This is Simple high water level alarm circuit diagram which we need to build it since: fish pond The water level will drop rapidly. We need to check water
This is 5V switching regulator circuit using transistor, decreases voltage or Step down voltage converter circuit, decrease 6-18V into 5V at current 100mA.
If you are looking for circuit that turns on relay when light comes. It is a simple light activated relay circuit. It will control load on-off with light.