And the collector will connect to the load that the transistor will turn on and the supply voltage of the circuit. The emitter will connect to ground of the circuit. To connect the transistor as a switch in a circuit, we connect the output of the device that will switch on the transistor to the base of the transistor. Looking at the back side of the transistor, the emitter is the first pin, the base is the middle, and the collector is the third. 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.Ī transistor is a 3-pin device composed of a base, collector, and emitter for bipolar junction transistors (BJTs). How to Connect a Transistor as a Switch in a Circuit Transistors are used when we want to switch devices on or off when only current can the on-off state of the transistor.Īs a prime example of where transistors function perfectly as electrical switches, we will go through a few example circuits below. Such as an ON-OFF switch for turning on or off a device, volume control, etc. Mechanical switches are used mostly outside of electronic circuitry where it is desired that humans control various functions Transistors, however, turn on and off, not by physical human intervention, but by electrical current.īoth have their own uses. Mechanical switches, such as knife switches, pushbutton switches, need human intervention- someone to press them down and pull them back up. Unlike all the switches above, which are mechanical switches, a transistor turns on or Transistors are an essentialĬomponent in many circuits and are sometimes used to amplify a signal.So why are transistors used so frequently as switches in circuits if these switches above have the same use?Īnd the reason is that transistors are electrical switches. The resistor is present to protect the transistor as they can be damagedĮasily by too high a voltage / current. The transistor has to receive a voltage at its ‘ base’ and until The transistor then allows current toįlow from the +9 volts to the 0vs, and the lamp comes on. When the switch is pressed a current passes through the resistor into The circuit shown in diagram B is based on an NPN transistor. A small current or voltage at the base allows a larger voltage to flow through the other two leads When buying a transistor, directions will normally state clearly which lead is the BASE, EMITTER or COLLECTOR.ĭiagram 'A' shows an NPN transistor which is The leads on a transistor may not always be in this arrangement. The ‘tab’ on the type shown to the left is usually next to the They are notĪlways set out as shown in the diagrams to the left and right, although The diagram below shows the symbol of an NPN transistor. The EMITTER - which is the negative lead. The COLLECTOR - which is the positive lead. The BASE - which is the lead responsible for activating the transistor. Transistors are manufactured in different shapes but There are hundreds of transistors which workĪt different voltages but all of them fall into these two categories. They areĬentral to electronics and there are two main types NPN and PNP. Technology Department does not contain at least one transistor. They are used in a variety of circuitsĪnd you will find that it is rare that a circuit built in a school Transistors can be regarded as a type of switch, asĬan many electronic components. PDF FILE - CLICK HERE FOR PRINTABLE VERSION OFĬLICK HERE FOR POWERPOINT VERSION OF WORKSHEET
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