The one you’re using for this lab is an NPN-type transistor called a TIP120. Working Explanation. Since the relay can draw as much as 80 mA when energized, you could damage the Arduino if you drive the relay directly from it. After everything is connected according to the wiring diagram above, upload this code to your Arduino: Although the simplest relay circuits would simply involve a switch closing a circuit, the applications of relays often require a small signal, possibly from some form of a microcontroller circuit or other device to actuate the relay. It will be saturated. See Figure 16 and Figure 17 for the pinout drawing and schematic symbol of the transistor. The areas of operation for a transistor switch are acknowledged as the Saturation Region and the Cut-off Region. In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil! In the following circuit, the transistor acts as a switch, in the same way, that a relay works. The transistor can handle higher voltages than most logic-level translator circuits. This is a schematic diagram for a circuit that uses an NPN transistor as a switch that turns an LED on or off. A transistor could translate between a 3.3 V circuit and a 12 V circuit, for example. In our case LED will glow. We can control the sensitivity of the circuit by adjusting VR1. A relay driver circuit is a circuit which can drive, or operate, a relay so that it can function appropriately in a circuit. Simple transistor circuits with LDR are very common and utilised in night lighting and emergency lighting systems. When we press the push button a small amount of current entered in the base of the transistor which turns it ON when we release the push button the base current is no more present so a transistor will enter into its cut-off region and finally turned OFF. The voltage level of the input can be changed by sliding the black arrow- head up and down on this vertical scale. The diode D1 protects the transistor against overvoltage. In the present design it’s around 1 minute. The driven relay can then operate as a switch in the circuit which can open or close, according to the needs of the circuit and its operation. Would a relay be better than a transistor switch? Here the "ground" refers to the negative line for an NPN and the positive line for a PNP BJT. The transistor may directly drive a low-current LED or bulb, or a relay coil, which then acts as a switch to control a high-current load. The values for Rb and Qs vary accordingly. Relays are a better choice for switching large currents (> 5A). How to Connect a Transistor as a Switch in a Circuit. The transistor is off when there’s no bias voltage or when the bias voltage is less than 0.7 V. The switch is on when the base is saturated so that collector current can flow without restriction. Transistor Switch’s Working Regions or Operating Modes Meanwhile, the saturation zone and cut-off area are known as the transistor switch’s working regions. If we set its resistance is low, the sensitivity of the circuit … They switch on, to turn on the relay, and LED1 grows. Transistor as a Switch. The relay circuits used for controlling relays often use semiconductor devices. The rate of ON and OFF per second can be changed by setting the 100 K pot. Relay control circuits. It is used to switch high loads or loads that needs AC current to operate. Diagram of Relay wired to a microcontroller and a lamp with a + 9 volt battery. As a result, the contacts of the relay switch. Now when Base of Transistor BC557 gets grounded, it starts conducting and energises the Coil of relay, which activates the Relay and turns ON the device connected to Relay. Then this transistor is capable of switching load that wil draw 4,6*20mA. Below is the pinout of a BJT: The advantage of a transistor to a relay is that a transistor is lighter, has less noise (if any at all), has no moving components, it is faster, consumes less energy and it is cheaper. For this simple circuit, arrangements are created that energizes the coil of relay and load connected with the relay is controlled. Below figure circuit configuration to control the relay through the transistor is shown. So now that we know the theory of why transistors are used as switches, let's go over how to connect a transistor to function as a switch in a circuit. Relay connection pins Figure 4. Transistor is working as switch. A transistor works best as a switch when it is connected with a common emitter configuration, meaning the emitter of the BJT must be always connected directly with "ground" line. The transistor allows you to control a circuit that’s carrying higher current and voltage from the microcontroller. The relay operation can be controlled by making the necessary changes in the circuit so that any external device is connected with respect to the relay and gets controlled. This is a simple photo switch suitable for home and industrial purposes.The circuit switches on a relay when the light intensity falling on the sensor crosses the set limit. The circuit