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How to Use Transistors

How to Use Transistors

If you have understood well how to use transistors in circuits, you might have already conquered half of  electronics and its principles. In this post we make an effort in this direction.


Transistors are 3 terminal semiconductor devices which is able to conduct relatively high power across their two terminals, in response to a significantly low power input at the third terminal.

Transistors are basically of two types: bipolar junction transistor (BJT), and metal–oxide–semiconductor field-effect transistor (MOSFET)

For a BJT, the 3 terminals are designated as base, emitter, collector. A low power signal across base/emitter terminal allows the transistor to switch a comparatively high power load across its collector terminal.

For MOSFETs these are designated as Gate, Source, Drain. A low power signal across Gate/Source terminal allows the transistor to switch a comparatively high power load across its collector terminal.

For the sake of simplicity we will discuss BJTs here, since their charcaeritics is less complex compared to MOSFETs.

Transistors (BJTs) are the building blocks of all semiconductor devices found today. If there wouldn’t be transistors there wouldn’t be any ICs or any other semiconductor component. Even ICs are made up of 1000s of closely knit transistors which constitute the features of the particular chip.

New electronic hobbyists usually find it difficult to handle these useful components and configure them as circuits for an intended application.

Here we’ll study the functions and the way of handling and implementing bipolar transistors into practical circuits.

How to use Transistors like a Switch

Bipolar transistors are generally a three lead active electronic component which fundamentally work as a switch for either switching ON or switching OFF power to an external load or an associated electronic stage of the circuit.

A classic example can be seen below, where a transistor is connected as a common emitter amplifier:

This is the standard method of using any transistor like a switch for controlling a given load. You can see when a small external voltage is applied to the base, the transistor switches ON and conducts heavier current across the collector emitter terminals, switching on a bigger load.

Remember, the negative or the ground line of the external voltage must be connected with the transistor ground line or the emitter, otherwise the external voltage will have no effect on the transistor.

Using Transistor as a Relay Driver

I have already explained in one of my earlier posts regarding how to make a transistor driver circuit.

Basically it uses the same configuration as shown above. Here's the standard circuit for the same:

If you are confused about the relay, you can refer to this comprehensive article which explains everything about relay configurations.

Using Transistor to Light Dimmer

The following configuration shows how a transistor can be used as a light dimmer using a emitter follower circuit.

You can see as the variable resistor or the pot is varied, the lamp intensity also varies. We call it emitter-follower, because the voltage at the emitter or across the bulb follows the voltage at the base of the transistor.

To be precise the emitter voltage will be just 0.7 V behind the base voltage. For example, if the base voltage is 6 V, the emitter will 6 - 0.7 = 5.3 V and so on. The 0.7 V difference is due to the minimum forward voltage drop rating of the transistor across the base emitter.

Here, the pot resistance along with the 1 K resistor forms a resistive divider network at the base of the transistor. As the pot slider is moved, the voltage at the base of the transistor is changed, and this correspondingly alters the emitter voltage across the lamp, and the lamp intensity changes accordingly.

Using Transistor as a Sensor

From the above discussions you might have observed that the transistor is doing one crucial thing in all the applications. It is basically amplifying the voltage at its base by allowing a large current to be switched across its collector emitter.

This amplifying feature is also exploited when a transistor is used as a sensor. The following example shows how it can used to sense the difference in ambient light, and switch ON/OFF a relay accordingly.

Here too the LDR and the 300 ohm/5 k preset forms a potential divider at the base of the transistor.

The 300 ohm is actually not required. It is included to ensure that the transistor base is never fully grounded, and thus it is never completely disabled or shut off. It also ensures that the current through the LDR can never exceed a certain minimum limit, no matter how bright the light intensity is on the LDR.

When it's dark, the LDR has a high resistance which is many times higher than the combined value of the 300 ohm and the 5 K preset.

Due to this the transistor base gets more ground side voltage (negative) than the positive voltage, and its collector/emitter conduction remains switched OFF.

However when sufficient light falls on the LDR, its resistance drops to a few kilo-ohm value.

This enables the base voltage of the transistor to rise well over the 0.7 V mark. The transistor now gets biased and switches ON the collector load, that is the relay.

