I would like to build a circuit consisting of a power supply, flashing light, and a reed switch. The purpose of the circuit is for the light to flash when the door the reed switch is connected to is opened. The light is rated for 12 volts and a current of 120 +/- 15 mA. How do I use that information to determine the voltage and amperage requirements of the power supply. I'd like to use an existing power supply such as USB charger and customize it for my needs. Thank you in advance.