What Everybody Ought to know about Basic Circuits in Electronics?

For all you eager to build your own electronic projects, the first thing you need to know is the basic electronics. There are many components in electronics which are used for applications like generating pulses, as an amplifier etc. We often require basic circuits for our electronic projects. These basic circuits can be a pulse generating circuit, an oscillator circuit or an amplifier circuit. Here I am explaining few electronics circuits. It is very useful for the beginners.

  • Astable Multivibrator using 555 Timer:

The 555 timer generates the continuous pulses in astable mode with specific frequency that depends on the value of the two resistors and capacitors. Here the capacitors charges and discharges at specific voltage.

When the voltage is applied the capacitors charges and through a resistors continuously and the timer produce continuous pulses.  The pin 6 and 2 are shorted together to re-trigger the circuit continuously.  When the output trigger pulse is high, it remains at that position until capacitor is completely discharged. Higher value of the capacitor and resistors are used to attain longer time delay.

These type of circuits could be used in switching the motors ON and OFF at regular intervals or for flashing lamps / LEDs.

Astable Multivibrator using 555 Timer
Astable Multivibrator using 555 Timer
  • Bistable Multivibrator using 555 Timer:

The bi-stable mode has two stable states which are high and low states. The high and low of the output signals are controlled by trigger and reset inputs pins , not by the charging and discharging of capacitors. When a low logic signal is given to the trigger pin , the output of the circuit go into the high state and when a low logic signal is given to the reset pin low the output of the circuit go into low sate.

These types of circuits are ideal for use in automated models such as railway systems and motor push to ON and push to OFF control system.

Bistable Multivibrator
Bistable Multivibrator
  • 555 Timers in Mono Stable Mode:

In the mono stable mode the 555 timer can produce one single pulse when the timer receives a signal at trigger input button.  The duration of the pulse depends on the values of the resistor and capacitor. When the trigger pulse is applied to the input through a push button, the capacitor is charged and the timer develops a high pulse and it remains high until capacitor completely discharges. If more time delay is required, higher value of resistor and capacitor are needed.

Monostable Multivibrator
Monostable Multivibrator
  • The Common Emitter Amplifier:

The transistors can be used as amplifiers where the amplitude of the input signal is increased. A transistor connected in common emitter mode is biased in such a way that its base terminal is given an input signal and the output is developed at the collector terminal.

For any transistor operating in active mode, the base emitter junction is forward biased, thus having a low resistance. The base collector region in reverse biased, having high resistance. The current flowing from the collector terminal is β times more than the current flowing into the base terminal. Β is the current gain for the transistor.

Common Emitter Amplifier
Common Emitter Amplifier

In the above circuit, current flows to the base of the transistor, from the AC supply source. It gets amplified at the collector. When this current flow through any load connected at the output, it produces a voltage across the load. This voltage is an amplified and inverted version of the input signal voltage.

  • The Transistor as a Switch:

The transistor acts as a switch when it is operated in saturated region. As the transistor is switched ON in the saturation region, the emitter and collector terminals get short circuited and the current flows from collector to emitter in an NPN transistor. Actually maximum amount of base current is given which results in a maximum amount of collector current.

The voltage at the collector emitter junction is such low such that it reduces the depletion region. This causes the current to flow from collector to emitter and they appear to be shorted.  When transistor is biased in cut off region, both the input base current and the output current are zero. The reverse voltage applied to the collector emitter junction is at its maximum level. This causes the depletion region at that junction to increase such that no current flows through the transistor. Thus the transistor is switched OFF.

Transistor as a Switch
Transistor as a Switch

Here we have a load that we wanted to turn ON and OFF with a switch. When the ON / OFF switch is in closed state, current flows in the base terminal of the transistor. The transistor gets biased such that the collector and emitter terminals are shorted and connected to ground terminal. The relay coil gets energized and the contact points of the relay closes such that the load gets the supply being connected in series through this contact acting like an independent switch.

  • Schmitt Trigger:

The Schmitt trigger is a type of comparator, which is used to detect whether the input voltage is above or below a certain threshold. It produces a square wave such that the output toggles between two binary states.  The circuit shows two NPN transistors Q1 and Q2 connected in parallel. The transistors are switched ON and OFF alternatively based on the input voltage.