As you can see, in this application too the transistors is basically amplifying the tiny base voltage such that a bigger load at its collector could be switched ON.

The LDR can be replaced with other sensors such as a thermistor for heat sensing, a water sensor for water sensing, a photodiode for IR beam sensing, and so forth.

Question for you: What happens if the position of the LDR and the 300/5 K preset are swapped with each other?

Transistor Packages

Transistors are normally recognized by their external package in which the particular device may be embedded. The most common types of package in which these useful devices are enclosed, are the T0-92, TO-126, TO-220 and TO-3. We will try to understand all these specifications of transistors and also learn how to use them in practical circuits.

Understanding Small Signal TO-92 Transistors:

Transistors like BC547, BC557, BC546, BC548, BC549, etc all come under this category.

These are the most elementary in the group and are used for applications involving low voltages and currents. Interestingly this category of transistors is used most extensively and universally in electronic circuits due to their versatile parameters.

NPN transistor BJT symbol

Normally these devices are designed to handle voltages anywhere between 30 to 60 volts across their collector and emitter.

The base voltage is not more than 6, but they can be easily triggered with a voltage level as low as 0.7 volts at their base. However the current must be limited to 3 mA approximately.

The three leads of a TO-92 transistor may be identified in the following manner:

Keeping the printed side toward us, the right side lead is the emitter, the center one is the base and the left hand side leg is the collector of the device.

UPDATE: Want to know how to use transistors with Arduino? Read it here

How to Configure a TO-92 Transistor into Practical The Designs

Transistors are mainly of two types, an NPN type and a PNP type, both are complementary to each other. Basically they both behave the same way but in the opposite references and directions.

For example an NPN device will require a positive trigger with respect to the ground while a PNP device will require a negative trigger with reference to a positive supply line for implementing the specified results.

The three leads of the transistor explained above needs to be assigned with specified inputs and outputs for making it work for a particular application which obviously is for switching a parameter.

The leads need to be assigned with the following input and output parameters:

The emitter of any transistor is the reference pinout of the device, meaning it needs to be assigned the specified common supply reference so that the remaining two leads can operate with reference to it.

An NPN transistor will always need a negative supply as the reference, connected at its emitter lead for proper functioning, while for a PNP, it will be the positive supply line for its emitter.

The collector is the load carrying lead of a transistor and the load which needs to be switched is introduced at the collector of a transistor (see figure).

NPN, PNP transistor wiring details

The base of a transistor is the trigger terminal which is required to be applied with a small voltage level so the current through the load can pass through, across to the emitter line making the circuit complete and operating the load.

The removal of the trigger supply to the base immediately switches OFF the load or simply the current across the collector and the emitter terminals.

Understanding TO-126, TO-220 Power Transistors:

These are medium type of power transistors used for applications which require switching of powerful relatively powerful loads lie transformers, lamps etc. and for driving TO-3 devices, typical egs are BD139, BD140, BD135 etc.

BD139 and TIP32 pinout diagram

Identifying BJT pinouts

The pinout are identified in the following manner:

Holding the device with its printed surface facing you, the right side lead is the emitter, the center lead is the collector and the left side lead is the base.

The functioning and the triggering principle is exactly similar to what is explained in the previous section.

The device is operated with loads anywhere from 100 mA to 2 amps across their collector to emitter.

The base trigger can be anywhere from 1 to 5 volts with currents not exceeding 50 mA depending upon the power of the loads to be switched.

Understanding TO-3 Power Transistors:

These can be seen in metallic packages as shown in the figure. The common examples of TO-3 power transistors are 2N3055, AD149, BU205, etc.

TO3 2N3055 pinout details base emitter collector

The leads of a TO-3 package can be identified as follows:

Holding the lead side of the device toward you such that the metal part beside the leads having larger area is held upward (see figure), the right side lead is the base, the left side lead is the emitter while the metallic body of the device forms the collector of the package.

The function and operating principle is just about the same as explained for the small signal transistor however the power specs increase proportionately as given below:

Collector-emitter voltage can be anywhere between 30 to 400 volts and current between 10 to 30 Amps.