with the 40k resistor produces a base current of about only 0.28mA which might not be enough (the datasheet for most little transistors say that the base current should be 1/10th the collector current when a transistor is used as a switch. The time delay is determined by the values of R1 and C1. In the circuit, IC is connected as an astable multivibrator and the 12-volt relay is extracted from the 2N4401 transistor that acts as a switch. The most frequently used practical application that is used for the transistor as a switch is functioning of LED. Transistor BC547 is probably the most elementary of the available electronic active components and yet becomes the basic, vital building block in most electronic circuits. Any other kind of solid state switch (bjt, scr, triac, igbt) will have some resistance and drop some voltage. The circuit is based on a NE 555 and two transistors for driving the relay. Pressing the push ON switch instantly charges the 1000uF capacitor and switching ON the transistors and the relay. The transistor will operate as an amplifier or other linear circuit if the transistor is biased into the linear region. The most important point to note here is that there are no moving contacts in a transistor and the conduction is by transistor action. The circuit with the 10k resistor produces a base current of about 0.41mA in the C1815 transistor. Relay driver Circuit using PNP BJT. Crystal Tester: If the circuit uses the BJT transistor as a switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the I-V characteristics curves shown below. A relay is a device by which electric current flowing in one circuit can open or close a second circuit i.e. R1 limits the current to LED1 with safe value. The Code. Advantages of relays: Relays can switch AC and DC, transistors can only switch DC. Transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In this circuit arrangement an Enhancement-mode N-channel MOSFET is being used to switch a simple lamp “ON” and “OFF” (could also be an LED). So in this arduino relay control tutorial we will simply learn How to interface a Relay with Arduino.Here we are not using any Relay Driver IC like ULN2003 and will only use an NPN transistor to control relay. Relays can switch high voltages, transistors cannot. To control the function of the relay transistor as a switch can be used. Its not working in linear region. The 2N7000 is normally used as a switch. The circuit has three distinct advantages, it requires only a few parts, always comes up with the relay deactivated, and doesn't need any switch debouncing. A transistor can be operated in three modes, active region, saturation region, and cut-off region. Here, the base circuit corresponds to relay coil and the transistor to the contacts of the relay. A transistor is a 3-pin device composed of a base, collector, and emitter for bipolar junction transistors (BJTs). The 100µF capacitor that is connected to the base of the transistor, is used to keep a stable base bias of the transistor so that there are no relay clicking. Note: The relay has a built-in transistor to drive the relay coil as well as a diode across the winding to protect the transistor. RELAY SWITCH CIRCUIT Relays are widely used to protect electronic devices from short circuits these are electromechanical devices which uses electromagnet and it is used to close the movable contacts when it is in open position Relays are used to control motors, heaters, or AC circuits which will draw a lot more electrical power. ... a 2N3904 is driving a relay. It acts as an electronic switch. The gate input voltage V GS is taken to an appropriate positive voltage level to turn the device and therefore the lamp load either “ON”, ( V GS = +ve ) or at a zero voltage level that turns the device “OFF”, ( V GS = 0V ). The protection diode Dp is used to protect the transistor from the reverse current generated from the coil of the relay during the switch off time. D1 absorbs the current spikes to protects the external relay driver. In Figure 4, you can see that there’s no electrical connection between the microcontroller circuit that’s controlling the coil of the relay and the lamp circuit. switch the current ON or OFF in the second circuit. Transistor to Operate the Relay. Whenever we need to connect any AC Appliance in our embedded circuits, we use a Relay. The circuit has a well-known NE555 timer IC which, due to its uses in the wide range of electronic circuits, is very popular. Transistor amplification factor also know as HFE in datasheets is variable to input current. This implies that, by switching between its “top-off” (saturation) and “absolute OFF,” the transistor is used as a switch to basically overwrite its Q-Point and the voltage dividers circuit that is needed for amplification. When switch S is OFF, the base current is zero. The relay will be actuated when the input of the circuit goes high. A relay switch circuit can be designed using NPN transistor. 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