So, if we want a lamp to be on when the relay is on, we connect our circuit from COM to NO. When the relay is turned on the electromagnet flips the switch up and COM is then connected to Normally Open. The diagram shows that COM is connected to the Normally Closed contact. This switch is "thrown" by the electromagnet inside. This shows the switch that is inside the relay. (Sometimes the relay board has a diagram rather than NO-COM-NC labels). The connections are labelled "NO", "COM", "NC". The relay contacts are brought out to screw-type terminal strips for easy connections to wires. Two relays are mounted on a small printed circuit board with added driver transistors, optical isolators and LEDs. On the right is a closeup of the contacts.Ĭonnecting to a relay board: See a typical relay board on the left. The Contacts are made of a special alloy of metals. ![]() Let's look at what is inside these relays: On the left is a relay with the usual plastic cover removed. The C (Common) contact down and it disconnects from the NC contact and connects to the NO contact. ![]() When the relay is activated the electromagnet (shown as a coil of wire) pulls The two possible contact positions are shown as NC (Normally Closed) and NO (Normally Open). The schematic diagram of this type of relay is shown on the right. The most common relays used with Arduino are SPDT switches that are Thrown by an electromagnet that can be activated by Arduino or other Microcomputers. Often we show switches by their Schematic Diagram. Sometimes you will see switches abbreviated: The two examples above would be SPST and SPDT. So it is a Single Pole - Double Throw switch. The switch on the right can be connected two different ways: to the contact on the left OR the contact on the right. So, the left switch above is a Single Pole - Single Throw switch: The simplest kind. Switches are labelled as having POLES (how many switches work together) and THROWS (How many different connections they can make). The simple switch on the left makes or breaks one connection. Switches make or break electrical connections. Photo above shows 1,2,4 and 8 relays on Opto-Isolated boards. 4.1 Using two typical SPDT (Single Pole Double Throw) relays to control a DC motor:Ī RELAY is: A Switch that is "thrown" by an Electromagnet. ![]() 1.1 Photo above shows 1,2,4 and 8 relays on Opto-Isolated boards.1 A RELAY is: A Switch that is "thrown" by an Electromagnet.
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