Base trigger should be optimally around 5 volts, with current levels from 10 to 50 mA depending upon the magnitude of the load to be triggered. The base triggering current is directly proportional to the load current.

Have more specific questions? Please ask them through your comments, I am here to solve them all for you.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

16 thoughts on “How to Use Transistors”

  1. Swag,

    Simple! Dead simple! Just by reading your lectures I find the connections I missed. Thank you very much for explaining the inner characteristics of these components.

    The lack of this knowledge made me afraid to build and test. You are giving us the ability to build in a safe manner so that we do not just blow things up. I have not seen any other electronics lectures where this is done so clearly.

    Without this all theoretical electronics knowledge is useless.

    Thank you again,

    • Hi Christo, I really appreciate your involvement with this website, please keep up the good work, I am truly glad this site is helping you learn something useful. Please continue posting your queries, I’ll be most happy to help!!

  2. Sir,
    As you stated for an NPN transistor, its emitter will connect to (-) of voltage source and giving a positive trigger voltage (resistorised) at its base, the transistor now conducts between collector and emitter.

    similar states for a PNP transistor with its emitter going to (+) of volt.source and passing a neg.trig signal at its base, the transistor conducts between collector and emitter.

    The rule is: As long as a transistor gets an appropriate input signal at its base, the path between emitter to collector gets to to conduct freely within its rated parameters.

    The above all is clear to me.

    Now i have some doubts:

    1. if we take an NPN transistor and give (+) to its collector, giving a negative trig.sig at its base and take the (+) output from its emitter, is this CORRECT to do??
    please guide me here.

    2. what do you mean by "The base triggering current is directly proportional to the load current". please do explain with some example.


  3. sir I made a generator self starter driving circuit. it was built around ic555 and 4017. 555 used in astable mode to drive 4017. pin 3 of 555 connected to pin 14 of 4017.
    I used pin 2,7 and 1 to drive a relay. each pin has its own resistor of 4k7 value. then i connected them to the base of a transistor (547/8050 I used both ) and connected a relay. I tested the circuit by connecting it to 12v 40Ah battery. it was all okay but when I connected the self starter wire to the relay(NO point) it started malfunctioning by tripping again and again disrupting the ic programming. I tried changing the base resistor and transistor as well. I observed that when high current pass through the relay circuit starts malfunctioning. why please solve

    • Mujahid, what is the function of the astable in your circuit? I could not understand this….also why did you use 3 pins in parallel of the IC 4017 for connecting the relay driver transistor?

      By the way did you connect a capacitor across pin5 and ground of the IC555? Do this if not done yet.

  4. how to control current and voltage to the base of a transistor and if your answer is Resistor then i would like to know how to determine the value of a resistor?

  5. hello , my name is sajeesh, i am from kerala. i am a cable tv technician. is it possible to work our optical receiving unit by the induction current produced from the television and the set top box from the customer end. optical receiving unit ratings 6volt 150mA . average co axial cable length to customer is 60 meters. now optical photo diode works well in this scenario but not sufficient signal gain so need a amplifier with optical photo diode with two fet in it. can you help me

    • optical receiving unit consists of a photo diode plus two fet . Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry
      There are several photodetector types:
      PhotodiOptical receivers convert optical signal (light) to electrical signal (current/voltage)
      Hence referred ‘O/E Codes, Phototransistors, Photon multipliers, Photo-resistors etc. Positive-Intrinsic-Negative (pin) photodiode
      No internal gain
      Avalanche Photo Diode (APD)
      An internal gain of M due to self multiplication
      Photodiodes are sufficiently reverse biased during normal operation  no current flow, the intrinsic region is fully depleted of carriers.The first receiver element is a pin or an avalanche photodiode, which produces an electric current proportional to the received power level.
      Since this electric current typically is very weak, a front-end amplifier boosts it to a level that can be used by the following electronics. normally five stage fet is used for amplification but ineed only two stage amplification

    • OK, but where is this unit installed, what is the function of this unit in the whole application?
      Is it kept on the TV set? If not then how should the induction toggle the unit? through a wire link?

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