The Sensor Inputs
Since we want to be able to track the sun we need some form of light detector. There are a number of alternatives here, but the simplest is an LDR (Light Dependant Resistor).
LDR light sensor (light dependant resistor)circuit for using a light dependant resistor for measuring light intensity The principle on which these work is that their resistance decreases as the light intensity increases. Typically it is about 1M ohm in darkness, 10K ohm in moderate light and 2K ohm or less in bright sunlight). Because of the relatively high impedance of the analogue inputs on the WASP we can connect the LDR’s directly using the circuit shown on the right, where the resistor used is 10K ohm.
This will cause the voltage appearing at the WASP input to vary between about 1v and 4v in average daylight conditions, which is more than enough to base our control system on. Note that we have 4 LDR’s connected to the WASP. This will allow us to mount two in the vertical plane and two in the horizontal. They will also be angled with respect to each other. As the sun rises into the sky the upper one of the vertical pair will receive more effective light since the sun is closer to being perpendicular to its surface while the lower will receive less. Similarly as the sun traverses east to west the two in the horizontal plane will receive different amounts of light. This is illustrated below