I recently purchased garden lights, you know the ones that have the panel in the roof and the light switches on when dark. I had or have been working on a solar lighting system myself and one aspect had been keeping me busy. The light sensor, where would you put it and what type would you use. When installing the garden lights to work I noticed something – there was no light sensor or day night sensor (yes, I did take them apart). That got me thinking and it revealed the obvious : the solar panel is in itself a day night sensor. There is no need for an additional sensor.
The dynamic then is using the voltage signal out to determine when to do what. So here is the breakdown. The solar panel produces a varying voltage depending on how much direct sunlight falls on the energy converting glass. The maximum voltage is produced when maximum sunlight falls on the panel, and as they day progresses and the light changes this varying voltage has two further trip points. The first trip point is the minimum voltage that can be used to charge the battery. For a lead acid cell no less than 13.5 volts is effective for charging.
The next trip point is when the sun is sufficiently set so that the light connected to your project can be switched on. So it clicks now, doesn’t it. Mulling over the sensor’s location, type connections and so on; really no need. The reduced component count also helps in terms of the device. 1 I/O line is not necessarily a game changer but as a system specification it’s one less thing to worry about.
The basic principle is therefore to utilise the voltage output from the panel to determine what state the sun is in, and this is exactly the same as what the LDR (Light Dependant Resistor) does.