The diode is an electronic component that allows the passage of current in only one direction.
They are solid state semiconductor devices and typically need more than 0.6 volts to operate. In a normal diode, current flows from the positive anode to the negative cathode.
The germanium point-contact diode, dates from the early days of radio, when the received radio signal was detected by means of a germanium crystal and a fine, pointed wire that rested on it. In modern germanium (or silicon) point-contact diodes, the wire and a tiny crystal plate are mounted inside a small glass tube and connected to two wires that are fused into the ends of the tube.
Junction-type diodes consist of a junction of two different kinds of semiconductor material. There are literally thousands of different specification diodes. They are usually identified with a number but some are color coded.
1N4148 Diode do the trick most of the time