LED555 Timer Based Solar Tracker
LED555 Timer Based Solar Tracker
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This tracker is a bit different from most other sensor based solar trackers. Those essentially work by directly measuring voltage, current, or resistance of the light sensor. I like using LEDs for these because they, generally, produce higher voltage than silicon sensors.
This tracker essentially measures the time it takes to charge a capacitor to a predetermined voltage. So its a timer based tracker.
All solar trackers have a characteristic I call Angle Gain Factor, AGF. AGF is in essence the amplification of aiming angle error to the sun into power to drive the motors. The motors are moved in a way to minimize the aiming angle. The more aiming error the more power to the motor.
LED pairs inherently are very sensitive to aiming angle. A major factor in AGF is in the characteristics of the LED pair. Of course, the other actor is in the power driver circuits.
In the example U2 and U3 are the 555 timer circuits, I usually use ICM7555IN by NXP Phillips which are superior to most because they are CMOS and have an input leakage current of less than 50pA and can operate from -40°C to +85°C. The LMC555CN is a bit better because it can output more current, but it costs 5 times as much.
Refer to the timing diagram:
U1 is an astable multivibrator, which can be any of the 555 variants. It periodically sets the timer Flip Flops, FF, by bringing TRG low and grounding capacitors C2 & C3 through diodes D2 & D3. When U1 releases the timers C2 & C3 start integrating charging currents. It's a race as to which reaches the Threshold, THR, first.
If C3 gets to THR first the output drives the motor forward until C2 catches up.
If C2 gets to THR first the output drives the motor reverse until C3 catches up.
If C3 & C2 get to THR at the same time the motor doesn't move.
Bright light causes faster charging of both C3 & C2.
In the dark charging will be slower. Slow enough so neither C3 nor C2 reach THR.
If even when dark the RC constant of C3 is short enough it will always reach THR and C2 is longer than that of C1 on U1 the motor will move in the parking direction until the Limit switch is reached.
The power supply voltage range of the ICM7555IN is 3V to 16V. I have tested this circuit throughout this range and up to 18V, the maximum allowed voltage.
The sensors I have used range from infra red photo diodes to ultra violet LEDs and they all worked fine. I tried CdS photo resistive cells, they worked OK but are highly non linear, not to good.
Now something really cool!
I tested it using a pair of 1N270 Germanium point contact diodes, worked just fine.
Then with a pair of 1N4148 switching diodes. That worked too. Cool huh!!!
Actually many, if not most, semiconductors of any type are affected by light. The glass packaged diodes, plastic LEDs, ICs with windows, and any other way that allows light to enter affects the semiconductor's characteristics. Usually leakage current but other things also. No wonder most have black opaque packages. Here is an article about this same thing. On the dark side.
Note! The CREE LED specification says the reverse voltage limit is 5V, other brands are similar as are the silicon photovoltaic cells. Even though this voltage is lower than the supply voltage they still work fine even at 18V.
Single Ended Sensors:
For the first attempt at this concept I used the single ended LED sensor circuits on the right. The capacitors get charged through the reverse biased LEDs and resisters. The charging rate is directly proportional to the intensity of light on the sensors and limited by the resisters. (The two resistors, which could be as low as 100KΩ, limit the maximum charging rate, but also limit current if the device shorts out. It's a safety thing.) When they reach the THR voltage the internal FF is reset.
Of course, one will get to the THR voltage first. The OUT pin voltages will be different from each other, one HI one LOW, due to the differences in charging times. As it gets closer to the light balance point the motor drive time duty cycle diminishes to zero. A kind of Pulse Width Modulation, PWM, circuit or variable speed motor drive. Cool huh again!!!
In most of the other simple solar trackers I've described "Parking" is usually difficult to do.
1. The use of a separate sensor to measure the "darkness" and park accordingly. This is not desirable as it adds complexity to the circuit.
2. Add a "weak" imbalance to the sensor circuits which causes motor movement when it's dark. This is desirable, if it can be done, as just the primary sensors are used.
The main difficulty has been that the LED circuitry operates at quite a high impedance. To introduce imbalance currents very high valued resistors were required. These, although available, are fairly expensive and tend to be impractical. One exception is the use of a "leaky" reversed biased diode in the LEDBlue tracker.
In this case I don't need high resistance resisters, instead I use the imbalance of "time" in the form of resister & capacitor, RC, time constants. The timing diagram helps illustrate this. (OK, in my example the two RC constant are the same.)
The single ended circuit simply stops moving when it gets dark, "No-Parking". I wanted to implement "Parking" when it gets dark. In the absence of the LED sensors the charging current is through the resistors. Depending on component values one RC circuit will always charge faster and tend to move in the Park direction.
(Note! Limit switch circuits are required when parking is involved.)
The back to back LED sensor circuit introduces some added differential voltage to the charging capacitors. Again one will win as with the single ended version. An alternate way is to use only one of the pullup resisters, charging current will be sent to the other capacitor through the LED pair. The rest of the circuit operates the same way.
Another advantage of this basic circuit is the output is not analog in nature. It can drive the output transistors into saturation which limits their power dissipation allowing for greater output currents. The analog qualities lay in the PWM nature of the timing.
Please note! I have tested these circuits as published. However, your application may need other timing or light sensitivity requirements. By suitably adjusting timing values a great variety of applications can be accommodated.
OK, I'm showing a generic 2N2222A/2N2907A low power driver good for a few hundred mA and
a high power MOSFET driver using the International Rectifier IR2111. Take your pick.
In essence the 555 is a FF. Why can't this be done with a FF. It can. It's just that the ubiquitous 555 timers are accurate and robust circuits.