Heliostat Sensors
Heliostats can be controlled using the light of the sun. alternatively they can be controlled by a very precise clock, or a clock signal. If they track the sun there needs to be a sensor that is light sensitive that produces a signal that can be used to drive the motors. There are several options.
Most Heliostat sensors use a light differential btween two sensors caused by shade. This eliminates most effects of overal light levels and enhances the sensitivity to the direction where the light comes from. One can use a blackened block between two sensors, or a flat piece of black aluminium, or simply orient the sensors in the opposite direction.
Some Examples
The signal from the light sensors is meant to drive the motors, so it needs to be conditioned into a clear signal. This can be done in several ways, f.i. amplification, using an a/d converter or a so called Schmitt Trigger or a series of opamps.
LDR
An LDR, a Light Dependent Resistor is a very old technology. One can use it to split a voltage which is then compared to that of another on the other side of the light barrier.
LDR signals need to be amplified.
Phototransistor
A Phototransistor is a transistor where the basis signal is replaced by light. It functions like a normal transistor and the (differential) signal needs to be amplified to be used.
LED
Leds can be used because they develop a potential when lit. There are two ways ot thinking about it. One is that the led actually functions a bit like a capacitor with a light dependent retention capacity. The more light the sooner the charge (and thus the potential) is lost. Others simply see a led as a potential source (like Redrok).
If you use the LED as capacitor you can combine it with a microcontroller and sense the ambient light by first 'charging' it and then measuring the potential as it falls. This may not work to well in bright environments. I have not seen the principle applied to a solar tracker.
If you use the LED as a pure potential generator you can put two of them in series and parallel, this gives you a positive/negative signal. This can be amplified, put into a Schmitt Trigger circuit or be fed into a sensitive A/D converter.
PV Cell
A photovoltaic cell can generate up to 9 volt (when it's 0,5 volt cells in series) dependend on the design. This can be very handy because in some cases you can feed it right into the A/D converter of a controller chip.
Light Sensing Chip
There are several types of light sensing chips. The signals they generate can vary, from a PWM signal (that can be integrated to create an abslute potential) to an I2C bitstream.
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