Once I purchased a China make AC-DC LED emergency lamp for Rs.70. I wonder ,how they are selling the device cheaply. Due to curiosity, I opened the case of thelamp. So simple circuit that cost only Rs.20, a small 6 volt battery and LEDs. This is the reason why they are selling the device at low cost. Reliability? Sometimes it will last years, otherwise few days. That depends on the use and conditions. Most of the China make devices use tranformerless power supply which can damage when spikes appear in the mains lines. Any way, simplicity and low cost can be considered. I have drawn the diagram and constructed one sample. It is still working. Let us see the circuit.