In this example I am just turning on and off a LED. * Once the NO terminal is connected a Lamp or any other load can be turned on. The electromagnet then pulls the switching contact and moves it to connect the COM terminal to the NO terminal. 07 amps starts flowing through the junction turning on the electromagnet. 0005 amps (500 microamps) flows and turns on the transistor. Basically, when +5v flow through the 1K resistor to the Base of transistor, a current of about. * Thats it our circuit is complete, now we can upload the code to the Arduino to turn on or off the relay. * Final, using a 1k resistor connect the Base of the transistor to the D2 pin of the Arduino. * The Emitter connects to the -ve rail of the breadboard. * Then connect the Collector of the NPN transistor to the 2nd pin of the coil. The diode across the electromagnet conducts in the reverse direction when the transistor is turned off to protect against a voltage spike or the backward flow of current. * Next we need to connect a diode across the electromagnetic coil. * Then connect one of the coils pin to the +ve 5v rail of breadboard. * Lets start by connecting the VIN and GND pins of the Arduino to the +ve and -ve rails of the breadboard. In this project the NPN transistor 2N2222 drives the relay when the NPN junction gets saturated. Therefore we need a device to amplify the current. * Now a question arises, Why do we need the extra bit of circuit to drive the relay? The coil of the relay needs a large current (around 150mA) to drive the relay, which an Arduino cannot provide. This process is called "Electrical Isolation". The switching contacts of a relay are completely isolated from the coil, and hence from the Arduino. A relay accomplishes this by using the 5V outputted from an Arduino pin to energize the electromagnet which in turn closes an internal, physical switch to turn on or off a higher power circuit. * A relay basically allows a relatively low voltage to easily control higher power circuits. You can use a 5V relay to switch the 120-240V current and use the Arduino to control the relay. A LED is easy enough, but large power items such as light bulbs, motors, pumps or fans required more sneaky circuitry. * The question is "Why to use a relay with an Arduino?"Ī micro controller's GPIO (general purpose input/output) pins cannot handle higher power devices. Only one of the pins will complete the circuit and will show activity on the multimeter. Then connect the other probe one by one to the remaining 2 pins. Now connect one on the multimeter's probe to the pin in between the coils (Common Pin) Turn the relay upside-down to see pins located at its bottom part. Set the multimeter to resistance measuring mode. If you ever get confused in figuring out which one is NO and which one is NC pin, follow the steps below to easily determine that: If we connect our battery to the right pins you can actually hear the *clicking* sound when the switch turns on. Hence, the positive output of DC power supply can be connected to any one of the coil pins while negative output of DC power supply will be connected to the other pin of the coil or vice versa. This relay has 'no' polarity marked on it since the internal suppressing diode is not present in it. You can do it either by connecting a multimeter to resistance measuring mode with a scale of 1000 ohm (since the coil resistance normally ranges between 50 ohm and 1000 ohm) or by using a battery. Let’s start by determining the relay coil pins. I have attached the datasheet of the relay for more information. L: is the "Coil Sensitivity" which is 0.36W 05VDC: Also known as "Nominal Coil Voltage" or "Relay Activation Voltage", it is the voltage necessary for the coil to activate the relay. It starts from and goes down till Finally the bottom bit says: SRD-05VDC-SL-C SRD: is the model of relay. Then we see the "Current and Voltage Rating": it is the maximum current and/or voltage that can be passed through the switch. Now, if we look at the top of the relay the first thing we see is SONGLE, it is the name of the manufacturer. The closing and releasing of the contacts results in powering on and off of the circuits. When the coil is de-energized the electromagnetic force disappears and the armature moves back to the original position turning on the NC contact. When the electromagnet is energized the NO is the one which is on and NC is the one which is off. When current flows through the coil of the relay, a magnetic field is created that causes a ferrous armature to move, either making or breaking an electrical connection. Middle one is COM (common) and the rest of the two are called NO (Normally Open) and NC (Normally Close). It is very popular relay among Arduino and DIY electronics hobbyists. The relay I am looking at is a SRD-05VDC-SL-C.
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