The diode in parallel on the relay plays a protective role to prevent the crystal triode from being damaged due to high voltage generated by self induction when the relay is powered off and released.

Component specification: J relay 4098 operating voltage 6V V1V2 crystal triode 9014 VD crystal diode 2CP10 or 1N4001 R adjustable resistor 100k C electrolytic capacitor 100u/6V K1 button switch.

The simplest switch has two pieces of metal called "contacts". When the two contacts contact, the current will form a loop. When the two contacts do not contact, the current will open. When selecting contact metal, the degree of corrosion resistance shall be considered, because most metals will form insulating oxides after oxidation, making the contact unable to work normally. Factors such as conductivity, hardness, mechanical strength, cost and toxicity shall also be considered when selecting contact metal. Corrosion resistant metals are sometimes plated on the contacts. Generally, it will be plated on the contact surface of the contact to avoid affecting its performance due to oxides. Sometimes non-metallic conductive materials, such as conductive plastics, are used on the contact surface. In addition to the contacts, there will also be movable parts to make the contacts conductive or non conductive. The switches can be divided into toggle switch, key switch, Rocker switch, etc. according to the different movable parts, and the movable parts can also be other types of mechanical links.