output of the two A to D converters.

The light tight box had slits made in the face of the surface that was to be perpendicular to the light source. Where the light entering the box when the source was dead center was carefully recorded om the back of the box/ A divider was constructed in the box center to form two equal chambers. The PC's were installed on the back wall of the box so that they were just outside the light beam. When the light moved left, or right of dead center, one, or the other PC would be illuminated, producing voltage. The PC's were connected to the A to D's. The outputs of the A to D's was connected to a computer through an IEEE bus. The program sampled the two A- D's alternately, and sent a digital pulse to a stepper motor, either positive, or negative depending on with PC was energized. The light tight box was connected to the shaft of the stepper motor and would move clockwise, or counter clockwise according to its input from the computer.

A margin of acceptable allowance had to be programmed in as the unit was so sensitive the the single step in either direction, when the light was perpendicular to the box would cause the program to send a correcting signal, and the box would chatter left and right, or hunt. The allowance stopped the hunting.

In addition, at the simulated Western most position, a micro-switch would be touched by the light tight box, causing a pulse that the program would see, and it would rotate the box to the East-most position to wait for the sun to arrive over the horizon. The device worked perfectly. This project was done in 1992, and I regret that I no longer have my schematics. You will need a DC voltage supply that will fit the parameters of you A to D, and that's about it for circuitry. With two sets of photo-voltaic cells, and a gimbal, you could track you light source in both the X and Y planes, to get accurate tracking. Covering the box face with a polarizing lens would allow this to track the sun, even on cloudy days.

I hope this was of use to you.

Answer 4

I would like to have this in differential mode utilizing both the photodiode in one amplifier.

Using a TIA (as per the linked circuit) is mainly done when you want to keep the current output from the photodiode (more) linear with incident light power.

A TIA (trans impedance amplifier) is also utilized to obtain higher bandwidths.

If neither are important to you then you can develop a voltage across a resistor in series with the photodiode and use a difference amplifier for calculating the erm.. difference!

A decent idea is to use an instrumentation op-amp: -

So, my advice is look for an InAmp with rail to rail capabilities (if running from a single voltage supply) or pretty much any old InAmp if you have dual supplies.

Answer 5

In addition to Whiskeyjack answer:

You can use 4 LDRs mounted on a reversed pyramid, then you detect pairs of LDR, which is more shadowed. It is easier to detect low brightness rather than high brightness of the Sun. In such way your gimbal moves until the LDRs are in equal shaded position. I guess, the best is to use a differential op amp to compute the brightness/shadowness difference.

^{simulate this circuit – Schematic created using CircuitLab}

EDIT: Similar solution as Olin

^{simulate this circuit}

Answer 6

You could use two LDRs, but a better approach may be to use a pair of more active components that are temperature regulated and less sensitive to aging such as the AV02-0512EN. Digikey 516-1719-1-ND ~$1.50

NOTE: This is a 3.3V Device. 100uA when on, but added benefit is it has a shutdown pin that drops that to 1uA. Since I assume you will only adjust the position every minute or more, this seems ideal.

The device is also logarithmic which helps even out the low light sensitivity / high light swamping issues.

R-load would likely be 15K in a 3.3V+ application. You may wish to add a variable resistor on one side so you can calibrate the unit.

Feed each of them to their own ADC pin on the Adrino.

Again, as others have mentioned, you need to arrange them such that there is some form of baffle where they both receive the same amount of light ONLY when the board is pointing "directly" at the sun. The exact geometry of this will dictate how close to the correct angle you can achieve. Having the baffle half "cover" both sensors when it is in-line produces the best directional accuracy.

You may also need to consider some form of lenses or conical light catchers in front of your sensors to give you more of a shaded/lit area to work with. If you do, then you may also need an infra-red filter so the magnifying effect does not cook your sensor.

BTW: Don't discount the temperature compensation feature. A sensor in the sun will have a considerably warmer temperature than one in the shade. That can cause you to slew too far, till the temperature difference inverts, then you drive back... If you are not careful, you end up perpetually "hunting".

And of course, it needs to be weather proof.

Answer 7

LDRs should work for your use case. However keeping LDR exposed to sunlight won't give you a considerable difference when sun moves. You can make an arrangement like this:

this gives a small access angle through which sunlight can reach the LDR. Use the LDR in a voltage divider configuration to figure out whether this LDR is in line with sun or not. If you increase the length of cylinder or decrease it's diameter, the access angle will reduce even further. Choose a shape which works.