This simple power supply circuit can reduce any voltage from 20vac to 120vac into 5 volts DC and can provide current up to 100mA. [Link]
This is an electronic fuse. It protects your circuit from shorts. Relays should be chosen depending on the input voltage. [Link]
Series resonance is a fundamental phenomenon in electronics. Series resonance can be achieved by connecting inductor L and capacitor C in series. To make things closer to reality there is also a resistor included that represents energy loss.