Schmitt Trigger Circuit
Schmitt Trigger Circuit

The transistor Q2 is biased through a potential divider arrangement. With the base being at a positive potential compared to the emitter, the transistor is biased in saturation region. In other words the transistor is switched on (the collector and emitter terminals are shorted). The base of the transistor Q1 is connected to ground potential through the resistor Re. Since there is no input signal given to the transistor Q1, it is not biased and is in cut off mode.  Thus we get a logic signal at the collector terminal of the transistor Q2 or the output.

An input signal is given such that the potential at the base terminal is more positive than the voltage across the potential divider. This causes the transistor Q1 to conduct or in other words the collector emitter terminals are shorted. This causes the collector emitter voltage to drop and as a result the voltage across the potential divider reduces such that the base of the transistor Q2 doesn’t get enough supply. The transistor Q2 is thus switched off.  Thus we get a high logic signal at the output.

  • H Bridge Circuit:

An H bridge is an electronic circuit that enables a voltage to be an applied across a load in either direction. The H bridge is a very effective method for driving motors and it finds a lot of applications in many electronic projects especially in robotics.

Here four transistors are used which are connected as switches. The two signal lines allow running the motor in different directions. The switch s1 is pressed to run the motor in forward directions and s2 is pressed to run the motor in backward direction.  Since the motor needs to dissipate the back EMF, the diodes are used to provide a safer path for the current. The resistors are used to protect the transistors as they limit the base current to the transistors.

H Bridge Circuit
H Bridge Circuit

In this circuit, when the switch S1 is in ON state, the transistor Q1 is biased to conduction and so is the transistor Q4. The positive terminal of the motor is thus connected to ground potential.

When switch S2 is also ON, the transistor Q2 and transistor Q3 are conducting. The negative terminal of the motor is also connected to ground potential.

Thus with no proper supply, the motor doesn’t rotate. When S1 is OFF, the positive terminal of the motor gets positive voltage supply (as the transistors are cut off). Thus with S1 OFF and S2 ON, the motor is connected in normal mode and starts rotating in forward direction. Similarly when S1 is ON and S2 OFF, the motor gets connected to reverse supply and starts rotating in reverse direction.

  • Crystal Oscillator Circuit:

A crystal oscillator uses a crystal to develop some electrical signal at a certain frequency. When mechanical pressure is applied on the crystal, it produces an electrical signal across its terminals with a certain frequency.

The crystal oscillators are used to provide stable and accurate radio frequency signals.  One of most common circuits is used for crystal oscillators is the colpitts circuit. They are basically used in digital systems to provide clock signals.

Crystal Oscillator Circuit
Crystal Oscillator Circuit

The crystal operates in parallel resonant mode and generates output signal. The capacitor divider network of C1 and C2 provide the feedback path. The capacitors also form the load capacitance for the crystal. This oscillator can be biased in common emitter or common collector modes. Here the common emitter configuration is used.

A resistor is connected between the collector and source voltage. The output is obtained from the emitter terminal of the transistor through a capacitor. This capacitor acts as a buffer to ensure that the load draws minimum current.

So these are the basic circuits you will encounter in any electronic project. I hope this article has given you ample knowledge. So there is this little task for you. For all the circuits I have listed above, there are alternatives. Kindly find that and post your answer in the comment sections below.

4 Comments

  1. ankit Lohiya says:

    Hi Tarun,

    Please let me know, how we can remove transformer from a circuit to reduce a voltage from 230V to 12V.

    Can we use resistors to reduce the voltages or can reduce DC voltage after conversion using the resistance…?

    Give me your valuable suggestions.

    1. Tarun Agarwal says:

      Hi Ankit
      There are two types of transformerless power supplies namely resistive and capacitive.

  2. V. Hari Hara Prasad says:

    Dear Mr. Sheri Naresh Reddy,
    First, let me wish you a happy and prosperous new year – 2014.
    Your articles are very nice. Though I am not an electronics master, as a hobbyist of electronics, I appreciate the content. Please include the applications of any circuit you write. This may help many hobbyists.

    I wish your articles will inspire more and more students of engineering get the basics, which they miss while studying in small colleges, which have mushroomed in the total Andhra Pradesh.
    All the very best, dear budding leader!
    V. Hari Hara Prasad
    Proddatur – Kadapa district. AP.

    1. Dear Hari Hara Prasad.
      Thanks for your feedback.
      For more detailed information on electrical and electronics please refer to our website https://www.elprocus.com
      For customization of projects please email us on team@elprocus.com

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